MODELING THE NONLINEAR ACTIVE COCHLEA: MATHEMATICS AND ANALOG VLSI Bo Wen A DISSERTATION

MODELING THE NONLINEAR ACTIVE COCHLEA: MATHEMATICS AND ANALOG VLSI Bo Wen A DISSERTATION
MODELING THE NONLINEAR ACTIVE COCHLEA:
MATHEMATICS AND ANALOG VLSI
Bo Wen
A DISSERTATION
in
Bioengineering
Presented to the Faculties of the University of Pennsylvania in Partial
Fulfillment of the Requirements for the Degree of Doctor of Philosophy
2006
Kwabena A. Boahen,
Christopher Chen,
Supervisor of Dissertation
Graduate Group Chair
COPYRIGHT
Bo Wen
2006
Acknowledgements
This dissertation would not have been possible without the help from many individuals. I
would like to acknowledge all of the people who in their own way helped and supported
me with the present work.
I am truly indebted to my thesis advisor, Kwabena Boahen, for his outstanding guidance, encouragement, and support, throughout the course of this work. I am grateful for
his accepting me in his laboratory, which has opened a door to many exciting opportunities
for my career. His exhortations of “develop your intuition” and “keep the faith” are what I
will always bear in mind in my future research.
I would like to express my deep appreciation to my thesis committee members, James
Saunders, Jan Van der Spiegel, and Daniel Bogen, for their insightful and constructive
advice throughout my doctoral studies. I am especially grateful to James Saunders for his
continuing encouragement in my career development.
I sincerely thank Elizabeth Olson at Columbia University, Michael Anne Gratton and
Virginia Richards at Penn, Paul Fahey at the University of Scranton, for their invaluable
suggestions and discussions with my study in the cochlear mechanics.
iii
Special thanks go to my lab fellows, Kareem Zaghloul, Kai Hynna, Brian Taba, John
Arthur, Paul Merolla, John Wittig, Rodrigo Alvarez, and Joseph Lin, who helped create
an intellectually stimulating and fostering environment. Their companionship has helped
make this endeavor enjoyable and rewarding.
I would like to thank my parents for their continuing love and support during my pursuit
of a doctoral degree. Finally, I thank my husband, Xinyu Liu, for his positive impact on me
by his curiosity and interest in science and engineering, for his belief in me for reaching
my research goal, and for his unconditional love and encouragement, before and during our
years at Penn.
iv
Abstract
MODELING THE NONLINEAR ACTIVE COCHLEA:
MATHEMATICS AND ANALOG VLSI
Bo Wen
Supervisor: Kwabena A. Boahen
The human auditory system vastly outperforms any machine in efficiency and robustness in perceiving sound. Complex and delicate, its front end, namely the cochlea, senses
and processes sound in a nonlinear active fashion, exhibiting remarkable sensitivity and
extraordinary frequency discrimination. The mechanism through which the mammalian
cochlea achieves its incredible capability is postulated as the cochlear amplifier, which is
generally believed to originate from outer hair cells electromotility. However, the detail of
the cochlear amplifier remains unclear.
Using both analytic and synthetic approaches, the present work provided a plausible basis for realization of the cochlear amplifier, thus advancing our understanding of cochlear
mechanics. We proposed a novel cochlear amplifier mechanism based on the cochlea’s
microanatomy, as well as outer hair cell (OHC) motility, to account for the cochlea’s
characteristic behavior. This mechanism, active bidirectional coupling (ABC), considers
both the basal tilt of OHCs and the apical tilt of phalangeal processes as critical for feeding OHC motile forces (with saturating property) forward and backward onto the basilar
membrane, thereby enhancing the cochlea’s functioning. The ABC-based mathematical
cochlear model produces responses that are comparable to physiological measurements.
Theoretical model analysis reveals that ABC leads to negative damping basal to the response peak over a small longitudinal cochlear region. The tilted structure works as a
v
spatial filter, amplifying the cochlear traveling wave only when its wavelength becomes
comparable to the tilt distance.
Inspired by the biology, we proceeded to build a nonlinear active silicon cochlea—a
very large scale integration (VLSI) physical cochlear model—for achieving real-time lowpower cochlear processing. Designed in current mode and operating in Class AB, this
microchip implements the ABC mechanism, together with a silicon auditory nerve, in 0.25
µm complementary metal-oxide-semiconductor (CMOS) technology. The resultant new
architecture addresses the shortcomings of existing silicon cochleae, filter banks in cascade
and in parallel. Analog current representing the basilar membrane’s velocity drives the
silicon auditory nerve, which encodes sound in digital pulses, mimicking neuronal spikes,
as the final output of the cochlea.
vi
Contents
Acknowledgements
iii
Abstract
v
Contents
vii
List of Tables
xii
List of Figures
xiii
1
Introduction
1
1.1
Organization of the Dissertation . . . . . . . . . . . . . . . . . . . . . . .
3
1.2
Original Contribution of Present Work . . . . . . . . . . . . . . . . . . . .
4
1.3
The Usage of “We” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5
vii
2
The Cochlea and Cochlear Modeling
2.1
3
6
The Cochlear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7
2.1.1
Cochlear Anatomy . . . . . . . . . . . . . . . . . . . . . . . . . .
7
2.1.2
Cochlear Mechanics . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.2
Mathematical Cochlear Modeling . . . . . . . . . . . . . . . . . . . . . . 22
2.3
Cochlear Modeling in VLSI . . . . . . . . . . . . . . . . . . . . . . . . . 28
The Active Cochlea: Mathematical Model
3.1
3.2
33
The Linear Model: Numerical Simulation . . . . . . . . . . . . . . . . . . 34
3.1.1
Outer Hair Cell Motile Forces . . . . . . . . . . . . . . . . . . . . 34
3.1.2
Active Bidirectional Coupling . . . . . . . . . . . . . . . . . . . . 35
3.1.3
Formulation of a Two-Dimensional Linear Model . . . . . . . . . . 40
3.1.4
Model Simulation Results Using Finite-Difference Method . . . . . 44
3.1.5
Bidirectional versus Uni-Directional Coupling . . . . . . . . . . . 47
3.1.6
Comparison with Physiological Data . . . . . . . . . . . . . . . . . 49
3.1.7
Frequency Responses . . . . . . . . . . . . . . . . . . . . . . . . . 54
The Linear Model: Semi-Analytical Solution . . . . . . . . . . . . . . . . 54
viii
3.3
4
3.2.1
Solution Procedures . . . . . . . . . . . . . . . . . . . . . . . . . 56
3.2.2
Wavenumber Loci of the Traveling Wave . . . . . . . . . . . . . . 58
3.2.3
Semi-Analytical Solutions . . . . . . . . . . . . . . . . . . . . . . 65
3.2.4
Impedance of the Basilar Membrane . . . . . . . . . . . . . . . . . 74
3.2.5
Comparison of Numerical and Analytical Solutions . . . . . . . . . 76
3.2.6
Spatial Filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
The Nonlinear Model: Formulation and Simulation . . . . . . . . . . . . . 82
3.3.1
Compressive Growth . . . . . . . . . . . . . . . . . . . . . . . . . 84
3.3.2
Two-Tone Suppression . . . . . . . . . . . . . . . . . . . . . . . . 87
3.4
Model Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
3.5
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
The Active Cochlea: Analog VLSI Implementation
4.1
4.2
94
Analog VLSI Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
4.1.1
Filter Design Techniques . . . . . . . . . . . . . . . . . . . . . . . 96
4.1.2
Mismatch in CMOS Circuits . . . . . . . . . . . . . . . . . . . . . 100
Cochlear Circuit Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . 102
ix
4.3
4.4
5
4.2.1
Cochlear Fluid Design . . . . . . . . . . . . . . . . . . . . . . . . 103
4.2.2
Synthesis and Analysis of a Single BM Stage . . . . . . . . . . . . 107
4.2.3
Analysis of the Design . . . . . . . . . . . . . . . . . . . . . . . . 111
4.2.4
Adding Active Bidirectional Coupling . . . . . . . . . . . . . . . . 124
A Log-Domain Class AB Low-Pass Filter . . . . . . . . . . . . . . . . . . 129
4.3.1
Nodal Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
4.3.2
Circuit Building Block . . . . . . . . . . . . . . . . . . . . . . . . 131
Log-Domain Class AB Circuit Implementation of the Cochlea . . . . . . . 133
4.4.1
BM Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
4.4.2
Spiral Ganglion Cell Circuit . . . . . . . . . . . . . . . . . . . . . 134
4.5
Chip Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
4.6
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
Cochlear Chip Responses to Sound
144
5.1
Chip Test Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
5.2
BM Frequency Responses: Linear Domain . . . . . . . . . . . . . . . . . . 147
5.2.1
Effect of Active Bidirectional Coupling . . . . . . . . . . . . . . . 147
x
5.2.2
6
Active Cochlear Frequency Responses . . . . . . . . . . . . . . . . 148
5.3
Nonlinear Frequency Responses . . . . . . . . . . . . . . . . . . . . . . . 153
5.4
Longitudinal Patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
5.5
Silicon Auditory Nerve’s Spike Activities . . . . . . . . . . . . . . . . . . 159
5.6
Nonideality in Chip Responses . . . . . . . . . . . . . . . . . . . . . . . . 163
5.7
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174
Conclusions
176
6.1
Summary of Present Work . . . . . . . . . . . . . . . . . . . . . . . . . . 177
6.2
Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
A List of Abbreviations
182
Bibliography
184
xi
List of Tables
3.1
Mathematical Model Parameters . . . . . . . . . . . . . . . . . . . . . . . 46
3.2
Mathematical Model Responses with Varying Outer Hair Cell Motility . . . 47
3.3
Bidirectional versus Uni-Directional Coupling: Simulation Results . . . . . 51
xii
List of Figures
2.1
Human Ear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
2.2
Mechanism of the Human Ear . . . . . . . . . . . . . . . . . . . . . . . .
9
2.3
Radial Segment of the Cochlear Duct
2.4
Cochlear Partition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.5
Basilar Membrane Vibration . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.6
Deflection of Outer Hair Cell Stereocilia . . . . . . . . . . . . . . . . . . . 16
2.7
Deflection of Inner Hair Cell Stereocilia . . . . . . . . . . . . . . . . . . . 18
2.8
Feedback System with Outer Hair Cell Electromotility . . . . . . . . . . . 20
2.9
Saturating Profile of Outer Hair Cells . . . . . . . . . . . . . . . . . . . . 21
. . . . . . . . . . . . . . . . . . . . 11
2.10 Nonlinear Compressions in the Cochlea . . . . . . . . . . . . . . . . . . . 23
2.11 Two-tone Suppression in the Cochlea
xiii
. . . . . . . . . . . . . . . . . . . . 24
2.12 Cochlear Microanatomy . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.1
Active Bidirectional Coupling Mechanism . . . . . . . . . . . . . . . . . . 38
3.2
Two-dimensional Cochlear Model . . . . . . . . . . . . . . . . . . . . . . 42
3.3
Effect of Outer Hair Cell Motility . . . . . . . . . . . . . . . . . . . . . . 48
3.4
Comparison between Uni- and Bidirectional Coupling . . . . . . . . . . . . 50
3.5
Basilar Membrane Displacement Responses . . . . . . . . . . . . . . . . . 52
3.6
Frequency-Position Map . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
3.7
Frequency Responses of Basilar Membrane Velocity . . . . . . . . . . . . 55
3.8
Wavenumber Loci of Traveling Wave . . . . . . . . . . . . . . . . . . . . 60
3.9
Real and Imaginary Parts of Wavenumber . . . . . . . . . . . . . . . . . . 62
3.10 Velocity of Traveling Wave . . . . . . . . . . . . . . . . . . . . . . . . . . 63
3.11 Simulated Traveling Wave in the Passive and Active Cochleae . . . . . . . 64
3.12 Basilar Membrane Effective Impedance . . . . . . . . . . . . . . . . . . . 77
3.13 Basilar membrane Effective Impedance (Normalized) . . . . . . . . . . . . 78
3.14 Comparison Between Numerical and Semi-Analytical Solutions . . . . . . 80
3.15 Concept of Spatial Filter . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
xiv
3.16 Phase Difference and Damping ratio . . . . . . . . . . . . . . . . . . . . . 83
3.17 Saturating Profile of Outer Hair Cell Force . . . . . . . . . . . . . . . . . . 85
3.18 Cochlear Nonlinearity—Compressive Growth in the Model . . . . . . . . . 86
3.19 Nonlinearity: Two-Tone Suppression in the Model . . . . . . . . . . . . . . 89
4.1
Fluid Diffusive Element Design . . . . . . . . . . . . . . . . . . . . . . . 105
4.2
Conceptual Electrical Model of a Single Basilar Membrane-Fluid Stage . . 115
4.3
Basilar Membrane-Fluid Interaction With Loading . . . . . . . . . . . . . 119
4.4
Transfer Function H(f ) of a Single Basilar Membrane-Fluid Stage and in
a Loaded Network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
4.5
Transfer Function G(f ) of a Single Basilar Membrane-Fluid Stage and in
a Loaded Network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
4.6
Transfer Function H(f ) with Loading in the Passive and an Active Case . . 125
4.7
Transfer Function G(f ) with Loading in the Passive and an Active Case. . . 126
4.8
Log-Antilog Multiplier . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
4.9
Class AB Low-Pass Filter and Basilar Membrane Circuit Design . . . . . . 135
4.10 Circuit Implementation of Saturation
. . . . . . . . . . . . . . . . . . . . 136
4.11 Input to Spiral Ganglion Cells . . . . . . . . . . . . . . . . . . . . . . . . 138
xv
4.12 Silicon Spiral Ganglion Cells . . . . . . . . . . . . . . . . . . . . . . . . . 139
4.13 Cochlear Chip Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . 142
4.14 Micrograph of Cochlear Chip Die . . . . . . . . . . . . . . . . . . . . . . 143
5.1
Frequency Responses in the Cochlear Chip with Varying Outer Hair Cell
Forces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
5.2
Frequency Responses in the Cochlear Chip Compared to Biology . . . . . . 151
5.3
Measured Frequency-Position Map in the Cochlear Chip . . . . . . . . . . 152
5.4
Nonlinear Compression in the Cochlear Chip . . . . . . . . . . . . . . . . 154
5.5
Saturating Input-Output Function in the Cochlear Chip . . . . . . . . . . . 155
5.6
Longitudinal Responses in the Cochlear Chip (I) . . . . . . . . . . . . . . . 157
5.7
Longitudinal Responses in the Cochlear Chip (II) . . . . . . . . . . . . . . 157
5.8
Longitudinal Responses in the Cochlear Chip (III) . . . . . . . . . . . . . . 158
5.9
Spectrum of A Complex Sound . . . . . . . . . . . . . . . . . . . . . . . . 160
5.10 Measured Cochleagram from the Cochlear Chip . . . . . . . . . . . . . . . 162
5.11 Rate-Intensity (RI) Functions Measured From the Cochlear Chip . . . . . . 164
5.12 Effect of Log-Normal Variations in Parameters on Model Responses . . . . 172
xvi
5.13 Effect of Log-Normal Variations and OHC Motility on Model Responses
xvii
. 173
Chapter 1
Introduction
The human auditory system exhibits extraordinary ability in sensing sound, distinguishing
frequencies, and maintaining satisfactory performance in adverse environments. Its frontend, the cochlea, converts fluid pressure fluctuations into neuronal spikes in the auditory
nerve, decomposing the mixture of frequencies, amplifying low-level sound, compressing
high-level sound, and producing interferences between different frequencies. For decades,
engineers have made efforts to build a hearing machine or computer that can emulate the
function and efficiency of the cochlea and the human auditory system. However, no engineered device to date is capable of performing as well as its biological counterpart.
Neuromorphic engineers have taken an unconventional approach in which they reverse
engineer the human brain, with the goal to morph or capture both the organization and
the function of the nervous system. For about two decades, neuromorphic systems have
modeled sensory systems, including the retina [1–4] and the cochlea [5–7], cortical functions, mainly including visual cortex [8, 9], developmental processes, based on axon guidance [10], and memory and learning [11, 12]. While artificial retinomorphic chips have
1
meticulously copied the detailed neuronal biophysics and complex synaptic organizations
in the retina, the silicon cochleas have benefited relatively less from morphing the biology
because the cochlear amplifier mechanism that accounts for the cochlea’s characteristic
behavior still remains a mystery.
Advances in physiological measurements and mathematical modeling in the past few
decades have revealed the cochlear mechanics to a larger extent, through much is yet to be
uncovered. In the mammalian cochlea, the non-uniform basilar membrane (BM) vibrates
in response to fluid pressure, providing broad frequency tuning which is then enhanced
and reshaped by the hypothesized cochlear amplifier. Bridging the gap between the dead
cochlea’s response and that of living cochleas, the postulated cochlear amplifier refers to the
selective amplification process occuring within the cochlear partition that enhances active
cochlear behavior: exquisite sensitivity, remarkable frequency selectivity, and nonlinearities. Thus, the BM preliminarily detects and filters sound signals and the cochlear amplifier
selectively amplifies the BM’s responses, resulting in nonlinear active cochlear behavior.
Since the discovery of outer hair cells’ (OHCs) electromotility in mammalian cochlea
[13, 14], cochlear modeling has been focusing on exploring the detailed manner in which
OHCs’ motile forces enhance the basilar membrane’s vibration. The OHC forces can be
transmitted, somehow through the complex organizations of the organ of Corti, onto the
BM to affect its motion. Enormous modeling efforts have been made to incorporate the
OHC motility in cochlear models, with differing hypotheses to account for the cochlear
amplifier [15–17]. Responses to sound in most active models were either unrealistic [18]
or missing important characteristic cochlear behaviors, for instance nonlinearity [15, 16].
Our project goal is to build a cochlear model, in mathematics and VLSI circuits, that
can reproduce the major characteristic cochlear behavior, including the active amplification
2
and nonlinear compression. We proposed a novel cochlear amplifier mechanism that takes
into account the cochlear micro-anatomy for transmitting OHC forces onto the BM. In
this mechanism, two tilted structures transmit OHC motile forces in the basal and apical
directions, respectively (namely, feedforward and feedbackward), onto the BM, resulting in
active bidirectional coupling (ABC) among BM transverse fibers. We enforced saturation
of ABC, which gives rise to nonlinear behavior. We proceeded to build a silicon cochlea
based on this nonlinear active cochlear model; the cochlear chip is the first to include the
model of cochlear amplifier mechanism, resulting in a novel silicon cochlea architecture
that addresses problems of existing designs.
1.1 Organization of the Dissertation
The dissertation thesis is organized as follows. In Chapter 2, we review cochlear modeling efforts in both mathematical models and VLSI implementations. The review emphasizes the models that explore the underlying mechanism of active amplification in the
cochlea. The content mainly encompasses the physiology of the cochlea including cochlear
microanatomy, outer hair cell (OHC) motility, and mathematical models that incorporate
OHC motility. This chapter also discusses advances in silicon cochlea designs and existing
problems.
Chapter 3 is dedicated to presentation of our hypothesized cochlear amplifier mechanism, active bidirectional coupling (ABC). We describe unique features of the cochlear
microanatomy that we believe play an essential role in manoeuvring OHC motile forces to
enhance the cochlea’s response gain and frequency tuning and to impart nonlinear processing to the cochlea. We formulate a two-dimensional (2D) nonlinear active cochlear model,
3
solve it numerically, and present the model’s simulation results. Analysis of a linear version
of the model follows, which provides insight into the mechanism underlying the selective
amplification.
In Chapter 4, we present the silicon cochlea design that is based on our nonlinear active
cochlear model. We present first the diffusive network for modeling the cochlear fluid, then
the synthesis of the BM filter design. We proceed to describe the transistor-level circuit
design of the BM. What follows is a silicon neuron circuit we have included in the silicon
cochlea for modeling the spiral ganglion cells, which convert the BM motion to auditory
nerve spikes. Last, we present the architecture of the entire cochlear chip.
In Chapter 5, we present chip responses to sound stimuli. The responses include both
analog currents that represent the BM’s velocity, and digital pulses that represent the auditory nerve’s discharge patterns.
Chapter 6 summarizes our efforts in modeling the nonlinear active cochlea, discussing
both the analytical and synthetical approaches.
1.2 Original Contribution of Present Work
The cochlear amplifier mechanism, active bidirectional coupling, is novel, the first to take
account into the tilted structure of both OHCs and phalangeal processes in a cochlear
model. The semi-analytical results of a linear version of the active cochlear model are
novel, including the notion of a spatial filter and the negative damping caused by the tilts
associated with delivery of outer hair cell motile forces.
4
The implementation of the nonlinear active cochlea model in analog VLSI is original.
The differential mode (Class AB operation) is the first among silicon cochlea designs. The
bio-inspired coupling between basilar membrane second-order sections is new, thus the
novel architecture of the cochlear chip.
1.3 The Usage of “We”
The personal pronoun “we” is used in a number of places throughout the thesis—though
in plural form, it is used with a singular sense. The present work was completed under
the guidance of my thesis advisor. However, despite the usage of “we” and/or “our(s)”, I
performed 99% of the work, if not all, including model development and simulation, circuit
design and simulation, and chip layout and test.
5
Chapter 2
The Cochlea and Cochlear Modeling
The human inner ear, namely the cochlea, functions as the front-end of the auditory system,
turning outside-world sound signals into neural impulses transmitted along the auditory
pathway for further processing. Complex and delicate, the cochlea does not just transduce
mechanical sound into electrical impulses (all-or-none neural spikes); it also processes and
amplifies sound by means of its unique structure and the physical properties of its comprising components. In this chapter, we first describe the cochlear anatomy and important
findings about the cochlea’s outer hair cells (OHCs) that make critical contributions to
characteristic cochlear behavior. We then review previous mathematical cochlear modeling
efforts. Last, we give a brief summary of existing cochlear models implemented in very
large scale integration (VLSI) circuits.
6
2.1 The Cochlear
2.1.1 Cochlear Anatomy
The periphery of the auditory system includes the outer ear, the middle ear, and the inner
ear (i.e., the cochlea) (sketched as in Figure 2.1). How is sound processed at each step of
the auditory periphery? As the first steps of sound detection, the sound transmission in the
outer and middle ear is basically linear [18]. In general, time-varying sound pressure in the
air (condensation and rarefication) is transmitted through the ear canal of the outer ear. The
basic role of the outer ear is to collect impinging sound waves and channel them toward
the inner ear’s sensory cells. The intercepted sound waves travel through the ear canal and
cause the eardrum to vibrate. The eardrum vibrations are transmitted through an air-filled
middle ear by the ossicles to the acoustic sensor, the cochlea, resulting in the motion of
the stapes at the oval window of the cochlea (Figure 2.2). The stapes movement sets the
cochlear fluid into motion, causing a traveling wave to form along the cochlear partition
through the pressure difference between the two main fluid chambers. The pressure caused
by the stapes motion gets released through complementary movement of the round window
at the bottom chamber of the cochlear duct due to the incompressibility of the cochlear
fluid.
The functional role of the middle ear is to match the acoustic impedance between the
end point of the external ear, the eardrum, and the sensor port of the inner ear. Acoustic
impedance, ZA , is defined as the ratio of acoustic pressure, P , to volume velocity (velocity
V times cross-sectional area, A) [21]:
ZA =
7
P
VA
(2.1)
Figure 2.1: Human Ear
The human ear comprises the outer, middle, and inner regions. The outer ear includes the pinna and the
external canal. The middle ear consists of three tiny bones, or ossicles, namely hammer (malleus), anvil
(incus), and stirrup (stapes). The inner ear, or the cochlea, is snail like and filled with fluid. Cochlear nerve
(i.e., the auditory nerve or the VIII th nerve), stems from the central axis of the cochlea and extends to the
central auditory system. From [19].
8
Figure 2.2: Mechanism of the Human Ear
Air pressure at the eardrum (Pd ) is transformed into stapes displacement (Xs ) and velocity (Vs ) by the ossicular chain of the middle ear. Stapes motion creates a pressure in the vicinity of the footplate, Pow , which
is related to the cochlea’s input impedance, Zow , and the stapes velocity, Vs . In the cochlea, P denotes the
pressure gradient across the cochlear partition, which produces a traveling wave in the cochlear fluid and
along the cochlear partition. Two full cycles of this wave are indicated by the wavelengths, λ1 and λ2. The
traveling wave reaches its peak with short wavelength. RW: Round window. From [20].
9
Impedance matching is of significance because only when the acoustic impedance of two
conducting media are identical can a sound wave be passed from one to the other without reflection [18, 22]. Since there is a ratio of 3880:1 between the measured acoustic
impedances of water and air, different sizes of the eardrum and the footplate of the stapes
come into play. Due to the fact that the velocities of the eardrum and the stapes are the
same, and the force acting on the eardrum and the stapes are equal, a ratio of about 62:1
would yield an equal impedance at the eardrum and the stapes [18]. More sophisticated
impedance calculations show that this ratio is a function of tonal frequency; it is actually
equal to 62 at about 2 kHz [23].
The cochlea is a much more complex and delicate organ than the external and middle ear. Its basic structure is revealed in the cross-sectional sketch of a radial slice taken
from the second turn of guinea pig (Figure 2.3). The cochlear partition that separates the
cochlear fluid duct into three chambers, comprises the basilar membrane (BM), and the
organ of Corti (OC). The BM consists of a number of fibers oriented in radial directions.
These transverse BM filers are inserted into the bony spiral lamina on the modiolus end
and the spiral ligament on the opposite side. The BM is narrower and thicker in the base
than it is in the apex, which imparts the stiffness gradient of the BM—This physical property is commonly believed to result in decreasing resonant frequency from the base to the
apex along the cochlea.
The OC is a complicated superstructure, containing both sensory cells and supporting
cells (Figure 2.4). Supporting cells mainly comprise pillar cells with two kinds, the inner
pillar (IP) and the outer pillar (OP) cells, which are the chief structural cells in the OC.
The trunks of pillar cells are composed of dense matrices of interlaced microfibrils and
microtubules [25]. One IP cell and one OP cell, together with a small portion of the BM,
form a relatively rigid structure.
10
Figure 2.3: Radial Segment of the Cochlear Duct
Filled with fluid, the cochlear duct is divided into three scalae—scala vestibuli, scala media, and scala
tympani—by the Reissner’s membrane and the cochlear partition. The cochlear partition comprises the basilar membrane and the organ of Corti. The modiolus is to the left. From [24].
11
Figure 2.4: Cochlear Partition
Resting on the basilar membrane (BM), the organ of Corti (OC) contains both sensory and supporting cells. In
mammal, there are three rows of outer hair cells (OHCs) and one row of inner hair cells (IHCs). The tunnel
of Corti is formed by the inner pillar (IP) cell, outer pillar (OP) cell, and a small portion of the BM fiber.
Deiters’ (D), Hensen (H), and Claudius (C) cells are also constitutional components of the OC. From [26].
12
The sensory cells are mainly the hair cells, namely outer hair cells (OHCs) and inner
hair cells (IHCs). IHCs encode sound information, for example, its frequency components,
intensity, timing, and other physical properties, into neuronal electrical discharge patterns,
carried to the central nervous system via afferent axons of spiral ganglion cells. Meanwhile, efferent axons originate from the brain stem and innervate the hair cells, mainly
OHCs.
Hair cells have hairs called stereocilia (Figure 2.4). IHCs, surrounded by supporting
pillar cells, have stereocilia that are free to deflect angularly. The deflection of IHCs’ cilia
is the final mechanical step in the ear’s transduction of acoustic signals into neural pulses
[18]. In contrast, OHCs have stereocilia that are embedded in the tectorial membrane,
particularly their tallest hair called kinocilium [18]. OHCs, supported only at their bases
by Deiters0 cells (DCs) and at their apexes by reticular lamina are free of cellular contact
on their sides; this is believed to be pertinent to OHC function.
2.1.2 Cochlear Mechanics
Sound stimuli elicit both mechanical and electrical responses in the cochlea. Driven by the
pressure difference across it, the BM vibrates at different cochlear locations depending on
the input frequency, forming an amplitude peak of its motion at certain cochlear location
called the best place, or the characteristic place; the corresponding input frequency is called
as the best frequency, or the characteristic frequency for that location [27–29]. The BM
responds maximally to high frequencies near the base as it is relatively stiff while it vibrates
most near the apex in response to low frequencies [30] as it is floppy there. Acoustic energy
is carried by the cochlear fluid, most of which is absorbed by the cochlear partition in the
vicinity of the characteristic place (Figure 2.5).
13
A mechanical traveling wave is formed both along the basilar membrane and in the
cochlear fluid, a fundamental discovery in auditory physiology by von Békésy [31]. The
traveling wave is a displacement wave on the BM or a pressure wave in the cochlear fluid
(near the BM), a wave much slower than the acoustic wave, which transverses the entire
cochlea (at a speed of 1,550 m/s) in a few microseconds [32]. The traveling wave propagates at a relatively large speed near the base, with a long wavelength, and slows down
near the characteristic place, where the wavelength becomes short [33]. Right after it peaks,
where almost all its energy is acquired by the BM, the traveling wave decays rapidly, forming a sharp cutoff.
BM’s up-and-down motion is transformed into the stereociliary deflections of OHCs
through the structural organization of the OC (Figure 2.6), as summarized in [18]. As
the BM is driven downward by acoustic pressure, the relatively rigid triangular pillar-cellcomplex keeps its shape and pivots clockwise about the foot of the IP cell, which is situated near the edge of the relatively immobile bony spiral lamina. The OHC, cradled by
the Deiters’ cell, is carried along with the BM, moving away from the spiral limbus. The
tectorial membrane—attached to both the stationary limbus and the stereocilia of the receding OHC—is stretched, resulting in shear force between the tectorial membrane and
the reticular membrane. Thus, the downward BM motion deflects the OHC stereocilia in
the counterclockwise direction, leading to forward transduction (i.e., a mechanoelectrical
process, in which mechanical force results in electrical transduction current in OHCs [34]).
Influx of transduction current depolarizes the OHC, causing changes in transmembrane
receptor potential, which in turn provide input to OHCs’ motor activity.
A further transduction stage in the cochlea occurs at the IHC, from fluid motion to
deflection of IHC stereocilia (Figure 2.7). The counterclockwise deflection of the OHC
stereocilia displaces the fluid (endolymph) underneath the tectorial membrane and above
14
Figure 2.5: Basilar Membrane Vibration
In this sketch of uncoiled cochlea, due to the fluid incompressibility, volume displacement of the round
window equals that of the oval window (stapes). Acoustic energy is carried by an acoustic wave that travels
from base (stapes) to apex (helicotrema), absorbed by the basilar membrane (BM) where the resistance to
the pressure is minimum. Thus, the BM vibrates up and down, driven by the pressure difference across it.
From [18].
15
Figure 2.6: Deflection of Outer Hair Cell Stereocilia
Basilar membrane (BM) motion causes deflection of outer hair cells’ (OHC) stereocilia through the microanatomy of the organ of Corti (OC). In the diagram, the downward BM motion is transmitted, through the
supporting pillar cells in the OC, to deflect the stereocilia to the counterclockwise direction, resulting in hyperpolarization of the OHCs. Similarly, upward BM motion will cause depolarization of OHCs. From [18].
16
the reticular lamina [35]. Unlike in OHCs, the stereocilia of IHC are not attached to the
tectorial membrane, thus they are free to move. Lined up in one row perpendicular to
the flow of fluid, the cilia present to it an almost uninterrupted barrier to be efficiently
deflected by the fluid motion [18]. The ciliary deflection of IHC leads to transduction
currents, which in turn give rise to neurotransmitter release at the IHC-spiral ganglion cell
synaptic interface.
Cochlear Amplifier
The cochlea exhibits remarkable acoustic sensitivity, high frequency selectivity and
(nonlinear) high-level compression in processing sound signals. The marked differences
in vibration amplitude/velocity and frequency tuning, in the measurements between dead
and living cochlea strongly suggested that a feedback amplification mechanism, namely the
cochlear amplifier, exists within the OC and injects mechanical energy into the BM locally
[36], thus enhancing the sensitivity and frequency tuning of the cochlea. In other words,
the cochlear amplifier, together with the passive frequency tuning of the BM, accounts for
the characteristic behavior of a living cochlea (i.e., large amplitude of motion, sharp tuning
at low sound levels).
OHCs have been widely believed to be the amplifying agent, thus the origin of the
cochlear amplifier, given their ability to change their cell body lengths under acoustic stimulation [13, 14, 37]. Although the upper limit has not yet been established [18], the speed
of electromotility, or at which OHCs alter their length, is as high as 24 kHz [38]. Cell
body length change of OHCs are relatively small, which are only a few percent of the cell’s
length at most, but in the same amplitude range as sound-evoked BM vibrations [18].
Upon their contraction or elongation, OHCs generate fast motile forces [39, 40], and
17
Figure 2.7: Deflection of Inner Hair Cell Stereocilia
Stereocilia deflection of outer hair cells (OHCs) cause the fluid in the gap between stereocilia and the tectorial
membrane to flow, which in turn deflects the nearby inner hair cells’ stereocilia in the same direction as that
of the OHCs’ stereocilia. From [18].
18
these forces are transmitted onto the BM to alter its motion. Deiters cells, which cradle
OHCs, most likely deliver these forces as they are formed by closely packed microtubular
arrays, thereby exhibiting columnar rigidity [18].
The prevailing view is that there exists a feedback system involving OHC electromotility that enhances the BM motion (Figure 2.8). The process consists of both forward transduction and reverse transduction [41]. Contrary to forward transduction, reverse transduction corresponds to the process from electrical cell membrane voltage to mechanical
force, an electromechanical process. However, details of the cochlear amplifier still remains largely unknown as it is not clear in what precise manner the OHC motile forces
feedback on the BM so that the BM motion is amplified in a frequency-selective fashion.
Cochlear Nonlinearity
It has been discovered that OHCs have a saturation property, thus yielding nonlinear
cochlear responses. The relation measured between sound pressure and receptor voltage
for outer hair cells shows the typical S-shape (Figure 2.9A) [42, 43]. Further, the cell
body length change of an OHC saturates with its transmembrane potential (Figure 2.9B).
Assuming that acoustic pressure causes proportional BM displacement and the OHC length
change is proportional to the cells’ receptor potential change, a saturation relation can be
derived in which OHCs’ motile forces saturate with respect to BM displacement produced
by acoustic pressure [17].
Cochlear nonlinear behavior includes high-sound-level compression [18], two-tone suppressions [32], distortion-product otoacoustic emissions [46], and more. One of the most
prominent nonlinear phenomena in the cochlea is high sound-level compression. Sound
signals at low intensities are amplified in frequency-selective fashion at certain cochlear
19
Figure 2.8: Feedback System with Outer Hair Cell Electromotility
This diagram presents a prevailing view of how outer hair cell (OHC ) motility affects basilar membrane
(BM) motion in general. BM motion causes deflection of OHCs’ stereocilia, resulting in a transduction
current, i(t), which in turn produces the receptor potential, Erec(t). The generated receptor potential changes
the OHCs’ length. Completing the loop, the length change of OHCs exerts a force F (t) on the BM. This
feedback system has been postulated as the cochlear amplifier. From [18].
20
A
B
Figure 2.9: Saturating Profile of Outer Hair Cells
A The relation between acoustic pressure and outer hair cells’ receptor potential is S-shape, saturating at high
pressure levels [44]. B Changes in the cell body length of an isolated outer hair cell in response to various
transmembrane voltage steps are also S-shape [45]. As can be seen, hyperpolarization elicited elongation
while depolarization caused contraction. Dots: Raw data. Solid line: Boltzmann function. Insert: Outer hair
cell.
21
position—the cochlea exhibits large gain, while high-level sound signals are nearly not
amplified—the cochlea exhibits small gain (Figure 2.10). Thus, the cochlear responses at
the peak show compressive growth with input intensity. From an engineering perspective,
the cochlea realizes automatic gain control, in which the gain of the cochlear amplifier
becomes attenuated with the increase of input intensity (Figure 2.10).
Two-tone suppression (2TS) is the phenomenon in which the cochlea’s response to one
tone (i.e., the probe) is suppressed by the presence of a loud tone (i.e., the suppressor). 2TS
occurs in the BM’s mechanical motion, in the receptor potential of IHCs, and in the electrical discharges on the auditory nerve (AN) [48]. Although there is debate as to whether 2TS
at the IHC and AN levels is the direct result of the suppression occuring at the BM level, it
is believed that two-tone suppression with the BM’s mechanical responses is attributed to
saturation of OHC forces [32].
One of the major characteristics of 2TS is its intensity dependence (Figure 2.11). In the
absence of a suppressor (or for low suppressor levels) the responses to the CF tone grow
at compressive rates. At higher suppressor levels, the responses to low-level CF tones are
reduced strongly, but only weakly at high levels. As a result, the BM input-output curve
for the CF tones is substantially linearized in the presence of moderately intense suppressor
tones. Another feature of 2TS is that it is characteristic frequency specific with regard to
both the probe tone and the suppressor tone.
2.2 Mathematical Cochlear Modeling
In the past few decades, enormous efforts in mathematical modeling have been made to
help uncover the nature of cochlear mechanics. Since the discovery of OHC electromotil22
A
B
Figure 2.10: Nonlinear Compressions in the Cochlea
Compression, or compressive growth in responses, with input intensity is one of the nonlinear phenomena in
the cochlea. A Families of isointensity curves representing the gain (velocity divided by stimulus pressure)
of basilar membrane responses to tone pips as a function of frequency (abscissa) and intensity (parameter, in
dB SPL). The thick line at the bottom indicates the average motion of the stapes. B The variations of basilar
membrane response phases as a function of frequency. Phases—BM displacement toward scala tympani
relative to condensation at the eardrum thus inward ossicular displacement—were computed. Each curve
represents data for a single stimulus intensity. From [47].
23
Figure 2.11: Two-tone Suppression in the Cochlea
Two-tone suppression at the basilar membrane’s velocity-intensity functions responding to a near-CF tone
(F1 = 18.8 kHz) presented simultaneously with a suppressor tone (F2 = 22.9 kHz). The parameter is the
intensity of the suppressor tone. These are data measured from Guinea pig cochlea in [49].
24
ity, cochlear modeling has become focused on incorporating the OHC motile forces in
order to reproduce characteristic cochlear behavior. Although the assumptions and mechanisms proposed differ somewhat in explaining how the cochlear amplifier works, all existing models shared a common goal, that is, to obtain BM responses to sound comparable to
measured physiological data.
Models have been modified and improved as more physiological findings and measurements have accumulated, especially by including micro-mechanical details within the
organ of Corti of the cochlea. Since the discovery of outer hair cell electromotility in the
mammalian cochlea [13], cochlear modeling has been focusing on exploring the detailed
manner in which OHC forces enhance basilar membrane vibration. It is commonly believed
that the OHC motile forces are transmitted, somehow through the complex organizations
of the OC, onto the BM to affect its motion.
Cochlear models that include OHC motility are referred to as active cochlear models;
those without OHC motility as passive cochlear models [50]. Building active cochlear
models have helped understand better how OHC motility contributes to amplification and
sharpening of traveling waves along the cochlear partition (CP) at low sound levels [51–53]
and nonlinear behavior at high sound levels. A number of active cochlear models have
introduced OHC motility into cochlear macromechanics (i.e., interaction between fluid
hydrodynamics and BM second-order sections) to account for the active amplification
[15–17, 54–59].
It is commonly assumed that OHCs are the origin of the active mechanism in active
cochlear models. With the same goal to account for the active amplification, they differ in
how the OHCs exert forces on the BM, and in the role of other structures and components
(e.g., sensory and support cells) within the CP. A common approach to most of the models
25
was adding a feedback loop to simulate the OHC motility [18].
The BM fibers are thought to be only weakly coupled to their adjacent neighbors
through the cochlear fluid [60] due to transverse (i.e., radial) orientation of these fibers.
Most cochlear models ignore the longitudinal elasticity (stiffness) because it is negligible
compared to the transverse stiffness. This assumption was mainly justified by the experiments performed by Voldrich [61], in which they probed the BM to study its stiffness.
However, the latest experiment carried out by Naidu and Mountain [62] provided consistent results with the earlier experiments done by von Békésy in 1960 [31], in conflict with
Voldrich results. These findings support the notion that there exists longitudinal coupling,
weak though, between BM transverse fibers and the coupling increases in strength from
base to apex.
Several cochlear modelers have implemented this longitudinal coupling, as a novel hypothesis of active mechanism, in order to produce more realistic cochlear responses. However, perspectives on how to include the coupling differ. Hubbard [55] included a second traveling wave mode that is coupled to the traveling wave formed along the BM, thus
coupling BM segments in the longitudinal direction. Wong [63] took a straightforward
approach by including weak stiffness between BM transverse fibers to generate distortion
product otoacoustic emission.
A few recently developed active cochlear models took into account the OHCs basal
tilt [15–17, 58, 59]; this arrangement was first pointed out by Voldrich [64]. These models
considered longitudinal coupling based on the observed anatomical structure of OC, in a
way that is compatible with the prevalent opinion on the contribution of OHC motility. This
perspective is that the motile OHC forces are delivered to neighboring cochlear locations,
thus coupling the adjacent BM fibers longitudinally. Along this line of modeling activity,
26
Sang and Geisler [15] proposed a feed-forward mechanism that considers the basal tilt of
OHCs, in which OHC forces affect the motion of the downstream neighboring BM; there
were also other models based in this idea [16, 65].
These models hypothesized, counter-intuitively, that this tilt sharpens tuning and amplifies BM motion through feedforward OHC forces that act on the downstream BM segment.
These models with feedforward OHC forces produce larger amplification than that in a passive model, but the overall responses are not realistic compared to physiological data (e.g.,
too much phase accumulation). As one of the remedies for the problem, the tectorial membrane was modeled as an additional resonant system in the feedforward model proposed
in [15].
Although Karavitaki and Mountain argued that this feedforward mechanism is not supported by the experimental results that the longitudinal component of OHC motion in the
apex is about ten times smaller than the radial component [66], we believe that OHC does
not really need to move in the longitudinal direction to a large extent, to give rise to the
feedforward OHC forces given the unique tilt arrangement.
Another striking anatomical feature within the organ of Corti, which has not been explicitly taken into account in a cochlear model, is the oblique orientation of the phalangeal
processes (PhPs) (Figure 2.12). PhPs are extended from Deiters’ cells, tilted toward the
apex along the longitudinal axis [26]. This PhP’s tilt has been only considered for its
constitutional role in the complex; its possible role in transmitting OHC forces onto the
BM in the process of cochlear amplification was left unexplored. Brownell and his coworkers [13] observed that an OHC, the PhP of the DC on which it sits, and the portion
of the RL between the apical end of the PhP and the stereocilia end of the OHC constitute a mechanical unit within the CP, and pointed out that increases in OHC length would
27
make this unit more rigid whereas decreases would make it more compliant. Our proposed cochlear amplifier mechanism takes into account both tilts in the OC, which we shall
present in detail in Chapter 3.
Cochlear nonlinearity is believed to originate from the nonlinear behavior of the OHC
[68], which was described in Section 2.1. Some nonlinear cochlea models focused on finding an efficient numerical method for solving nonlinear equations [16, 69–75]. Modeling
efforts have been made to model the nonlinearity of the cochlear behavior, including compressive growth at high sound levels, two-tone suppression, and distortion product otoacoustic emissions [18, 76–79]. In most nonlinear models, the nonlinearity was obtained by
introducing a nonlinear element, through certain nonlinear functions (e.g., the hyperbolic
tangent tanh).
2.3 Cochlear Modeling in VLSI
Engineers have long attempted to build physical cochlear models that behave similar to the
biological cochlea. The major motivation of this approach stems from its great potential
for a myriad of sound-processing related applications, for example, acoustic front-end of
automatic speech recognition systems and signal processing component of auditory prosthetic devices. Meanwhile, these physical models also function as real-time computational
tools due to their neuromorphic nature, complementary to physiological measurements and
mathematical modeling, to help uncover the cochlear mechanics. When built for such
purpose, they share common goals with mathematical simulations, which is to emulate the
nonlinear active cochlear behavior in order to shed light on how the cochlea works. Among
physical cochlear models, silicon cochleas, particularly analog VLSI (very large scale in-
28
Figure 2.12: Cochlear Microanatomy
Scanning electron micrograph of the organ of Corti of the mole rat cochlea. Each outer hair cell (OHC) is
cupped at its base by a Deiters’ (D) cell. Each Deiters’ cell extends a stalk up and expands into a phalangeal
process (PhP), inserted into the reticular laminar (not labeled) at some distance from the base of the Deiters
cell. The outer hair cells are tilted toward the base (on the left) while the phalangeal process is titled toward
the apex (on the right). From [67].
29
tegration) implementations, have exhibited advantages over computer software simulations
in computation efficiency and over digital cochlear chips based on analog to digital converter and digital signal processor in power consumption [6, 80].
We shall only review analog VLSI cochlear models in this thesis due to the nature of our
present work. Several generations of silicon cochleas have been built since Lyon and Mead
developed the first analog electronic cochlea [5]. The first silicon cochlea modeled the biological cochlea as a one-dimensional bank of second-order low-pass filters (LPFs) whose
resonant frequencies decreased exponentially. The design employed transconductance amplifiers [81] and capacitors to implement the second-order sections and these second-order
sections were cascaded; the output of one section was fed as input to the following section.
With exponentially increasing time constants along the cascade, the silicon cochlea was
able to generate the characteristic frequency-position map in the biological cochlea.
Despite improvements by other researchers [82, 83], the silicon cochlea based on a cascade structure continues to suffer from several shortcomings, which have been found to be
mainly associated with the cascaded structure [6, 84]. In this design, the peak gain (at a
certain position in the cascade) is obtained through accumulation along the cascade, which
imposes a constraint on the number of filters that can be included in a cascade because
too much accumulated peak gain would drive filters out of their linear operating range. In
addition, phase accumulates along the cascade, resulting in too much phase delay at the resonant peak, which is unrealistic compared to biology. Meanwhile, noise and distortion with
the output of preceding filters undesirably propagate along the cascade. It is also obvious
that the cascade structure has poor fault tolerance because dysfunction of one second-order
section caused all of its following sections to fail.
Adopting a more biology-inspired approach, Watts built a silicon cochlea with parallel
30
structure to address these shortcomings of the cascade structure [6]. This parallel structure
was based on the biological cochlea’s mechanics in which the basilar membrane interacts
with the cochlear fluid. Recent designs have improved Watts original design through circuit
innovation [84–86]. The cochlear chip with a parallel structure integrated the cochlear fluid
model using a resistive network, resulting in a two-dimensional passive cochlea design.
The basilar membrane sections are coupled through the fluid resistive network, thereby
eliminating the problems of phase delay and poor fault-tolerance. The inclusion of cochlear
hydrodynamics reproduced the transition from long-wave to short-wave propagation in the
cochlear duct.
Nevertheless, the parallel structure of silicon cochleae elicited a new problem. In a
second-order system, a high (desirable) quality factor (Q) corresponds to phase frequency
responses that have a steeper slope, resulting in phase differences close to π among neighboring basilar membrane sections (due to the inherent frequency-position map). Coupling
among basilar membrane sections through the fluid network, therefore, counteracted one
another eliminating the response gain due to their out-of-phase motion [7]. In other words,
this interference has destructive nature, leading to little response gain.
Advances in analog VLSI circuit design technique has also been employed to design
better silicon cochleas. These mainly include the transition from voltage mode [5, 6, 82]
to current mode [80, 84, 85]. Using current to represent signals instead of voltage, currentmode circuits offer the advantages of larger dynamic range, lower power consumption,
and smaller chip area, over their voltage-mode counterparts [87]. The recently proposed
log-domain filter design approach has also been adopted to design a silicon cochlea [84],
which uses the logarithmic characteristic of the weak-inversion MOSFET (metal-oxide
semiconductor field effect transistor) to extend dynamic range.
31
Despite all the progress that has been made in developing silicon cochleas, responses
of existing designs fall short in achieving responses comparable to their biological counterpart. None of existing silicon cochleas was able to reproduce all the characteristic cochlear
behavior, including large dynamic range, large gain and sharp tuning at low sound levels,
and low gain and broad tuning at intense sound levels. The major factor to account for poor
model responses is that none of the designs include implementation of the cochlear amplifier mechanism—an indispensable component for generating characteristic cochlear behavior. In addition, the constraints imposed by the physical medium for a silicon cochlea, for
example, device (transistor) mismatch, cause discrepancy between the model implementation and the biological cochlea properties. It is obvious that the biological cochlea also has
mismatch (i.e., variations in its biophysical properties), however, it somehow preserves its
extraordinary ability in sensing and processing sound signals. Discovering the biology’s
secret or trick for fighting mismatch and variation will no doubt lend itself to improving
artificial cochlea-like sound processing system. Building a better silicon cochlea requires a
physiologically plausible cochlear amplifier mechanism and desires advanced circuit technique. In order to match the brain’s power efficiency, lowering the power consumption in a
silicon cochlear design should also be taken into account as one of the design goals.
32
Chapter 3
The Active Cochlea: Mathematical
Model
In Chapter 2, we have reviewed existing cochlear modeling effort in both mathematics
and VLSI circuits. Complementary to physiological measurements, these cochlear models have made enormous contributions to help uncover the cochlear mechanics, providing
explanation for physiological measurements. In this chapter, we present a nonlinear active cochlear model that implements a novel cochlear amplifier mechanism. The idea of
our proposed mechanism stems from the assumption that the cochlea’s micro-architecture
serves its functioning effectively.
We shall focus our modeling effort on numerical simulation and mathematical analysis
as follows. We first describe the cochlear microanatomy and the active mechanism, then
we formulate a two-dimensional (2D) mathematical cochlear model, followed by model
simulation carried out through both numerical and semi-analytical methods. To verify
the proposed cochlear amplifier mechanism, we compare the simulation results obtained
33
through both solution methods with physiological data measured from mammalian cochlea.
Through mathematical analysis, we further characterize the traveling wave formed along
the basilar membrane (BM) and in the cochlear fluids. Last, we extend this linear active
model to a nonlinear active one to simulate the nonlinearity of cochlear responses, including the compression at high sound levels and two-tone suppression. The mathematical
model described in this chapter serves as a blueprint for the design of a physical model
using analog VLSI techniques, which will be discussed in Chapter 4.
3.1 The Linear Model: Numerical Simulation
In this section, we first discuss the outer hair cell (OHC) forces, including their generation,
property, and its possible role in cochlear mechanics. Then we describe our cochlear amplifier mechanism that is based on both cochlear microanatomy and outer hair cell motility.
3.1.1 Outer Hair Cell Motile Forces
OHCs have unique cortical structure in the lateral surface [36], which are believed to
be somehow responsible for their ability of mechanical deformation—changing their cell
body length—at acoustic frequencies when they are acoustically or electrically stimulated.
Roughly speaking, the lateral surface of an OHC consists of three layers, the outmost
plasma membrane, the innermost lateral cisternae, and the helical circumferential filaments
in the middle [36]. The array of cytoskeletal filaments are crosslinked by thinner strands
and attached by pillars to the outmost lateral cistern and to large particles in the inner leaf
of the plasma membrane.
34
It is widely held that among various cortical elements of OHCs, the array of large particles bound in the plasma membrane plays an important role in OHC motility [88]. The
idea is that the membrane-bound particles undergo conformational changes when the transmembrane voltage changes thereby causing the cell to hyperpolarization or depolarization.
Upon depolarization, the effective diameters of the particles are reduced or their packing
density becomes increased, resulting in shrink of the surface area of the OHC. Similarly,
hyperpolarization causes expansion of OHCs. The change of surface area of OHCs are then
translated into a change in its cell body length.
Deformation of OHCs can be deemed to be “rapid” although the upper limit to the
speed of OHC electromotility has not yet been determined [18]. Therefore, it is reasonable
to assume that OHC forces can be generated following acoustic frequency and on a cycleby-cycle basis. In other words, we assumed that OHC force are exerted onto the BM
instantaneously and not selective with input frequency.
3.1.2 Active Bidirectional Coupling
It has been widely accepted that OHCs are the origin of the cochlear amplifier and that
they make essential contributions to characteristic cochlear behavior. However, the exact
manner or details about how OHC motile forces lead to active amplification in the cochlea
still remains a mystery due to the difficulties in accessing and exploring the functional
roles of different components in the cochlear partition. In our effort pursuing the cochlear
amplifier mechanism, we believe that the complexity and uniqueness of organization and
architecture of the cochlear partition, especially of the organ of Corti (OC), play a direct
role in transmitting OHC forces onto the BM, thus enhancing the BM’s motion.
35
We hypothesized a novel cochlear amplifier mechanism in which we considered the
exquisite microstructure of the OC. However, this mechanism did not intend to include
every anatomical details of the OC. Instead, it postulated a possible mean of implementing
the feedback of OHC forces to account for the active cochlear amplification.
The basic idea of the mechanism is that OHC forces, due to OHC mechanoelectro
motility, can be delivered bidirectionally in the longitudinal direction through the cochlear
micro-architecture; if this additional energy is applied at the appropriate cochlear location,
it will enhance the vibration of the basilar membrane (BM), thus generating desired amplification. These forces are exerted by the OHC on the BM segment through the Deiters’ cell
(DC), realizing active bidirectional coupling (ABC), which results in high sensitivity, high
frequency selectivity, and nonlinear compression.
We hypothesized that the mechanical unit, consisting of the OHC, the phalangeal process (PhP) and the reticular membrane (RL), plays a direct role in transmitting OHC forces
onto the BM, thus actively amplifying BM responses. As the main vibrating organ within
the cochlea, the BM is composed of transverse fibers that are weakly coupled longitudinally [18]. The weak coupling is From the base to the apex, BM fibers increase in width
and decrease in thickness. The resulting exponential decrease in stiffness gives rise to the
passive frequency tuning of the cochlea.
The cochlear partition is divided into a number of radial segments from the base to the
apex with OHCs and PhPs arranged within the OC (Figure 3.1). For simplicity, we define
each BM segment as consisting of one DC sitting on the BM, the apical end of one PhP,
and the stereocilia end of one OHC—the latter two both attached to the RL. The stereocilia
end of one OHC lies in one segment, say segment i − 1, while its basolateral end lies in
the immediately apical segment i. The DC in segment i sends a PhP that angles toward the
36
apex of the cochlea, with its apical end inserted just behind the stereocilia end of the OHC
in segment i + 1. The RL is made up of the stereocilia end of the OHCs and the apical end
of PhPs of DCs.
Our hypothesized cochlear amplifier mechanism comprise effect of two distinct OHC
forces due to the tilted structure, namely the feedforward and feedbackward mechanism.
The general idea of the feedforward mechanism is that the motile OHC force due to OHCs’
contraction or expansion is exerted onto an adjacent downstream BM segment due to the tilt
of the OHC. And the novel insight of the feedbackward mechanism is that the OHC force is
delivered onto an adjacent upstream BM segment due to the tilt of the PhP. Downstream and
upstream represent the longitudinal direction with respect to the forward (i.e., from base to
apex) traveling wave—downstream corresponds to apicalward while upstream corresponds
to basalward.
In the case of rigid OHCs (without motility), each BM segment moves up and down as
the cochlear fluids’ pressure difference between the scala vestibuli (SV) and the scala tympani (ST) acts on the BM. Figure 3.1B depicts a transverse view of the cochlear partition,
which reveals a critical structure called arches of Corti. The arches of Corti are formed by
the inner pillar cell (IPC), the outer pillar cell (OPC), and part of the BM transverse fiber.
The base of the IPC works as a rigid pivot point when the BM fiber vibrates up and down.
Given the micro-architecture, the movement of the BM segment i − 1 causes the stereocilia
of the same segment to deflect due to the radial shear motion between the RL and the TM.
In the case of OHCs with electro-motility, the deflection of the stereocilia in turn results
in the contraction or expansion of the OHC because ion flow across the membrane depolarizes or hyperpolarizes the OHC. Due to its longitudinal tilt, the OHC in segment i − 1
exerts a force on the BM (through the DC) that sits in the downstream segment i, which is
37
IHC
RL
OHC
PhP
DC
BM
A
d
Stereocilia
Basal
i -1
i
TM
i+1
Stereocilia
Apical
OHC
IHC
IPC
B
Spiral
lamina
RL
Tunnel
of Corti
OPC
DC
BM
Spiral
ligament
Figure 3.1: Active Bidirectional Coupling Mechanism
A Longitudinal view. B Transverse view of the Organ of Corti. In A, three segments are labeled (i.e., i − 1,
i, and i + 1) for illustration. Only the outermost (from the modiolus) row of OHCs is shown; the tectorial
membrane (TM) is not shown. In B, the TM is included. d denotes the longitudinal tilt distance.
38
pulled to or pushed away from the RL. Thus, the vibration of BM segment i amplifies the
displacement of its upstream neighbor, segment i − 1, due to the feedforward OHC force.
Besides this feedforward mechanism, our model features a novel feedbackward mechanism, which is implemented through the PhP extending from each DC’s main trunk [89].
Similar in structure to pillar cells within the OC, the DCs and their PhPs are presumably
rigid. As a result, with the apical end of each PhP immediately adjacent to that of an OHC,
it is reasonable to propose that the PhP transmits mechanical forces effectively—both tensile as well as compressive.
An pertinent phenomenon about the RL is the premise for the feedbackward mechanism. At a step of current injection, both the BM and the RL moved toward each other,
which is consistent with the relationship between the direction of BM’s motion and the
property of OHC forces (i.e., tensile or compressive). In other words, when the BM is
driven upward, corresponding to depolarization of OHCs, thereby contraction of the OHCs,
which in turn pull down the RL, resulting in opposite direction of motion of the BM and
the RL. More importantly, it was reported that the RL moves 5 to 10 times farther than
the BM [90]. This not only showed clearly that the OC is indeed deformable to a physiologically significant extent [18], but also provided us a rather useful hint on the relative
magnitude of feedbackward OHC forces to that of feedforward OHC forces.
Process of feeding back OHC forces is described as follows. In segment i + 1 of the
cochlear partition, the deflection of stereocilia due to the upward or downward displacement of BM segment i + 1 causes the OHC whose stereocilia end lies in the segment i + 1
to contract or expand, thus pulling down or pushing up the RL to some extent (meanwhile,
BM segment i + 2 (not labeled in Figure 3.1) is pulled up or pushed down due to the feedforward OHC force). The driven RL segment i + 1 then pushes down or pulls up the rigid
39
PhP and the whole DC, exerting a force on the BM segment i. Thus, BM segment i is
additionally pushed or pulled by the upward or downward displacement of the downstream
BM segment i + 1, due to the feed-backward OHC force.
In summary, the micro-architecture of the cochlear partition gives rise to feedforward
and feedbackward OHC forces that introduce active bidirectional longitudinal coupling
between BM segments. The motion of BM segment i − 1 reinforces that of segment i while
the motion of segment i + 1 opposes that of segment i.
3.1.3 Formulation of a Two-Dimensional Linear Model
We implemented a two-dimensional (2D) cochlear model in which the cochlear fluids are
included. This physical model is illustrated in Figure 3.2A. In the 2D model, both the
length of the uncoiled cochlea and the finite height of the cochlea duct (i.e., the scala
vestibuli (SV) and the scala tympani (ST)) are discretized into a number of sections with
the original aspect ratio of the cochlea maintained. The stapes movement sets the cochlear
fluid into motion and causes a vibratory deformation of the cochlear partition.
In the 2D model (Figure 3.2B), both the length of the uncoiled cochlea and the height of
the cochlea duct (i.e., the SV and the ST) are discretized into a number of sections with the
original aspect ratio of the cochlea maintained. The variable x represents the distance from
the stapes along the cochlear partition, with x = 0 at the stapes and the round window and
x = L (uncoiled cochlear duct length) at the apex and the helicotrema. And y represents
the vertical distance from the wall, with y = 0 at the top/bottom wall of cochlear duct and
y = h (cochlear duct radius or height) at the BM.
We first describe the formulation of a linear 2D mathematical model based on our
40
cochlear amplifier mechanism—active bidirectional coupling (ABC), and present its numerical solution. Next, we shall extend the linear model to a nonlinear model that simulates
cochlear nonlinear responses by incorporating OHCs’ saturation property.
Assuming incompressibility of the cochlear fluids, we modeled only the bottom cochlear
chamber (ST) in our 2D model, which is mathematically equivalent to modeling both chambers because the fluid movements in the SV and ST are complementary (Figure 3.2B). Now
let φ(x, y, t) represent the difference between the velocity potentials in the SV and ST:
φ(x, y, t) = φSV (x, y, t) − φST (x, 2h − y, t),
(3.1)
where φSV is the velocity potential in the SV while φST is the velocity potential in the ST.
From here on, we omit the time variable t in the expressions and equations for the linear
version of the model so that we will deal with the model in the frequency domain.
Due to the assumed incompressibility of the cochlear fluid, the velocity potential difference φ of the fluids is required to satisfy
52 φ(x, y) = 0,
(3.2)
where 5 is the Laplacian operator. This potential is related to fluid velocities in the x and
y directions:
Vx = −
∂φ(x, y)
,
∂x
and
Vy = −
∂φ(x, y)
.
∂y
(3.3)
Considering the pressure difference, Pd (x, y), thereby the force, imparted on a small
element of fluid and its resultant velocity, Vx and Vy , we obtain the following relations
41
Top wall
Oval
window
Stapes
Basilar
membrane
Base
Round
window
A
Helico- Right wall
trema Apex
Bottom wall
Top wall
y
2h
Stapes
Fluid
Scala tympani
Scala vestibuli
Basilar membrane
h
Fluid
0
B
Scala vestibuli
0
Bottom wall
Right wall
Scala tympani
L
x
Figure 3.2: Two-dimensional Cochlear Model
The uncoiled cochlea is represented by a rectangle, which includes the two major fluid ducts, scala vestibuli
(SV) and scala tympani (ST). As an approximation for the passive-cochlea case, the cochlear partition was
reduced to only the basilar membrane. A The stapes motion causes the oval window moves inward (solid-line
half circle) and outward (dashed-line half circle), which in turn pushes or pulls the round window due to the
incompressibility of the fluid. B In the two-dimensional (2D) cochlea model, because the fluid movements
in the SV and ST are complementary (i.e., equal in magnitude but with opposite directions), only the bottom
compartment was considered in order to reduce simulation time and computation load as well as taking
advantage of simpler coordinates. Thus, the basilar membrane forms one of the boundaries at y = h. The
longitudinal length of the cochlea at the apex is denoted by L.
42
according to the Newton’s second law:
−
∂Vx
∂Pd(x, y)
= 2ρ
∂x
∂t
and
−
∂Pd(x, y)
∂Vy
= 2ρ
,
∂y
∂t
(3.4)
where ρ is the fluid density and the factor 2 accounts for the same amount of fluid mass
in both SV and ST. Combining the relation with Equation 3.3 yields the relation between
pressure difference Pd and velocity potential φ:
Pd (x, y) = 2ρ
∂φ
∂t
(3.5)
The BM’s vibration is in the vertical direction, with downward displacement (i.e., towards the ST) being positive. The BM has mass, damping, and stiffness. Its motion is
directly driven by the pressure difference across it, which can be described as follows.
Pd (x, y) + FOHC (x) = S(x)δ(x) + β(x)δ̇(x) + M(x)δ̈(x),
FOHC (x) = αS(x)(γδ(x − d) − δ(x + d)),
where M(x) is the mass, S(x) is the stiffness, and β(x) = ζ
at y = h,
(3.6)
p
S(x)M(x) is the damping,
per unit area, of the BM, and ζ is the damping ratio; δ is the downward displacement of
the BM while the first time-derivative of δ is its velocity and the second is its acceleration.
Pd (x, y) at y = h is the pressure difference at the BM. FOHC (x) combines feedforward and
feedbackward OHC forces, expressed as a fraction α of the BM stiffness (i.e., OHC motility
factor). γ is the ratio of the feed-forward to the feed-backward coupling, representing
relative strengths of the OHC forces exerted on the BM segment through the DC, directly,
and through the tilted PhP. d denotes the tilt distance, which is the horizontal displacement
between the source and the recipient of the OHC force, assumed to be equal for feedforward
43
and feedbackward cases.
The stapes and the round window at the base are assumed to move identically except in
opposite directions. The basal boundary condition is described as follows.
−
∂φ
= f (t),
∂x
at x = 0,
(3.7)
where f (t) is the velocity of the stapes (defined to be positive for inward velocity).
At the apex, the horizontal velocity is zero due to the existence of hard wall. Therefore,
the apical boundary condition is described as follows.
∂φ
= 0,
∂x
at x = L.
(3.8)
At the bottom wall, the fluids do not have a vertical motion, thus the velocity in the y
direction is zero, which is described as follows.
∂φ
= 0,
∂y
at y = 0.
(3.9)
3.1.4 Model Simulation Results Using Finite-Difference Method
Having obtained the mathematical formulation of an active cochlear model, we shall explore its responses or behavior in responding to any sound stimulus. We first employed a
numerical approach to obtain model responses, or simulating the model on a digital computer. The major advantage of numerical simulation is that it provides a straightforward
and stereotyped means for obtaining model behavior—discretization in space and/or time,
44
transformation/approximation of the original set of governing equations, and solving the
equivalent equation set for responses.
The numerical method we used is Neely’s finite difference (FD), which has been most
widely employed in solving linear cochlear models [73, 91, 92]. Other numerical methods in the frequency domain included integral-equation method proposed by Allen [93]
and Sondhi [94]. In the FD method, derivatives in the ordinary differential equations that
describe the fluid hydrodynamics and all the cochlea’s boundary conditions were approximated by their finite-difference counterparts. Thus, a set of differential equations becomes
a set of linear algebra equations, which can be solved at moderate computational cost with
digital computers.
Specifically, we simulated the linear active model numerically in the frequency domain
and a nonlinear active model in the time domain. In order to investigate the effect of OHC
motility on the BM motion, we simulated the model using different OHC motility factors
α. The model parameter used in our simulations are listed in Table 3.1.4. Part of the
parameters are chosen to match the empirical data of frequency-position map of cat [95].
In this simulation, the cochlea is discretized into 350 segments longitudinally, amounting
to radial segments approximately 71 µm long (for a cochlear duct length of 25 mm). In the
vertical direction, 13 segments were used to represent the fluids at each cochlear position.
We obtained the BM displacement responses—amplitude (normalized by the stapes displacement) and phase (with respect to the stapes motion)(Figure 3.3). The input excitation
was a 2 kHz pure tone. The OHC motility factor α varied from 0.0 (passive case) to 0.20
(active case), resulting in different values of amplitude gain in the BM displacement.
Response characteristics were summarized in Table 3.1.4 in terms of peak gain (BM
45
Parameters and denotation
Cochlear duct height h
Cochlear duct length L
Fluid density ρ
BM mass per unit area M(x)
BM stiffness per unit area S(x)
BM damping ratio ζ
Tilt distance d
Segment length 4
OHC motility factor α
Forward-to-backward ratio γ
Values
1.0
25.0
1.0 × 10−3
3.0 × 10−5
5.0 × 106 e−0.4x
0.2
71.0 × 10−3
1.0 × 10−2
0.0 − 0.2
0.3
Unit
mm
mm
g/mm3
g/mm2
g/(mm2 s2)
mm
mm
Table 3.1: Mathematical Model Parameters
This set of parameters were chosen for simulating the cat cochlea.
response peak amplitude normalized by the stapes motion amplitude), peak phase (phase
lag relative to the stapes motion), Q10 (Q10 is used to quantify the sharpness of tuning,
calculated as the ratio between the CF and the frequency span of the curve 10 dB below
the peak. In other words, the larger the Q10, the sharper the tuning.), and peak location, or
characteristic place (CP).
With the varying α from 0.0 to 0.20, the response peak became larger and tuning gets
sharper. When α = 0.15, the amplitude of the BM displacement was realistic while the
phase accumulation at the peak was larger compared to the physiological data (around −1
to −6π radians [32]).
As was evident in the simulation, inclusion of OHC motility in the passive cochlear
model elicited active behavior of the cochlea. As can be seen from Figure 3.3A and Table 3.1.4, amplitude amplification of BM motion only occured in the vincinity of peak,
giving rise to frequency-selective amplification of BM vibration. Phase response curves almost overlap with one another (see Figure 3.3B): phase accumulation did not change much
near the base while phase lag at the peak varied a little simply due to the shift of peak loca-
46
α
0
0.1
0.15
0.2
Peak gain (dB)
17.6
46.2
79.5
114.6
Peak phase (π radians)
-5.1
-18.7
-19.2
-22.0
Q10
0.6
3.5
6.1
6.1
Peak location (mm)
11.4
14.6
14.6
14.6
Table 3.2: Mathematical Model Responses with Varying Outer Hair Cell Motility
This table summarizes the characteristics of the basilar membrane responses in the cochlear model with
different values of outer hair cell motility factor.
tion with the varying OHC motility factor α. This further suggested the restricted property
of the location of the ABC-based cochlear amplifier.
3.1.5 Bidirectional versus Uni-Directional Coupling
In our model, a forward-to-backward ratio, γ, is used to describe the contribution of feedforward and feedbackward OHC forces to the BM’s motion. Researchers have previously
shown how an active cochlear model behaves with feedforward forces [15,17]. It is then instructive to investigate what roles of forward and backward OHC forces play in enhancing
the BM’s motion.
In order to demonstrate the effects of adding feedforward and/or feedbackward OHC
forces into a passive model (without OHC motility), we simulated three variations of a
linear active model (i.e., with only feedforward, with only feedbackward, and with both
forces). The same parameter values were used for these simulations; the OHC motility
factor α was 0.15 and the ratio between feedforward coupling and feed-backward coupling
γ was 0.3 (in the case that feedforward OHC forces were present).
Peak amplitude, phase accumulation, and Q10 are used to evaluate the model response
quantitatively. Figure 3.4 shows the amplitude and phase of the BM displacement, nor47
BM Displacement
Amplitude H20 Log10 L
125
Α=0
Α = 0.1
Α = 0.15
Α = 0.2
100
75
50
25
0
-25
0
5
10
15
20
Distance from Stapes HmmL
25
0
5
25
A
BM Displacement
Phase HΠ radiansL
0
B
-20
-40
10
15
20
Distance from Stapes HmmL
Figure 3.3: Effect of Outer Hair Cell Motility
BM displacement, normalized by the stapes displacement, for different outer hair cell motility factor ( α)
values, to a 2 kHz pure tone. A Amplitude. B Phase. When α = 0.0, the model shows responses of a passive
cochlea; as α increases, the model generally shows larger amplification, sharper tuning and more phase lag
at peak. The parameter values used for the model simulation are listed in Table 3.1.4
48
malized by the stapes displacement, when driven by a 2 kHz tone for the three different
configurations as well as the passive case. The BM response is least amplified (peak < 20
dB) and broadest (Q10 is as small as 0.6) in the passive case, where the cochlear amplifier
mechanism—ABC—is absent; feedforward forces alone increase the amplification (peak
reaches around 54 dB) and sharpen the tuning (Q10 is about 2.6) to a limited extent; feedbackward forces alone increase the amplification a little more (peak reaches about 57 dB)
and sharpen the tuning to a much larger extent (Q10 is 7.0); the BM displacement is largest
(peak above 80 dB) and more realistically tuned (Q10 is 6.1) when both feedforward and
feedbackward forces are included.
Regarding the phase, the four cases show different phase accumulation at the peak. The
passive case and the feed-backward forces result in the smallest phase lag, and the case with
both forces has moderate phase lag at the peak, while the feedforward forces causes the
largest phase accumulation at -25.6 π radians. Thus, feedforward OHC forces alone result
in too much phase shift, feedbackward OHC forces alone give rise to too sharp tuning with a
not-large-enough peak, and when both of them, feedforward and feedbackward OHC forces
are present with different strengths (i.e., when the cochlear amplifier mechanism—ABC—
is included), the model produces the most realistic response in terms of peak amplitude,
phase, and frequency tuning of the BM responses compared to physiological measurements
[32].
3.1.6 Comparison with Physiological Data
The responses of the 2D linear active model at different input frequencies separated by
octave intervals are shown in Figure 3.5. Due to the logarithmic frequency-position relation, the peak locations corresponding to each input frequency are spaced evenly at 3.5
49
BM displacement
amplitude HdB SPLL
80
No ABC
Forward only
Backward only
With ABC
60
40
20
0
-20
0
A
5
10
15
20
Distance from stapes HmmL
25
5
25
BM displacement
phase HΠ radiansL
0
-20
-40
0
B
10
15
20
Distance from stapes HmmL
Figure 3.4: Comparison between Uni- and Bidirectional Coupling
Basilar membrane displacement responses, normalized by the stapes displacement, at 2 kHz pure tone. A Amplitude. B Phase. Four cases are simulated for comparison: No ABC, Feedforward only, Feedbackward only,
and with ABC. ABC: Active bidirectional coupling.
50
Model
No ABC
Forward only
Backward only
With ABC
Peak gain (dB)
17.6
53.9
57.1
79.5
Peak phase (π radians)
-5.1
-25.6
-17.4
-19.2
Q10
0.6
2.6
7.0
6.1
Peak position (mm)
11.4
15.1
14.5
14.6
Table 3.3: Bidirectional versus Uni-Directional Coupling: Simulation Results
This table compares the simulation results in the passive cochlea model (No ABC), the active cochlea model
that includes unidirectional outer hair cell (OHC) forces (Forward only and Backward only), and the active
cochlea model that include bidirectional OHC forces (With ABC). ABC: Active bidirectional coupling.
mm, which is just the octave span of cats (14% of cochlear length [95]). As the damping
ratio ζ remains the same, the tuning for different input frequencies are the same across the
cochlea.
A frequency-position map can be obtained from the peak locations for different input
frequencies. The frequency-position map extracted from the model responses was plotted
in Figure 3.6 and compared with the empirical function regressed from cat physiological
data [96]. The model data are in reasonable agreement with the regressed function (their
slopes are fairly similar), especially in the medium and high frequency region near the base.
The model data demonstrated a near-linear relationship between cochlear positions and
their corresponding CF in the base and middle of the cochlea. Discrepancy in slope between
the model responses and the physiological data mainly lied in the apical low-frequency
region, which may be attributed to the fact that differences between the apical and other
regions of the cochlea was not included in the present model. It has been observed that the
tuning curves of low-frequency and high-frequency fibers have different shapes [97, 98].
The factors to account for the basal-apical differences is yet to be discovered.
51
BM Displacement
Amplitude HdB re StapesL
16k
80
8k
4k
2k
1k
500Hz
60
40
20
0
-20
0
5
10
15
20
Distance from Stapes HmmL
5
10
15
20
Distance from Stapes HmmL
A
25
BM Displacement
Phase HΠ radiansL
0
-20
-40
0
B
25
Figure 3.5: Basilar Membrane Displacement Responses
Longitudinal patterns of basilar membrane displacement to six octave-intervaled frequencies. A Amplitude.
B Phase. The outer hair cell motility factor α was set to 0.15 for all the simulations in the plot.
52
x = Proportional
Distance from Apex
1
Empirical function
Model simulation
0.8
0.6
0.4
0.2
0
0.1
0.2
0.5
1
2
5
10
Characteristic Frequency HkHzL
20
Figure 3.6: Frequency-Position Map
The red dots denote generated from simulation results the two-dimensional (2D) linear active model. The
blue curve is the regressed function based on physiological data of cat; the empirical function is f(Hz) =
456(102.1x − 0.8) (x is the proportional distance from apex) [95].
53
3.1.7 Frequency Responses
We obtained the frequency responses of the BM velocity gain and phase at a cochlear
position around 6.9mm from the stapes; its CF was around 9.3 kHz. The OHC motility
factor was set to 0.17. The amplitude response compares favorably to the biological data,
while the phase accumulation at the peak (−6.7 cycles) is larger than biological data (−0.5
to −3 cycle) [32].
3.2 The Linear Model: Semi-Analytical Solution
While numerical methods can solve for model responses, they do not provide much insight
into the cochlear mechanics and the role of OHC motility in the active amplification; they
also require prohibitive computation time and resource, especially in time-domain simulations of nonlinear cochlear models and for computation with fine discretization which is
always desirable in the short-wave region to reduce the discretization error.
An alternative to numerical simulations is to solve the model analytically—that is, to
obtain explicit mathematical form or expression to describe the fluid and/or BM’s motion.
However, since transcendental equations were involved in the problem, our analytical approach had to incorporate numerical calculation at certain point in the solution process,
resulting in a semi-analytical solution (SAS). In addition, due to the difficulty in obtaining
approximation in solving the nonlinear version of the model, our analytical approach only
dealt with the linear version of the active cochlear model.
54
BM Velocity Gain
HdB re StapesL
80
60
40
20
0
-20
2
3
4
5 6 7 8 9 10
Frequency HkHzL
2
3
4
5 6 7 8 9 10
Frequency HkHzL
A
20
BM Velocity Phase
HCycles re StapesL
0
-5
-10
B
20
Figure 3.7: Frequency Responses of Basilar Membrane Velocity
A Response gain. B Phase. Both were relative to the stapes motion. The outer hair cell motility factor α was
set to 0.17 to match the peak amplitude in the physiological data. Solid line: model simulation results. Dots:
Physiological data [47].
55
3.2.1 Solution Procedures
Our analytical approach employed the LG (Liouville-Green), or WKB(Wentzel-KramersBrillouin) or WKBJ (Wentzel-Kramers-Brillouin-Jeffereys) method, a well-known approximation for describing wave behavior on shallow canals. Steele and Taber [99] and de Boer
and Viergever [100] first applied this approach for solutions to cochlea models.
The WKB method is based on the idea that the wavelength can be estimated based on
the properties of the medium in which the wave travels; it deals with the nonuniformity
along the cochlear duct due to the varying BM boundary properties. WKB assumes that
when the wavelength is short enough, the properties of the medium can be treated as uniform within the span of a wavelength [101]. As Steele and Taber pointed out [99], this
short-wave approximation can still be applied to the long-wave region of the cochlea because the long wavelength region is defined relative to the height of the cochlear duct, and
for the cochlea, the wavelength must be very long before the WKB solution becomes invalid. Further, the comparison that Zweig [102] made between the WKB and direct numerical integration solutions for a one-dimensional (1D) model confirmed that it is reasonable
to use only exponentials in the analytical approximations.
Following the approach in [6], we assumed a solution form of the velocity potential φ
that represented a wave traveling in the +x direction with y dependence:
φ(x, y, t) = Y (y)ei(ωt−kx),
(3.10)
where k is the complex wavenumber and ω is the real angular frequency of the wave. This
solution form must satisfy Laplace’s equation (Equation 3.2) and the bottom wall boundary
56
condition (Equation 3.9). Thus, the solution form can be further determined as:
φ(x, y, t) = B(x) cosh(ky)ei(ωt−kx),
(3.11)
where B(x) varies with cochlear position x. The form of φ applies to any cochlear position
(x, y) and time t while B and k take on different values due to the varying stiffness and
damping of the BM along the cochlea, which forms one boundary of the fluid body. In
other words, the wavenumber and wave amplitude are mainly determined by the physical
properties of the BM.
As Vy = ∂δ/∂t = −∂φ/∂y at y = h, the BM displacement δ can be computed as
R
δ(x, y, t)|y=h = − ∂y φ(x, y, t)dt|y=h
=⇒ δ(x, t) = iB(x)ei(ωt−kx)k sinh(kh)/ω.
(3.12)
By substituting these forms in Equation 3.12 and their corresponding derivatives, together with Equation 3.5, into the BM boundary condition (Equation 3.6), we derived a
dispersion relation, which describes the relation between the wave’s energy (i.e., input frequency ω) and its momentum (i.e., wavenumber k).
k tanh(kh)
2ρω 2
=
. (3.13)
1 + αS(x)k tanh(kh)(γeikd − e−ikd )/(2ρω 2 )
S(x) + iωβ(x) + (iω)2M(x)
Given any input frequency, we can solve this dispersion relation for k, which leads to the
solution of fluid velocity potential φ as well as the BM displacement δ.
Furthermore, we had Vstapes = −∂φ/∂x at x = 0 and we can derive the stapes displace-
57
ment as follows.
δstapes(y, t) =
Z
Vstapesdt,
(3.14)
And the average stapes displacement is the integration over the height of the oval window
(i.e., from y = 0 to y = h as an approximation and simplification). That led to
1
δstapes(t) =
h
Z
h
δstapes(y, t)dy
(3.15)
0
This gives a stapes displacement expression, which is a function of the input frequency ω
and the wavenumber at x = 0 (denoted by k0 ):
δstapes(ω, k0 ) = |
B0 sinh(k0 )
|,
ω
(3.16)
where B0 is B(x) at x = 0. And for simplicity, we assume the phase of the stapes displacement is zero. This stapes displacement will be needed for calculating the gain of BM
responses (i.e., BM amplitude normalized by stapes motion).
3.2.2 Wavenumber Loci of the Traveling Wave
Now we can solve the dispersion relation for the wavenumber k at different cochlear position x. The parameters used for the semi-analytical solution is exactly the same as the
numerical method (Table 3.1.4). Since the dispersion relation is transcendental, we have to
solve it numerically. The solution is the loci of k along the cochlea, starting from x = 0 to
x = L. During this process, we trace the locus of k by using the value of k at the previous
position as the initial value.
For wave propagation in the +x direction, the real part of the wavenumber, kr , is in-
58
versely related to the wavelength, whereas the imaginary part, ki , is related to the waveamplitude. These relations are evident if we rewrite the Equation 3.12 by replacing k with
kr + iki as
δ(x, t) = iB(x)ekix ei(ωt−kr x) (kr + iki ) sinh((kr + iki)h)/ω.
(3.17)
Thus, wave propagation in the passive and active models may be compared by plotting the
real and imaginary parts of the wavenumber. As the wave propagates, it transitions from
long wave to short wave, where its wavelength becomes much smaller than the cochlear
duct’s height.
Different OHC motility factor α in the simulation corresponded to strikingly different
loci of k, especially in the short-wave region where kr is large (Figure 3.8). The input was
a 2 kHz pure tone. The wavenumber locus stayed in the fourth quadrant in the passive case
where the OHC motility factor α = 0 (Figure 3.8A; only the fourth quadrant was shown),
while in the active cases where the OHC motility factor α > 0, the wavenumber loci
entered the first quadrant, right before the peak position at different extent with different
OHC motility factors, and then returned to the foruth quadrant (Figure 3.8B; only the first
and the fourth quadrants were shown). It is this positive value of ki that produces large
amplification of BM displacement in the active case.
We plotted kr and ki , scaled by the tilt distance d, seperately (i.e., kr d and ki d), with
respect to the distance from the stapes x, to understand more clearly the relation between
wavenumber loci and BM responses (Figure 3.9). Only the passive case and one active case
(α = 0.15) were shown for clear illustration.
In both passive and active case, kr started from a near-zero value and gradually in-
59
0
ImHkL HCyclesmmL
ImHkL HCyclesmmL
10
-20
Α = 0.0
0.2
0
0.15
0.1
-10
-40
0
5
10
ReHkL HCyclesmmL
0
10
20
ReHkL HCyclesmmL
Figure 3.8: Wavenumber Loci of Traveling Wave
Wavenumber loci are different at different OHC motility factors. The input is 2 kHz pure tone. A When the
outer hair cell motility factor α = 0.0. B When α = 0.1, 0.15, and 0.2. The dots mark the characteristic
place, where the BM vibration peaks.
60
creased, indicating that the wave travels at a high speed, then gradually slows down at
cochlear positions where the peak occurs. A dramatic difference between the passive and
the active cases occurs after the peak: in the passive case, kr increases to a large value
then drops to a near-zero value, while in the active case, it asymptotes at non-zero value—
indicating that while the wavelength remains short in the active case it becomes long again
in the passive case. As for ki , whereas it keeps negative in the passive case, we observed
positive values right before the peak location in the active case.
Having obtained the locus of the wavenumber k, we shall now investigate other characteristics of the TW. The traveling velocity v of the wave can be calculated as ω/k and the
wavelength λ is equal to 2π/kr or 2πv/ω (i.e., v/f); it is proportional to the velocity. (Figure 3.10) shows the TW velocity for both a passive and an active case. The velocity at the
CP is 1.94, and 0.36 m/s for these two cases, respectively. The corresponding wavelengths
are 969, and 180 µm for the passive and active cases, respectively.
To visualize the traveling wave behavior, we plot the velocity potential in the fluid and
the BM displacement, obtained using the wavenumber k we just solved for. Figure 3.11
shows the traveling wave for both the passive and an active cases. In the passive case, the
BM amplitude is only about 23 dB, and the Q10 is calculated to be 0.6, while the active
response reaches 85 dB and the Q10 is about 5.4. The wavelength decreases as the wave
travels; near the peak (in the short-wave region), the wave also becomes more localized
in the vertical direction, which cannot be simulated in a long-wave approximation model.
In particular, the wave extends from the BM for about 77 µm into the fluids (from model
semi-analytical simulation), which is comparable to its longitudinal wavelength.
61
ReHkLd HCyclesL
1.5
Α = 0.0
1
Α = 0.15
0.5
0
0
A
5
10
15
20
Distance from stapes HmmL
Α = 0.15
0
ImHkLd HCyclesL
25
-0.5
Α = 0.0
-1
0
B
5
10
15
20
Distance from stapes HmmL
25
Figure 3.9: Real and Imaginary Parts of Wavenumber
The plots show the loci of kr and ki multiplied by the tilt distance d. The passive case (α = 0) and the active
case (α = 0.15) are compared. A Real part of k. B Imaginary part. Input frequency is 2 kHz. Dots mark the
peak locations.
62
Velocity v Hm  sL
50
Α = 0.0
25
Α = 0.15
0
0
5
10
15
20
Distance from stapes HmmL
Velocity v Hm  sL
A
25
Α = 0.0
2
Α = 0.15
0
10
B
15
20
Distance from stapes HmmL
Figure 3.10: Velocity of Traveling Wave
Model simulation result using semi-analytical solutions for passive and active cases are shown. A Traveling
wave velocity for 2 kHz pure tone input. B Zoom-in around the characteristic place. Red curve represents
the passive case (α = 0.0), while blue curve represents the active case (α = 0.15). Dots mark the peak
locations.
63
A
B
C
D
Figure 3.11: Simulated Traveling Wave in the Passive and Active Cochleae
The input frequency is 2 kHz. Passive case (α = 0): A & B. Active case (α = 0.15): C & D. Traveling wave
(TW) in the fluid: A & C. TW along the basilar membrane: B & D. The plots for the passive and active cases
are on different scale.
64
3.2.3 Semi-Analytical Solutions
We now proceed to perform further analytical treatment to the model in order to derive
closed-form expressions that relate the traveling wave behavior, namely the wavenumber
k, to cochlea’s biophysical parameters. This effort will no doubt lead to more insight about
the contribution of the active mechanism, ABC, to the active amplification in the cochlea.
The analysis shall focus on seeking answers to three questions that highlight the characteristic features of the wavenumber locus in the active case. First, why does the wavenumber enter the first quadrant before the peak? Second, what is the asymptotic value of the real
part of the wavenumber (i.e., in the cut-off region)? Last, when is the imaginary part equal
to zero? Answer to the first question will shed light on how active amplification arises.
The second addresses why the wavelength remains short in the cut-off region, unlike the
passive case. The third reveals how the CP depends on the model’s parameters.
For convenience, we repeat here the dispersion relation (Equation 3.13) of the cochlear
traveling wave in the active case (i.e., with ABC):
k tanh(kh)
2ρω 2
=
. (3.18)
1 + αS(x)k tanh(kh)(γeikd − e−ikd )/(2ρω 2 )
S(x) + iωβ(x) + (iω)2M(x)
We shall now seek solutions to the dispersion relation in the three characteristic regions of the traveling wave, namely the long-wave, short-wave, and the cut-off region. In
these three regions, different assumptions and approximations will be introduced due to the
varying properties of the traveling wave in order to simplify the solution process.
Long-Wave Region
65
The dispersion relation (Equation 3.18) can be transformed into
2ρω 2
k tanh(kh)
=
.
√
√
1 + α γS(x) sinh (ikd + log γ)k tanh(kh)/(ρω 2 )
S(x) + iωβ(x) + (iω)2M(x)
(3.19)
√
√
where the term in the denominator on the left γ sinh (ikd + log γ) replaced the term
γeikd − e−ikd in Equation 3.18.
The wave in the cochlear fluid and along the BM travels at a large speed near the stapes,
corresponding to a long wave length. When the wave length is larger than the cochlear duct
height, the cochlea region is referred to as the long-wave region. Therefore, in the longwave region, we have hk 1 since wavenumber k is small. Therefore, tanh (hk) can be
√
approximated to be hk. In addition, the relation |kd| |log γ| holds in this region due
to the small k and the choice of d and gamma (see Table 3.1.4). Thus, Equation 3.19 is
simplified as:
2ρω 2
k2h
≈
.
√ 2
1 + α γS(x) sinh (log γ)k h/(ρω 2 )
S(x) + iωβ(x) + (iω)2M(x)
√
Also, the term
(3.20)
√
√
γ sinh (log γ) is equal to (γ − 1)/2. Substituting this into Equa-
tion 3.20 yields an expression of the wavenumber k:
k≈ω
s
2ρ/h
.
S(x) + iωβ(x) + S(x)α(1 − γ) − ω 2 M(x)
(3.21)
As the BM impedance in the long-wave region is dominated by its stiffness (far away from
the resonance), ignoring the damping and mass gives the approximate solution:
k≈ω
s
2ρ/h
S(x)(1 + α(1 − γ))
66
(3.22)
As we can see, the wave number k is proportional to the input frequency ω at any position
in the long wave region. In the passive case where α = 0, the solution of k reduces to:
k≈ω
s
2ρ/h
.
S(x)
(3.23)
Even in the active case, since α is much less than 1 (< 0.2 given the chosen model parameters), the effect of the term α(1 − γ) is rather minor—evidence that ABC does not take
much effect in the long-wave region.
In terms of traveling velocity, the group velocity of the fluid is equal to the phase velocity c:
c = ω/k ≈
s
S(x)(1 + α(1 − γ))
,
2ρ/h
(3.24)
which corresponds to a non-dispersive wave.
Short-Wave Region
Switching to the short-wave region, we shall seek the answer to the question of why
the wavenumber locus enters the first quadrant. In the short-wave region, in the vicinity of
the peak location, we observed that the real part of the wavenumber, kr is far greater than
the imaginary part, ki . In addition, since |kr h| 1, we have tanh(kh) ≈ tanh(kr h) ≈ 1.
Therefore, Equation 3.18 can also be tranformed into:
2ρω 2
k
=
.
√
√
1 + iα γS(x) sin (kd − i log γ)k/(ρω 2 )
S(x) + iωβ(x) + (iω)2 M(x)
(3.25)
In the following derivations, we replace Ω for ω/ωn , where ωn (x) is the resonant anp
gular frequency at the cochlear location x (ωn = S(x)/M(x)). In addition, we have
67
β=ζ
p
S(x)M(x) = ζM(x)ωn . With these substitutions, Equation 3.25 becomes:
(1 + iζΩ − Ω2 )k =
Since |kd−i log
2ρ
√
√
Ω2 + i2αk γ sin(kd − i log γ).
M(x)
(3.26)
√
√
√
γ| is close to 0, we have sin(kd−i log γ) ≈ kd−i log γ. In addition,
Ω < 1 as the peak occurs well before the resonance and ζ is chosen to be 0.2, so we have
ζΩ 1. Thus, Equation 3.26 becomes:
√
√
√
2αd γk 2 + i(1 − Ω2 − 2α γ log γ)k − i2ρΩ2 /M(x) = 0,
(3.27)
√
√
Further, we ignore the term α γ log γ since it is far less than 1 based on our parameter
choice. Since Ω2 1, we ignore the 1 − Ω2 term, and we have
2αdγk 2 + ik − i
2ρ 2
Ω = 0.
M(x)
(3.28)
Hence, we find the solution to the wavenumber in the short-wave region as:
q
√
√
i16α γρd/M (x)Ω − 1 − i /4αd γ.
k=
(3.29)
From the expression of k above, we obtain its imaginary part, ki as follows.
√
ki =
2
2
q
p
1 + 1 + 256γd2 α2 ρ2 Ω4 /M(x)2 − 1
.
√
4αd γ
(3.30)
Ω increases as the wave propagates, due to the decrease in ωn . Thus, if α > 0, the imaginary
part of the wavenumber k, ki , becomes positive, pushing the wavenumber locus into the first
quadrant. Therefore, it is evident that a positive OHC motility factor is the source of active
amplification.
68
Cut-Off Region
Now we shall look at the cut-off region, for which we transfrom Equation 3.18 into:
2ρω 2
k tanh (kh)
=
.
√
1 + iα γS(x) tanh (kh) sin (kd − i log γ)k/(ρω 2 )
S(x) + iωβ(x) + (iω)2 M(x)
(3.31)
√
Substituting k with kr + iki in Equation 3.31 yields:
√
√
√
α γS(x) (i cosh(ki d − log ( γ)) sin(kr d) − cos(kr d) sinh(ki d − log( γ)))
+
ρω 2
(kr − iki ) = (S(x) − ω 2 M(x) + iωβ(x))/2
kr2 + ki2
(3.32)
Substituting Ω for ω/ωn and decomposing Equation 3.32 into the real and imaginary
part yield:
√
√
ρΩ2
k
−
α
γM(x)(cos(k
d)
sinh
(k
d
−
log(
γ))) = (M(x)(1 − Ω2))/2, (3.33)
r
r
i
2
kr2 + ki
and
√
√
ρΩ2
k − α γM(x)(sin(kr d) cosh (ki d − log( γ))) = −(ζΩM(x))/2.
2 i
2
kr + ki
(3.34)
These two equations can be further reorganized as:
ρ
kr
√
√
M(x)
2
2 γα(cos(kr d) sinh (ki d − log( γ))) = Ω + 2
Ω2 ,
kr + ki2
2
69
(3.35)
and
ρ
ki
√
√
M(x)
Ω2 .
2 γα(sin (kr d) cosh(ki d − log( γ))) = ζΩ + 2
kr + ki2
2
(3.36)
Dividing Equation 3.36 by Equation 3.35 results in
2 Mρ(x) ki
ζ
+
tan(kr d)
Ω kr2 + ki2
=
√
2 Mρ(x) kr
tanh(ki d − log( γ))
1+ 2
kr + ki2
(3.37)
√
Since |ki d| 1 in the cut-off region, and log( γ) ≈ −0.6, we have tanh(ki d −
√
log( γ)) ≈ −1. Thus, we have:
ρ
ki
ζ
M(x)
+ 2
Ω
kr + ki2
− tan(kr d) =
ρ
2
kr
M(x)
1+ 2
kr + ki2
2
(3.38)
Since Ω 1 near the apex, ζ/Ω ≈ 0. Also because |ki | |kr | near the apex, we have:
ρ
2
ki
ζ
M(x)
+ 2
Ω
kr + ki2
≈ 0.
ρ
2
kr
M(x)
1+ 2
kr + ki2
(3.39)
tan(kr d) ≈ 0,
(3.40)
Therefore, Equation 3.38 leads to:
which gives the asymptotic value of kr as π/d. This result predicts that the wavelength gets
70
no shorter than twice the tilt distance d, and explains why kr asymptotes to the same value
independent of α. For a tilt distance d = 71 µm, this estimate yields the value of 44.2,
which is close to the simulation result of 36.1.
At the Characteristic Place
It is also instructive to investigate the model’s responses at the CP as both the magnitude
and phase of the BM response at the CP reflect the characteristics of the cochlear mechanics, as well as encode the input signal’s property. As we already know, ki = 0 corresponds
to the peak in the active case. Therefore, Equation 3.33 and Equation 3.34 become:
√
√
ρΩ2
− α γM(x)(cos(kr d) sinh (− log( γ))) = (M(x)(1 − Ω2 ))/2,
kr
(3.41)
and
√
√
α γM(x) sin(kr d) cosh (− log( γ)) = ζΩM(x)/2.
(3.42)
Around the peak, we have sin(kr d) ≈ π − kr d since kr d ≈ π (by linearization sin(x)
√
√
√
√
around π). Also, we have γ sinh (− log γ) = (1 − γ)/2 and γ cosh (− log γ) =
(1 + γ)/2.
Substituting m = M(x)/2ρd (the ratio between the BM mass and fluid mass of depth
d), Equation 3.41 becomes
ρΩ2
1−γ
cos kr d = (M(x)(1 − Ω2))/2,
− αM
kr
2
1
)−1
=⇒ α(1 − γ) cos kr d = Ω2 (1 +
mkrd
71
(3.43)
for the real part and Equation 3.42 becomes
α(1 + γ)(π − kr d) = ζΩ
(3.44)
for the imaginary part of the dispersion relation.
Solving Equation 3.44 for kr , we obtained its solution at the peak position:
kr =
ζΩ
π
−
.
d αd(1 + γ)
(3.45)
With cos(kr d) ≈ −1, Equation 3.43 becomes:
Ω2
+ αmkr (1 − γ) = mkr (1 − Ω2 ).
d
(3.46)
Substituting the expression of kr at the peak (Equation 3.45) into Equation 3.46 yields:
−ζΩ3 + α(1 + γ)(π + 1/m)Ω2 + ζ(1 − α(1 − γ))Ω + π(α(1 − γ) − 1) = 0
(3.47)
Ignoring the Ω3 term due to Ω 1 at the CP, we obtained a second-order polynomial
equation in Ω:
α(1 + γ)(π + 1/m)Ω2 + ζ(1 − α(1 − γ))Ω + π(α(1 − γ) − 1) = 0
(3.48)
Solving Equation 3.48 for Ω, we then obtained:
Ω=
ζ(α(1 − γ) − 1) +
p
(α(1 − γ) − 1))2 ζ 2 + 4πα2 (1 + γ)2(1 − γ(1 − γ))(π + 1/m)
2α(1 + γ)(π + 1/m)
(3.49)
Substituting model parameters in the expression of Ω above gives values of Ω at the CP:
72
0.48, 0.49, and 0.49 when α is chosen to be 0.1, 0.15, and 0.20, respectively. These values
verified the assumption we made earlier that Ω2 1 at the CP. Since α(1 − γ) 1 and
γ(1 − γ) 1, the solution above can be further simplified as:
p
ζ 2 + 4πα2 (1 + γ)2 (π + 1/m) − ζ
Ω=
2α(1 + γ)(π + 1/m)
(3.50)
This simplified solution of Ω yields 0.49, 0.52, and 0.53 for α is chosen to be 0.1, 0.15,
and 0.20, respectively. On the other hand, the results from the semi-analytical simulation
approach described in the previous section (Figure 3.9) were 0.51, and 0.51 for α = 0.1
and 0.15, respectively (diverged at α = 0.2). Therefore, the predictions in Equation 3.50
are in good agreement with the simulation based on numerical solution of the wavenumber
k.
Substituting the expression of Ω in Equation 3.50 into the expression of kr (Equation 3.45) yields:
π
kr = − ζ
d
p
ζ 2 + 4πα2 (1 + γ)2 (π + 1/m) − ζ
.
2α2 d(1 + γ)2(π + 1/m)
(3.51)
Qualitatively speaking, Equation 3.51 clearly shows that at the CP, the wave propagation
speed is determined by the BM-to-fluid mass ratio m, BM’s damping ratio ζ, the tilt distance d, the OHC motilty factor α, and the forward-to-backward ratio γ. Quantitatively, kr
is a little smaller at the CP than the asymptote value of kr (i.e., π/d in the cut-off region).
This solution for kr evaluates to 23.8, 26.5, and 27.4 when the α is set to be 0.1, 0.15, and
0.20, respectively. In comparison, the semi-analytical simulation gives 23.6 and 25.9 for α
= 0.1 and 0.15, which indicates that the approximations in Equation 3.51 are quite good.
We have solved a linear version of the active cochlear model that includes our cochlear
73
amplifier mechanism, ABC, using a semi-analytical approach. The analysis provided answers to several important questions about how the parameters of ABC, namely the OHC
motility factor α and the forward-to-backward ratio γ, give rise to distinctive locus of the
wavenumber k, thereby the active amplification in the cochlea. We shall now proceed to
investigate the impedance of the BM based on some of the results we have obtained through
the SAS.
3.2.4 Impedance of the Basilar Membrane
Impedance quantifies resistance to motion. While mechanical impedance is defined as the
ratio of the driving force to the velocity it imparts, effective acoustic impedance of the BM
is calculated as the pressure difference acting on a BM segment divided by the velocity of
that segment. For a passive BM (without OHC forces exerted on it), the real part of the
acoustic impedance is the BM damping and the imaginary part is the difference of BM
mass and stiffness. In contrast, for an active BM (with OHC forces exerted on it), the real
part and the imaginary part of the impedance will be altered due to the additional OHC
forces. Thus, exploration of the acoustic impedance of the BM shall give us more insight
about the property of OHC forces, or what contribution OHC forces would make to the
BM’s vibration, which eventually leads to large amplification and sharp tuning.
We derived the expression of the effective impedance of BM, zm , based on its definition.
For convenience, we repeated the solution form of the velocity potential of the fluid φ:
φ(x, y, t) = B(x) cosh(ky)ei(ωt−kx),
(3.52)
where all the variables have the same physical meaning as that in the previous section.
74
From here, we obtained the BM velocity, VBM , in the vertical axis as:
VBM (x, y, t) = −
∂φ(x, y, t)
|y=h
∂y
= B(x) sinh(kh)ei(ωt−kx) .
(3.53)
Also, based on Equation 3.5, we obtained the pressure difference that is applied to the BM
as:
Pd (x, y, t) = 2iρωB(x) cosh(ky)ei(ωt−kx)|y=h
= 2iρωB(x) cosh(kh)ei(ωt−kx).
(3.54)
Then we derived the expression of zm as:
zm =
Pd
VBM
= −
2iρω
.
k tan(kh)
(3.55)
Based on the loci of wavenumber k that we have obtained previously, we obtained and
plotted the real and imaginary components of the BM impedance (Figure 3.12) at 2 kHz
pure-tone input. The reactive components of the effective BM impedance in the passive
and active cases are almost the same (Figure 3.12B), indicating that BM stiffness and mass
are not altered by OHC motility. Meanwhile, the major difference between the passive and
the active case lies in the resistive component of the BM impedance (Figure 3.12A), which
implies that the inclusion of ABC actually modifies the damping. Specifically, compared to
the passive case, the damping in the active case is reduced (by half) before the characteristic
place and is increased after the CP. Furthermore, close to the peak location, at about 14.6
mm, the damping becomes negative, then rises abruptly to a positive value at the CP. As a
75
result, the BM gains its large peak amplitude around this position and then the vibration’s
amplitude is cut off sharply.
We plotted the normalized impedance (Figure 3.13) by dividing it by the critical dampp
ing (i.e., mass and stiffness component at resonance; S(x)M(x)). The resistive component of the normalized impedance yields the damping ratio. Clearly, the damping ratio
in the active case is no longer constant; instead, it reduced from 0.2 to a value closer to
zero and becomes negative right before the CP. In contrast, the reactive components of
the impedance in both cases are very similar—their CPs are far from the mass-dominated
region.
3.2.5 Comparison of Numerical and Analytical Solutions
We have solved the linear active model using both numerical and semi-analytical methods.
It is helpful to check how well the results of the two methods agree with each other. Comparing the results in Section 3.1.4 and in this section, the BM responses obtained using the
finite-difference (FD) method and the semi-analytical (SA) method are quite close to each
other (Figure 3.14).
While the BM’s amplitude responses are quite close to each other, the major difference
between the two approaches is in the phase responses. The FD method results in phase
plateau in the cut-off region while the SA approach does not. This is because of the fact
that in our SA method when solving the wavenumber k, we only track the traveling wave
mode while in the FD simulation, all the modes were found through solving the set of
differential equations. In an attempt to seek quantitative agreement between numerical
simulation and analytical solution, Watts concluded that inclusion of an additional degree
76
ReHZmL
2
1
Α = 0.0
0
Α = 0.15
0
5
A
0
10
15
20
Distance from Stapes HmmL
25
Α = 0.0
ImHZmL
Α = 0.15
-200
-400
0
B
5
10
15
20
Distance from Stapes HmmL
25
Figure 3.12: Basilar Membrane Effective Impedance
The basilar membrane impedance at 2 kHz pure tone input. A Re(zm ), the resistive component of the
impedance. B Im(zm ), the reactive component of the impedance. Red: the passive case (α = 0.0). Blue:
the active case (α = 0.15). The dots mark where the peak occurs in both cases. The difference between
passive and active cases mainly lies in the resistive component of the impedance, in which negative damping
is responsible for the active cochlear responses.
77
!!!!!!!!!!!!!!!!!!!!!!
ReHZmL S HxL M HxL
2
1
Α = 0.0
0
Α = 0.15
0
!!!!!!!!!!!!!!!!!!!!!!
ImHZmL S HxL M HxL
A
5
10
15
20
Distance from Stapes HmmL
25
0
Α = 0.15
Α = 0.0
-20
-40
0
B
5
10
15
20
Distance from stapes HmmL
25
Figure 3.13: Basilar membrane Effective Impedance (Normalized)
The basilar membrane impedance normalized by the ctritical damping,
p
(S(x)M (x)). The input and data
are the same as that in Figure 3.12. A Resistive component, or the damping ratio. B Reactive component. Red
curve represents the passive case (α = 0.0), while blue curve represents the active case (α = 0.15). The dots
mark where the peaks occur. As expected, the damping ratio in the passive case is a constant (i.e., 0.2). In
contrast, the damping ratio in the active case becomes negative right before the peak while it is only affected
slightly near the base by the inclusion of ABC.
78
of freedom, a second wave mode, addresses the issue effectively [6].
Using the SA method, we observed a bifurcation at α = 0.2 (see Figure 3.8). The
wavenumber locus goes into the first quadrant and does not return to the fourth quadrant,
corresponding to an exploding BM displacement response. Our numerical simulations,
however, do not show this bifurcation at larger values of α. When α increases, the numerical solution simply gives rise to increased peak height. We think that the bifurcation in the
SA solution might be due to the way the roots of the dispersion relation are found, which
is very sensitive to initial values.
3.2.6 Spatial Filter
An instructive perspective on the ABC mechanism is to think of ABC as a spatial filter
that is imposed on the traveling wave along the BM (Figure 3.15). The basal tilt of outer
hair cells and the apical tilt of phalangeal processes help selectively amplify the traveling
wave along the basilar membrane. Near the base, the wavelength is long relative to the
cochlear duct height and also the tilt distance. Positive feedforward outer hair cell forces
and negative feedbackward forces tend to cancel each other, and no extra energy is pumped
into the traveling wave. In contrast, near the characteristic place, the wavelength is short
and comparable to the tilt distance, thus forward and backward outer hair cell forces more
or less reinforce each other, enhancing the basilar membrane motion, thereby producing
amplification near the peak.
To validate the spatial filter concept, we investigated the phase difference between the
source and the target BM segments of OHC forces. In our simulation, this corresponds to
the phase difference between adjacent stages (Figure 3.16A). Clearly, when the traveling
79
BM displacement
gain HdB SPLL
80
FD
SA
60
40
20
0
-20
A
5
10
15
20
Distance from stapes HmmL
25
5
25
0
BM displacement
phase HΠ radiansL
-10
-20
-30
-40
-50
-60
0
B
10
15
20
Distance from stapes HmmL
Figure 3.14: Comparison Between Numerical and Semi-Analytical Solutions
The basilar membrane responses are in good agreement with each other using the two approaches. FD:
Finite-difference. SA: Semi-analytical.
80
Apex
Base
Short wave region
Long wave region
Cut off region
Figure 3.15: Concept of Spatial Filter
The tilts of both outer hair cells and the phalangeal process (see Figure 3.1 for structure details) is shown,
overlapping with a snapshot of the traveling wave (its envelop is denoted by the dashed line). In the longwave region near the base, feedforward and feedbackward forces due to outer hair cell motility tend to oppose
or cancel each other, resulting in no amplification. In the short-wave region, they tend to reinforce each other,
resulting in large amplification. In other words, feedforward and feedbackward forces work in concert to
amplify the traveling wave, only when the wavelength is short and comparable to the tilt distance. Therefore,
the tilt along the cochlea work as a spatial filter.
81
wave is short, the phase difference between neighboring BM segments increases, reaching about π/2. This increasing phase difference activates the ABC because feedforward
and feedbackward OHC forces now act in concert, instead of in opposition, such that the
BM receives net positive energy. Note that the π/2 phase difference does not occur at
the peak position, but a little basal to it, which concurs with the notion that this energy
boosts the BM’s motion. Moreover, the π/2 phase difference occurs in the region where
negative damping is achieved in the cochlea. In addition, it is worth noting that the phase
difference becomes closer to π at cochlear positions away from the peak toward the apex
(Figure 3.16A), thus feedforward and feedbackward forces will oppose each other and get
canceled again.
Figure 3.16B plots the damping ratio (normalized real part of the BM effective impedance)
with the two zero-crossing points indicated. The region between the two lines, exhibiting
negative damping, delimits the region where the cochlear amplifier takes effect.
3.3 The Nonlinear Model: Formulation and Simulation
In order to model nonlinear cochlear behavior, we extend the linear active model to a nonlinear active model by incorporating the saturating property of OHC forces. The saturation
of OHC forces is assumed to be the origin of the cochlear nonlinearity. The assumption that
the OHC forces have a saturation property is supported by the physiological data presented
in Chapter 2: the OHC’s receptor potential saturates with the acoustic pressure and the
OHC’s cell body length change saturates with the receptor potential. Based on these observations, we use the saturating profile shown in Figure 3.13 to model the relation between
OHC force and BM displacement.
82
Phase difference between
neighboring BMs HΠ radiansL
1
0.5
0
0
5
10
15
20
Distance from the stapes HmmL
0
5
10
15
20
Distance from stapes HmmL
!!!!!!!!!!!!!!!!!!!!!!
ReHZmL S HxL M HxL
A
25
1
0.5
0
-0.2
B
25
Figure 3.16: Phase Difference and Damping ratio
A Phase difference between neighboring stages. Dashed lines label π/2, the basilar membrane (BM) characteristic place, and the peak location. B Damping ratio (Real part of the BM effective impedance, normalized
by critical damping) with the zero-crossings labeled. The longitudinal span between the two dashed lines is
where the cochlear amplifier mechanism, active bidirectional coupling (ABC), takes effect.
83
For simplicity, the present model assumes that OHC forces saturate with a symmetric
profile for both contraction and elongation. This curve is a hyperbolic tangent (tanh). It is
applied to both the feedforward and feedbackward OHC force terms in Equation 3.6. Thus,
the saturated OHC force was modeled as:
FOHC (i) = A(tanh(
αS(i)δ(i + d)
αS(i)γδ(i − d)
) − tanh(
)),
A
A
(3.56)
where A determines the saturation level of OHC forces.
3.3.1 Compressive Growth
In our nonlinear active model, we investigated the compressive growth of BM responses at
high input levels, a phenomenon that is intensively studied, and accounted for by the saturation of OHC forces. We stimulated the model with a 5 kHz pure tone at various intensities
and obtained BM velocity responses (Figure 3.18). The input level was increased in 20 dB
steps. Only 150 BM segments were used to save computation time. The integration time
and the size of the integration step were chosen to be 40 ms and 4 µs (i.e., 10,000 steps),
respectively, such that the transient responses decayed, at least within the region concerned
(i.e., from the base to the position a little apical to the peak).
Clearly seen from Figure 3.18A, the BM responses becomes more broadly tuned with
increasing input levels. With input level at 0 or 20 dB (corresponding to stapes displacement at 1 pm), the BM velocity response shows a large peak and sharp tuning (Q10 = 4.2
and 5.3, respectively); as the input increases, the BM response grows in a compressive
manner and becomes more broadly tuned. When input level is 100 dB, the BM response
resembles the passive cochlea’s response (Q10 = 1.6). As a result, the input dynamic range
84
OHC forces
BM displacement
Figure 3.17: Saturating Profile of Outer Hair Cell Force
This profile between outer hair cell (OHC) forces and basilar membrane (BM) displacement approximates
physiological data: the OHC receptor potential saturates with acoustic pressure and the OHC’s cell body
length change saturates with the receptor potential. For simplicity, a symmetric form was used for the saturation profile.
85
100
80
60
40
20
0 dB SPL
0
2
4
6
Distance from Stapes HmmL
8
104
100 dB SPL
103
80
60
102
0
0
40 20
2
4
6
Distance from Stapes HmmL
B
8
104
Amplitude HΜmsL
BM Velocity Phase HΠ radsL
A
-10
103
5.7mm
102
Chinchilla
1
6.5
1
4
10
Linear
10-1
-20
0
C
BM Velocity Amplitude HΜmsL
BM Velocity Amplitude HΜmsL
104
103
102
101
1
10-1
10-2
2
4
6
Distance from Stapes HmmL
0
8
D
20
40
60
80
Input Level HdB SPLL
100
Figure 3.18: Cochlear Nonlinearity—Compressive Growth in the Model
Basilar membrane (BM) velocity responses at various input intensities. A BM velocity amplitude grows
in a compressive manner, exhibiting sharp tuning at low input levels while showing broad tuning (passive
behavior) at high levels. Only part of the cochlea (0-8mm) is shown. B Partial plots that highlight the small
localized cochlear region that is stimulated by low-level sound, compared to much broader activity at high
sound levels. C BM velocity phase responses. They are almost independent of input levels. D BM velocity
amplitude versus input level at three different cochlear locations: 4 mm, 5.7 mm (where the BM peaks at 0
dB SPL), and 6.5 mm. Near the base, the BM shows linearly increasing responses with input levels; at and
apical to the peak, the BM shows nonlinear (saturating) responses (i.e., compression at high input levels).
86
of 100 dB is compressed by the cochlea, yielding 50.1 dB compression, which is comparable to physiological data [47]. Note that in the region basal to the peak location, the BM
response increases proportionally with the input level.
This peak-specific high-level compression is further evident when BM responses at
three different cochlear positions were plotted against the input intensity (Figure 3.18D).
These cruves are also called the input-output function of the BM. At a cochlear location
before the peak where x = 4 mm (near the base), the input-output function is a straight
line with a close-to-unity slope (1.01-1.06). In other words, the BM response near the base
increases proportionally with the input intensity. At the peak where x = 5.7 mm (the CP at
the lowest input level in the simulation), the input-output function showed varying slopes
at low, medium, and high sound levels, which are 0.62, 0.40, and 0.70, respectively. The
BM response around the peak shows saturation at medium to high input levels, evidence of
compressive growth. At a position that is a little apical to the CP where x = 6.5 mm, the
BM velocity also shows saturation with a slope of 0.48 at high input levels.
For comparison, we also plotted the input-output function of the BM at the peak of
chinchilla cochlea’s responses (from [47]; Figure 3.18D). As can be seen, our model simulation is in good agreement with physiological data with the similar saturation profile.
Therefore, the nonlinear cochlear model with saturation of OHC forces was shown to be
capable of model the compressive growth in the biological cochlea.
3.3.2 Two-Tone Suppression
In a nonlinear cochlear model, frequency components of the input sound are expected to
interact with one another. When two tones are presented to the cochlea, suppression is
87
observed, known as two-tone suppression. That is, the response to one tone is reduced in
the presence of the other one. This phenomenon occurs at three levels: the BM vibration,
the IHC membrane voltage, and the auditory nerve discharge (action potentials). The main
features of mechanical two-tone suppression are at least qualitatively similar to that of twotone rate suppression in the auditory nerve [48]. Therefore, we investigate this interference
phenomena at the BM level in our cochlear model (i.e., with saturating ABC).
Our model showed two-tone suppression both when the suppressor frequency is higher
(high-side) and lower (low-side) than the probe frequency. We used an 8 kHz probe tone,
a 10 kHz high-side suppressor tone, and a 6.4 kHz low-side suppressor tone, of various
intensities. The BM input-output curves (BM velocity versus input level) at the CP (Figure 3.19) show that for both the high-side (Figure 3.19A and Figure 3.19B) and low-side
(Figure 3.19C and Figure 3.19D) cases, compression decreases with increasing intensity.
Consequently, the saturating BM input-output curve obtained for the probe tone alone becomes almost linear when an intense suppressor tone is simultaneously present.
The mechanism underlying two-tone suppression has been ascribed to the saturating
profile of the OHC input-output curve (i.e., the OHC receptor potential versus sound pressure) [18, 79]. In our model, the louder suppressor drives the OHC into saturation so that
the probe response is reduced. For low-side suppression (Figure 3.19C& D), the probe
tone response is directly suppressed by the suppressor tone. In contrast, for high-side suppression (Figure 3.19A & B), the suppressor response acts by suppressing the probe-tone
response basal to the CP of the probe-tone response, where negative damping occurs for
the probe tone.
88
40
8kHz with 10kHz
0
1
BM Velocity Amplitude
HdB re StapesL
A
2
3
4
Distance from Stapes HmmL
80
60
Linear
40
40
B
40
8kHz with 6.4kHz
2
3
4
5
Distance from Stapes HmmL
CF: 8kHz
30
Probe 8kHz Alone
0
100
5
Suppressor 6.4kHz
80
1
C
BM Velocity at the Peak
HdB re StapesL
Probe 8kHz Alone
BM Velocity at the Peak
HdB re StapesL
BM Velocity Amplitude
HdB re StapesL
80
Suppressor 10kHz
100
50
60
70
80
Probe Level HdB re StapesL
90
CF: 8kHz
80
60
Linear
40
30
6
Suppressor
10kHz
0 dB
40 dB
60 dB
80 dB
100 dB
D
40
Suppressor
6.4kHz
0 dB
40 dB
60 dB
80 dB
100 dB
50
60
70
80
Probe Level HdB re StapesL
90
Figure 3.19: Nonlinearity: Two-Tone Suppression in the Model
A The basilar membrane (BM) velocities (in dB re Stapes) at the characteristic position at which the CF is
near the probe tone frequency 8kHz versus the probe tone intensity. Increasing suppressor tone intensity(from
zero to 100 dB) turns the curve from nonlinear to linear relation. B The BM velocity amplitude for the probe
8kHz alone, the suppressor 10kHz, and 8kHz with 10kHz. C The BM velocities at the characteristic position
at which the CF is near the probe tone frequency versus the probe tone intensity. Increasing suppressor tone
intensity(from zero to 100 dB) turns the curve from nonlinear to linear relation. D The BM velocity amplitude
for the probe 8kHz alone, the suppressor 6.4kHz, and 8kHz with 6.4kHz.
89
3.4 Model Evaluation
We have presented our model simulation results and showed that the present active cochlear
model—by incorporating the cochlear amplifier mechanism, ABC—is able to reproduce
the characteristic cochlear responses. We shall now evalaute the model through discussing
several issues that are related to the model’s performances.
Given the fact that the cell diameters of OHCs and DCs are much larger than the BM
transverse fiber width [18], we need to explore the effect of discretization on model simulation results. Discretizing the BM into larger number of segments does not affect the
simulated BM responses as long as the tilt distance d is fixed. Without loss of generality,
we always use an BM segment width that is an integer fraction of the tilt distance d. For example, we doubled the discretization step number, which decreases the BM segment length
∆ to be 0.035mm, while we kept the tilt distance d to be the same as 0.071 mm. This resulted in the configuration that each BM segment sends and receives coupling to and from
BM segments that are two-segment upstream (basalward) and downstream (apicalward)
away from the segment in question, respectively, instead of to and from its immediate
neighbors. Reaults of the simulation under such configuration showed that discretization
does not affect model responses.
Previous models (e.g., [15]) included the OHC low-pass filtering in the cochlear amplifier. However, such concern has been largely cleared by newly accumulated evidence
showing that the limiting frequency of the operating motor in the mammalian OHCs is up
to 25 kHz [103]. Thus, OHC electromotility is fast enough, especially for human hearing. Theoretical analysis of OHC receptor potentials showed that the corner frequency of
low-pass filtering could be extended into the audio range due to the membrane-voltagedependent capacitance [104]. According to these findings, there is no need in our model to
90
include low-pass filtering effect in OHCs.
The focus of this model is to help explore a plausible way in which OHCs forces are
delivered through the unique architecture of the OC. Our model seeks to capture the major
anatomical features of the cochlear partition and to verify their role in reproducing realistic cochlear responses. We did not attempt to include all the possible roles that every
contitutional component in the OC plays in the cochlear amplifier. For example, we did
not include the tectorial membrane’s inertial mass against which outer hair cells can exert
forces [105]) nor its essential role in generating the shear force that deflects OHCs’ stereocilia. However, the functional role of the tectorial membrane was implicitly included; it
was just not modeled by mathematical variables.
There is room for the model to be extended to include more details in the Oc that have
been brought to the attention of cochlear physiologists. For example, the organ of Corti
shows morphological differences along the length of the cochlea—difference in OHCs’
properties between the basal and apical regions are particularly relevant. First, despite fairly
constant diameter, OHCs’ length varies: relatively short near the base while relatively long
in the apex [106]. Second, OHCs are oriented at varying angles along the radial direction
near the base and apex: they are oriented almost in parallel to the modiolus near the base
while their apical ends are more slanted toward the modiolus near the apex [107]. These
differences would possibly result in differences in OHC forces. Therefore, it is reasonable
to postulate that these differences in OHCs at different cochlear locations will contribute to
the differences in BM’s vibrations along the cochlea, such as magitude, tuning, and cut-off
slope. The present model did not include these distinct aspects between the base and apex,
which will be addressed in its improvement as the next step.
91
3.5 Summary
OHC motility was discovered more than two decades ago in the mammalian cochlea and
has since been considered to be the basis of the cochlear amplifier. The exact manner in
which OHC motility contribute to characteristic cochlear behavior, or the cochlear amplifier mechanism, however, remains a mystery. The principle idea of our proposed cochlear
amplifier mechanism stems from the notion that the cochlear anatomy, especially the microanatomy of the cochlear partition, must somehow serve its functioning. Our observation
of tilts of OHCs and phalangeal processes has suggested that this unique arrangement of
sensory cells and supporting cells work in concert to transmit OHC motile forces onto
the basilar membrane, thus enhancing its motion. This delivery of active forces from one
cochlear position to another results in bidirectional coupling between basilar membrane
fibers, and thus active bidirectional coupling.
Specifically, in the active bidirectional coupling mechanism, OHCs’ motile forces are
delivered, through the basal and apical tilt of the OHCs and PhPs, respectively, to up- and
downstream BM fibers. ABC also includes the saturation of OHCs forces with BM displacement. Our 2D mathematical cochlear model produces large amplification, sharp tuning, high input-intensity compression near the characteristic place, and two-tone suppression, which are the characteristic cochlear responses, and compares favorably with physiological measurements. Our model analysis shows that ABC results in negative damping,
which accumulates energy to produce a large and localized peak. By its dependence on the
traveling wave’s wavelength, we also demonstrate that the bidirectional tilts function as a
spatial filter that selectively amplifies BM motion selectively—only when the wavelength
is appropriate (i.e., comparable to tilt distance).
Our model was not built to describe the details of the cochlear micro-mechanics; it is
92
only inspired by them. Our model captures the major anatomical features of the cochlear
partition needed to produce realistic cochlear responses. The model could be further extended to address other issues that have already been brought to attention of cochlear physiologists. For example, OHC’s tilt distance is less than that of the PhP, which can be easily
incorporated in the model. Further, OHC length increases from the base to the apex, which
can be modeled by a varying OHC motility factor. Also, the saturation profile of OHC
forces versus BM displacement is asymmetric, whereas the model uses a symmetric profile.
In summary, our mathematical cochlear model based on a novel cochlear amplifier
mechanism, ABC, emulates active cochlear behavior. With ABC, the cochlea is able to
realize nonlinear amplification by virtue of clever arrangement of its sensory and supporting
cells, rather than a complex arrangement of multiple resonant systems. Indeed, our model
provides a plausible physical basis, the microanatomy of the cochlear partition, for the
hypothesized cochlear amplifier that has not been described in such detail until now.
93
Chapter 4
The Active Cochlea: Analog VLSI
Implementation
In the previous chapter, we elaborated our efforts of exploring the cochlear amplifier mechanism through developing a mathematical cochlear model. We proposed a novel cochlear
amplifier mechanism based on the cochlear microarchitecture, namely, active bidirectional
coupling (ABC), to account for the active amplification in the cochlea. The resultant nonlinear active cochlear model yields characteristic responses comparable to physiological
measurements. In this chapter, we will proceed to morph the cochlear microarchitecture as
well as mechanics into a silicon very-large-scale-integration (VLSI) chip.
The chapter is organized as follows. First, we begin with discussing circuit design techniques, specifically continuous-time filter design, as background reference to our cochlear
circuit. We shall also discuss the effect of transistor mismatch in analog CMOS (Complementary metal-oxide semiconductor) circuits. Second, we synthesize a second-order section that models the dynamics and properties of the basilar membrane (BM) in the cochlea
94
and the interactions between the BM and the cochlear fluids. Third, we show the logdomain (current mode) implementation of our Class AB design. In addition, we describe
circuit design that emulates the cochlear fluid and circuitry that realizes the cochlear amplifier mechanism, ABC. Last, we present the architecture of our silicon cochlea.
4.1 Analog VLSI Approach
We employ analog VLSI circuit technique for developing microelectronic chips to mimic
the nervous system’s functions—these microchips are known as ”neuromorphic systems”.
Toward the goal of modeling the brain functions, this approach offers several advantages
over software simulation and/or digital hardware modeling. First of all, hardware implementation runs in real time while computer software simulation usually requires tremendous amount of computation time and large memory space.
Second, analog MOSFET (metal-oxide-semiconductor field-effect transistor) transistorbased circuits, rather than digital implementation, perform continuous-time computation
in which continuously varying electrical signals, voltages and/or currents, represent analog
properties of the biological system. This analog computation is especially suitable and beneficial for the task of modeling the sensory periphery of the nervous system because input to
the sensory models is usually real-world analog signals—light or sound, for example—so
that the need for analog-to-digital converters is eliminated.
Finally, operating transistors in subthreshold region leads to compact and low-power
designs which are desirable in both engineering and clinical applications. Two major factors lead to the features of small size and low power consumptions. On one hand, simple
exponential relationship between drain current and gate voltage of a MOS transistor work95
ing in weak inversion usually gives rise to simple designs, meaning smaller chip area. On
the other hand, low current level results in lower power consumption than their digital
counterparts. Specifically, we use current-mode low-pass filter (LPF) as the building block
of the cochlear chip design.
In this section, we shall first discuss the log-domain filter design technique that we
employed in our cochlear chip, then we will touch on an important issue that is closely
associated with subthreshold analog circuit design—effect of noise and variation of parameters on circuit and system performance.
4.1.1 Filter Design Techniques
Electronic filters are circuits that perform signal processing for specific purposes. From
the perspective of frequency-component processing of signals, there are five basic filter
types:low-pass, high-pass, bandpass, all pass and notch filters. Based on the way signals
are processed, filters fall into three different categories: Continuous-time filters process
analog signals in real time, and fully-digital filters perform filtering operation on digital
representation of samples of signals, often not in real time, while sampled data filters operate on samples themselves [108]. Digital filters can be reliable and insensitive to noise,
but require analog-digital converter and digital-analog converter when dealing with analog signals or when interfacing with real world; they also consume high power due to fast
switching. Switched capacitor filters and switched current filters are the two best known
sampled data filters; they use switches (usually transistor switch) together with capacitors
and active devices to provide filter functions [108]. Sampled-data filters need sampling and
switching, which may have problems in signal settling, clock feedthrough, and charge injection [109]. In our circuit design, we used continuous-time filter design because it meets
96
the requirement for low power and moderate dynamic range [110]. In other words, it serves
our goal to design a cochlear chip whose performance and efficiency are comparable to its
biological counterpart.
Aside from passive continuous time filters (classic RLC filter as an example) which are
still much in use but are unsuitable for implementation in the ubiquitous integrated circuit
[108], active continuous time filters have been extensively explored, in which active-RC
and active-C filter form major conventional continuous time filters. Opamp-RC is the major
form of active-RC filters that use opamp as active elements. Operational transconductance
amplifier-C (OTA-C, or gm-C) filters use only active element, OTA, and capacitors, making
them suitable for integration [108]. Two most popular forms of active-C filters are MOS-C
and OTA-C filters [109].
Most of the existing silicon cochlea employed gm-C filter design [6], [82], [111]. A
gm-C filter is suited for high-speed application due to the fact that it is configured in open
loop such that there is no stability constraint. Its major drawback is that with an OTA
used in open loop, it has a very small input level range, about 60 mV, to keep it relatively
linear [81].
Among all active filter design approaches are only a few of the circuit topologies well
suited to VLSI implementations, one of which, the state-variable filter is the most general
in form and most widely used in VLSI continuous-time filters [110]. The key advantage
of state-variable filters is that they require only two basic building blocks: integrators and
weighted summers [110]. By decoupling the poles of the system, a N-th order biquadratic
(biquad) filter structure is generally realized as a cascade of second-order circuits, followed
by a first-order circuit, if N is odd. The biquad approach is widely used for its simplicity,
ease of design, and ease of debugging.
97
Log-domain Filters
As a novel form of current-mode active continuous-time filter, Log-domain filters have
been recently proposed and developed and have demonstrated the potential to accomplish
both high-frequency and low distortion levels [112]. A Log-domain filter directly utilizes
the inherent exponential I-V relationship of a bipolar transistor or a MOSFET transistor operated in subthreshold. It realizes a linear filter from outside although the internal filtering
done in Log-domain is nonlinear. Log-domain filters using MOS transistors has become
increasingly attractive in current-mode design due to its zero gate-leakage current and low
power consumption. As there is no need to include linearization in a Log-domain filter,
unlike in a gm-C filter, the topology is usually quite simple, resulting in fast operation.
The Log-domain technique shares certain similarities with the classical translinear principle, and can be viewed as a dynamic extension to this principle [109]. Logarithmic compression in log-domain followed by exponential expansion is referred to as companding
(compress-expand) technique. Log-domain filters achieve the advantageous potential of
companding signal processing, which mainly includes larger dynamic range than that of
a linear signal processing circuit. The compression raises weak signals further above the
noise floor while shrinking strong ones away from the distortion-prone level; the expansion
restores the signal’s original dynamic range by attenuating the weak signals and amplifying
the large signals [109].
Current versus Voltage Mode
Traditional voltage-mode circuits have faced difficulties in meeting high-speed, highly
linear operation towards low-power and low-voltage designs [113]. Current-mode circuits
excel in their high-speed response, generally low power supply requirements, and low sen-
98
sitivity to various parasitics. Design of current-mode filters requires current-mode building
blocks, rather than voltage-mode ones. Recent advances in analog integrated circuits and
signal processing have shown that the current-mode approach is superior to the voltagemode in terms of its wide bandwidth, high speed, low voltage and power, large dynamic
range, and simplicity in circuit structure [108]. Current-mode operation offers large dynamic range if the nonlinear device transconductance is compensated for in the filter design,
such that the operation remains linear outside the small-signal region 1 [4].
Class AB versus A Operation
Classification principle in analog electronic amplifiers has also been introduced in classifying analog filter designs. Among Class A, B, AB and C, Class A and AB are the two
mostly often encountered circumstances. In a Class A filter design, the input signal drives
the output signal throughout the entire input wave cycle, while in a Class B design, only
half cycle of the input drives the output. The “off” status of the device in Class B design
yield high power efficiency; however, it suffers from a drawback that there is some distortion caused by switching between “on ” and “off” status—called crossover distortion.
Employment of a complementary pair or “push-pull” arrangement of Class B design, but
with a slight “on” bias (a little above zero) for the device when it is not in use, results in
Class AB design.
Compared to Class A operation, Class AB implementation offers the advantages of
increased dynamic range, reduced effect of transistor mismatch, and lowered power consumption. This is mainly due to the fact that a Class AB design only maintains a relatively
low baseline level (current/voltage), thus suppressing this major source of variation and
additional power consumption.
1
For small time-varying signals carried over a constant bias, a nonlinear device or system can be considered as linear as an approximation.
99
The Class AB design not only inherits the power efficiency of Class B design, but also
gets rid of the crossover distortion problem by means of the non-zero baseline. In a Class
AB design, there are two differential signal paths in which signals are always kept positive
relative to the low baseline signal. The final full-wave output of a Class AB design can
be obtained through combining the output signals in the two filtering paths. A commonmode constraint is required to determine the differential output signals together with the
differential mode. This configuration eliminates the need for a large baseline (a DC current
in current-mode design), thus also decreasing power consumption and moderating the effect
of transistor mismatch introduced by matching of the baseline which is a requirement in a
Class A design.
According to the noise behavior analysis for Log-domain filters in both class A and
class AB operation, the dynamic range of class AB companding circuits can be substantially extended compared to that obtained with class A circuits without increasing the maximum signal-to-noise ratio and therefore without increasing the power consumption [114].
However, the requirement of an additional class AB conditioner for the input signal (e.g.,
on-off circuit [4] or a current splitter) might result in a higher noise floor, which, in turn,
will somewhat reduce the extension of the dynamics range. Despite the drawback, Class
AB design is favorably chosen in our cochlear chip design because it meets our goal of
reduced variations between circuit components as well as the low power consumption.
4.1.2 Mismatch in CMOS Circuits
Device variations can be categorized into systematic and random [115]. Differential circuit topology and proper biasing techniques can make the integrated circuits performance
largely insensitive to these systematic variations. Device to device variation is refereed
100
to as device mismatch, which is random. In subthreshold operation, variation in threshold
voltage is the major source of channel current difference for a matched transistor pair. It has
a normal distribution with zero mean and a variance dependent on the device area (channel
width W times channel length L) [115]. This simply suggests that increasing transistor size
will decrease the extent in threshold voltage matching, thus improving device matching and
circuit performance.
Several approaches can be taken to reduce the effect of transistor mismatch. First,
proper layout technique can be employed, such as symmetry and common centroid technique, to obtain better matching between transistors that are supposed to match. Second,
transistor size can be increased such that area variation becomes smaller. However, this
size increase is always related to increase in parasitic capacitances, and the extent of area
increase is also limited by the fabrication cost. When taking the approach of increasing
transistor size in our design, we first investigated though SPICE (simulation program with
integrated circuit emphasis)
2
simulation of transistor-level circuits to find the transistors
whose size change leads to most significant change in output signals, and then we only
increase the size of transistors that fall into this category, rather than using large-area transistors everywhere blindly, in order to achieve cost-effective design.
Imperfection in circuit performance can also be caused by the Early effect
3
of MOS
transistors. This occurs mostly with current mirrors, the most frequently used circuit primitive. In order to achieve better matching between the original current and the copied one in
a current mirror, an implementation improvement can be made by connecting one transistor
2
SPICE is a general purpose analog circuit simulator. It is a powerful program that is used in integrated
circuit and board level design to check the integrity of circuit designs and to predict circuit behavior.
3
Early effect, also the channel length modulation effect, describes the phenomenon that the drain current
increases slightly with drain voltage for a saturated MOSFET. Early voltage is defined to be the drain-tosource voltage (negative) at which the drain current is zero. In an ideal MOSFET, the Early voltage would be
infinite.
101
in series on both sides of the mirror and using diode connection on the source-current side.
This results in a “complex current mirror”. The insertion of these two additional transistors
brings the source-to-drain voltage closer between both sides, thus reducing the effect of
Early voltage. Our SPICE simulations show that if a complex mirror accompanied by increasing the channel area of the transistors that are connected to power supply rails would
make the gain of the mirror closer to 1 while decreasing the standard deviation of the gain
distribution to a large extent.
Class AB design could suffer from mismatch between the differential paths introduced
by device mismatch. An ideal differential-mode circuit does not respond to common-mode
signal (i.e., a signal that is present on both paths) and only responds to differential signals.
That is, an ideal Class AB design has zero common-mode gain and large differential-mode
gain. The ratio between the differential-mode signal gain and the common-mode gain,
called common-mode rejection ratio (CMRR), should be equal to infinity in the ideal case.
However, existence of device mismatch will cause the CMRR to decrease as it increases the
common-mode gain. To address this problem, proper and careful layout technique should
be employed to reduce the effect of mismatch between the two differential paths to make
the CMRR as large as possible.
4.2 Cochlear Circuit Synthesis
We shall synthesize the cochlear circuit in this section which will be the circuit analog
of our mathematical cochlear model, thereby the biological cochlea. We synthesize a circuit model of a 2D active cochlea, which comprises the cochlear fluid design, the basilar membrane (BM) design, and implementation of our hypothesized cochlear amplifier
102
mechanism—active bidirectional coupling (ABC). We first start by synthesizing a passive
cochlea, which mainly includes the design of the BM boundary condition. The synthesis
result was a second-order section that is formed by two first-order LPFs, and the interaction between the input and output signals of the BM. We then synthesize the BM boundary
condition in the active case by adding ABC, achieving an active cochlear circuit model.
4.2.1 Cochlear Fluid Design
VLSI implementation of a passive cochlear model consists of two circuit elements: the
cochlear fluid and the BM. In this section, we present the design of a cochlear fluid element; connecting a number of such elements will form the fluid network that emulates the
cochlear fluid body. In other words, we will implement a discrete fluid network, at a large
scale (i.e., small discretization step), as the approximate analogy of the continuous cochlear
fluid.
Modeling the two dimensions of the cochlear fluid chamber offers advantages over a
one-dimensional model in which the height of the fluid chamber is not considered. A 2D
fluid model allows for fluid motion in the vertical direction, and is thus capable of capturing
more biologically compatible responses. Our mathematical modeling investigation on the
choice of a 1D or 2D model also revealed that a 2D model generates more realistic cochlear
behavior than a 1D model, in terms of the amplitude and the phase of the BM vibration.
Therefore, we implemented a 2D cochlea model in analog VLSI, also taking advantage of
the 2D planar nature of a silicon die.
The first silicon cochlea to implement a 2D cochlear fluid was constructed by Watts
[6]. Following Watts’s approach, we treat the cochlear fluid as a resistive sheet [6]. Our
103
implementation is different from his though as we employ current-mode design, rather than
voltage mode.
In current mode, we achieve a simpler implementation of the fluid element, described
as follows. In discrete form, the fluids can be viewed as a grid of elements with certain
resistance that corresponds to the fluid density or mass. As we know from Chapter 3, the
fluid velocity potential, φ, satisfies the Laplace’s equation assuming the incompressibility
of the inviscid fluids. Since electrical charge is conserved for a small resistance sheet while
flux is conserved for a small fluid volume, current variables shall be used to represent the
velocity of the fluid, and also of the BM.
Following the approach in [116], we implement the cochlear fluid network using a diffusor network formed by a 2D grid of nMOS transistors. A MOS transistor in subthreshold
ohmic region acts as a diffusive element as the source-to-drain current flowing through the
channel is caused by the diffusion of charge carriers. Thus, such a MOS transistor is called
a diffusor and a network formed by diffusors is called a diffusive network.
We now show in detail the operation of a diffusor. We rewrite the drain current, Ids ,
of a nMOS transistor as follows (voltages are in the unit of uT = kT /q, where k is the
Boltzmann constant, T is the absolute temperature (in Kelvins), and q the magnitude of
the electrical charge (in coulombs) on the electron. At room temperature ( T = 300K), the
value of uT is approximately 26 mV):
Ids = I0eκVg (e−Vs − e−Vd )
= D(Qs − Qd ),
(4.1)
where Vg , Vs , and Vd are the gate, source, and drain voltages, respectively, and I0 is the
104
Vin0
A
R
Id
Vin
Iin0
Vin0
B
Vg
Vin
Id
Iin
Figure 4.1: Fluid Diffusive Element Design
Analogy between resistive element and its nMOS implementation. A A resistor with its two terminal voltages
denoted by Vin0 and Vin . B An nMOS transistor acts as a diffusor with its gate voltage denoted by Vg .
The drain and source voltage of the nMOS is also Vin0 and Vin , respectively, and the reciprocals of their
exponentials are lineally related to the currents Iin0 and Iin , respectively. An analogy can be drew between
the resistance of a resistor and the gate voltage of a MOS transistor. In other words, the resistance can be
simulated by the diffusive coefficient of a MOS transistor working in subthreshold ohmic regime.
leakage current at zero gate voltage. D = eκVg denotes diffusion strength of charge carriers
(electrons in the case of nMOS transistors) while Qs = I0 e−Vs and Qd = I0 e−Vd represent
their concentrations at the source and the drain, respectively.
We derived the relation between the resistance of a resistor and the diffusive coefficient
of a MOS transistor by drawing an analogy between a resistor and a nMOS transistor in
subthreshold ohmic regime (Figure 4.1). For the resistor R (in Figure 4.1A), the current Id
was calculated as
Id =
Vin0 − Vin
.
R
(4.2)
For the pMOS transistor that is gated by Vin0 (in Figure 4.1B) with assumed small drain
voltage (4uT smaller than Vdd ), its drain current Iin0 can be described as I0e−κVin0 eVdd ;
similarly, Iin is equal to I0 e−κVin eVdd . By assuming κ ≈ 1, we obtain the following as an
105
approximation:
Iin0 = I0e−Vin0 eVdd ,
Iin = I0e−Vin eVdd .
(4.3)
And for the nMOS transistor in Figure 4.1B, according to Equation 4.1, the channel current
Id can be represented by D(Qin − Qin0) where Qin = I0e−Vin and Qin0 = I0e−Vin0 . Thus,
Equation 4.3 becomes
Iin0 = Qin0 eVdd ,
Iin = Qin eVdd .
(4.4)
Therefore, for the diffusive element in Figure 4.1B, we have
Id = D(Qin − Qin0)
= De−Vdd (Iin − Iin0)
=
Iin − Iin0
eVdd /D
(4.5)
By comparing Equation 4.5 with Equation 4.2, we obtained the expression as follows
that describes the relation between the magitude of the fluid-element resistance and the
circuit variables:
R ≡ eVdd /D
= eVdd e−κVg
(4.6)
(4.7)
Therefore, the resistance due to the fluid density was implemented by controlling the gate
106
voltage Vg of an nMOS transistor thus its difference from the power supply Vdd . Increasing
Vg thus decreasing the difference results in larger channel current, thus corresponding to
smaller resistance, and vice versa.
4.2.2 Synthesis and Analysis of a Single BM Stage
We shall now synthesize the circuit design for realizing the BM boundary condition in the
log domain (current mode). This BM circuit will interact with the diffusor fluid network
that we have discussed in the previous section.
For reference, we repeated the boundary condition of the BM in the passive case as
2ρφ̇ = S(x)δ(x) + β(x)δ̇(x) + M(x)δ̈(x),
where M(x) is the mass, S(x) is the stiffness, and β(x) = ζ
at y = h,
(4.8)
p
S(x)M(x) is the damping
per unit area of the BM, and ζ is the damping ratio; δ is the downward displacement of
the BM while the first time-derivative of δ is its velocity and the second is its acceleration.
φ(x) is the velocity potential at the BM boundary where y = 0, ρ is the fluid density, and
the factor of 2 accounts for fluid mass in both top and bottom cochlear ducts. For this
second-order system, the input is the fluid velocity potential, φ, which is the driving force,
and the output is the BM velocity, δ̇.
The design goal for the passive case is to determine a relation between the input and the
output in terms of circuit variables (electrical signals: current in our current-mode design)
that implements the BM boundary condition as shown in Equation 4.8. Once we obtain the
BM circuit design equation, we shall employ a number of BM circuits to model a number
of BM transverse fibers, and make the BM circuits interact with the fluid diffusive network,
107
mimicking the biological cochlea.
We employed current to model the velocity as in our fluid element design. We used
the current flowing through the BM impedance, Imem , to represent the BM velocity δ̇ in the
vertical direction. Thus, the BM displacement δ is represented by the integral of the current
Imem while the BM acceleration δ̈ is represented by the first time-derivative of Imem . We
also use the quantity Iin to represent the velocity potential ρφ.
Substituting Imem and Iin in Equation 4.8 yields
I˙in = S(x)
Z
˙ .
Imem + β(x)Imem + M(x)Imem
(4.9)
Taking the time derivative of Equation 4.9 in order to remove the integration operation,
we obtain
˙ + M(x)I¨mem .
I¨in = S(x)Imem + β(x)Imem
(4.10)
This equation thus becomes the target of our circuit synthesis. Synthesis of the above
equation means decomposing the second-order section into two first-order sections and
possible another equation that describes the relation between the input, the output, and
intermediate variables.
We tackled the synthesis task for the passive case starting with rewriting the design
target equation. Since the passive model is a linear system, we transformed the design
equations in the time domain to the frequency domain to get rid of time-derivative expressions. Using s = jω, Equation 4.10 becomes
Iins2 = S(x)Imem + β(x)Imems + M(x)Imem s2 ,
108
(4.11)
which is a new version of our target design equation.
Our synthesis process was a variation of the state-space synthesis approach proposed by
Frey [112]. The general procedures of the approach are firstly to determine a state-space
representation of the system, secondly to explore the equilibrium condition and obtain a
transformation that yields appropriate operating condition, and finally to derive a set of
nodal equations for capacitors, which suggests an interconnection of transistors, thus the
transistor-level implementation of the design.
We developed a Mathematic code to facilitate the synthesis process, which mainly involves determining the coefficients of the state variables, input, and output current signals.
The choice of coefficients is based on whether the chosen set of coefficients could lead to
realization of the target design equation and which realization is the most economical solution, meaning the simplest implementation and smallest chip area among other possible
options.
We found that a few solutions to the synthesis equations were possible. For example, the
two first-order sections could be chosen to be both integrators, both LPFs, or one integrator
and one LPF. Adding gain factors for the state variables as well as input (Iin) and the output
(Imem ) introduces additional flexibility. We need to determine what choice yields the best
functionality, flexibility, and implementation cost.
We shall describe both the synthesis result involving two integrators and the result with
two LPFs. Through analyzing both cases, we shall explain why we reached our decision,
based on considerations of DC operating point and stability.
Implementing the second-order system of the BM requires two state-space variables,
which were represented by currents Is and Io . Synthesis result of a second-order system
109
that consists of two integrators is described as follows, with s = jω.
τ1 Iss = Imem
τ2 Ios = Imem + Is
Imem = Iin − Io
(4.12)
Gain factors can be easily added to the results above thus adding more flexibility to the
design. We shall describe two of the possible variations as follows. We added gain factors
a and b at different positions, which result in
τ1 Is s = Imem
τ2 Ios = Imem + bIs
Imem = a(Iin − Io)
(4.13)
and
τ1 Is s = Imem
τ2 Ios = Imem + bIs
Imem = Iin − aIo,
(4.14)
For the design described in Equation 4.12, Equation 4.13, and Equation 4.14, their DC
operating point were found as follows.
Imem = 0,
Is = 0, and Io = Iin.
110
(4.15)
We also synthesized this second-order system using two interacting LPFs, and the design equations are described as:
τ1 Iss + Is = −Iin + Io ,
τ2 Ios + Io = Iin − bIs ,
(4.16)
Imem = Iin + Is − Io,
The two first-order systems are both LPFs, with time constants τ1 and τ2 , respectively; b in
the second LPF is a gain factor, which is necessary for realizing reasonable BM stiffness
given practical choice of τ1, τ2 and capacitance (capacitor size is limited by silicon area).
This design realizes the passive cochlea model, as was described by Equation 4.11.
This design uses a loop of two LPFs connected with negative feedback to realize the poles.
The zeros of the second-order system are achieved by creating an output signal, Imem , that
is the weighted summation of the two LPFs’ outputs, Is , and Io , as well as the input signal,
Iin. The design’s DC operating point, shown below, was somewhat different from that of
the integrator-based design we just described above.
Imem = Is ,
1
Is = Iin, and Io = Iin.
b
(4.17)
4.2.3 Analysis of the Design
We performed some analysis on both the integrator-based design and the LPF-based design
that we presented in the previous section. This analysis associates the circuit variables
and their expressions with physiological parameters in the biological cochlea. We also
obtained the transfer functions of a single BM section, when stages are combined to form a
111
1D network (single cochlear stage with loading), and when stages are combined and ABC
is included. Analysis and result comparison revealed that the output of a combined BMfluid network cannot be simply derived from single-stage responses due to the effect of
network loading. We will also present the effect of ABC on the transfer functions.
Circuit Parameters vs. Physical Properties
Given Equation 4.16 in the LPF-based design, Iin, Is, and Io can be expressed in terms
of the output current Imem as follows.
(b + 1) + (τ1 + τ2 )s + (τ1 τ2 )s2
Imem ,
τ1 τ2 s2
1
= − Imem ,
τ1s
(b + 1) + τ1 s
=
Imem .
τ1 τ2 s2
Iin =
Is
Io
(4.18)
By comparing the expression of Iin with the design target Equation 4.8, we obtain the
circuit counterparts of the BM ’s stiffness S(x), damping β(x), and mass M(x):
S(x) =
b+1
,
τ1 τ2
β(x) =
τ1 + τ2
,
τ1 τ2
M (x) = 1.
(4.19)
As we can see, three circuit parameters, b, τ1 , and τ2 , were used to model the BM’s physical
properties. This configuration was the same as in the mathematical model simulation,
in which the BM mass was a constant. Based on these relations, it is required that τ1
and τ2 increase exponentially from the stapes along the cochlea in order to simulate the
exponentially decreasing BM stiffness and damping.
Furthermore, we can derive the quality factor of the second order section based on the
112
above relationships.
p
S(x)M(x)
Q =
β(x)
r
b + 1 τ1 τ2
=
τ1 τ2 τ1 + τ2
√
b+1
p
= p
.
τ1 /τ2 + τ2 /τ1
(4.20)
Therefore, Q was electrically tunable and it varied from stage to stage as τ1 and τ2 varied
exponentially along the cochlear position. If τ1 = τ2 , Equation 4.21 reduced to:
√
b+1
,
2
Q=
(4.21)
which implements a constant-Q along the cochlea if the gain factor b is implemented as a
constant.
Similarly, the integrator-based design (Equation 4.12) yields the relationship between
Iin and Imem as follows.
Iin =
1 + τ1 s + τ1 τ2 s2
Imem ,
τ1 τ2 s2
(4.22)
based on which, we obtain the circuit analogs as follows,
S=
1
,
τ1 τ2
β=
and the quality factor as follows,
Q=
1
, and M = 1
τ2
r
τ2
.
τ1
(4.23)
(4.24)
Its two variations have the following circuit analogs, respectively:
S=
ab
,
τ1 τ2
β=
a
, and M = 1
τ2
113
(4.25)
and
S=
b
,
τ1 τ2
β=
1
1
, and M =
τ2
a
(4.26)
And the quality factors are the same,
Q=
r
bτ2
.
aτ1
(4.27)
For the BM design, we chose the design with two low-pass filters (LPFs), rather than
two integrators or one LPF and one integrator (synthesis results not shown). The main reason was that through circuit simulations, we found that the response stability of a secondorder system with internal interaction between its two first-order systems was less sensitive
to parameters (e.g., time constants, input amplitude, etc.) when the two comprising firstorder systems were both LPFs instead of integrators. In addition, in terms of realizing
a relatively large quality factor, an LPF-based design does not require the two time constants, τ1 and τ2 , to have a large ratio to achieve certain large quality factor, as required in
the integrator-based design.
Transfer Functions
Investigating the characteristics of a single BM stage, especially when it is embedded
in the network that a cochlear circuit requires, was instructive for understanding the response of the whole 2D cochlea. We calculated the transfer function of a BM-fluid stage,
in an isolated condition and in the context of a network (with interaction between previous
and following BM sections and fluid diffusive elements), and compared them in order to
develop insight into the effect of network loading on a BM section.
In a single BM-fluid stage, the BM was viewed as a finite impedance, zm , to the driving
114
Iin0
Vin0
Vfd
Fluid
R
BM
Vin
zm
Iin
Imem
Figure 4.2: Conceptual Electrical Model of a Single Basilar Membrane-Fluid Stage
A single basilar membrane (BM)-fluid stage includes a fluid diffusive element realized by an nMOS transistor,
and an impedance zm , representing the BM circuit. Iin0 and Iin represent the velocity potential at the two
terminals of the fluid element while the corresponding voltages, Vin0 and Vin , represent the logarithm of the
velocity potentials, respectively. The fluid diffusive element, an nMOS transistor, is gated by the voltage Vfd ,
which determines the resistance given a supply voltage. The current flowing through the BM impedance is
denoted by Imem , emulating the BM’s velocity in the vertical direction.
115
force of the fluid (Figure 4.2). Iin0 and Iin represent the velocity potential of the two fluid
terminals. The impedance zm can be calculated as follows, based on the BM design we
have described in the previous section.
Iin
Imem
(b + 1) + (τ1 + τ2 )s + (τ1τ2 )s2
=
.
τ1τ2 s2
zm =
(4.28)
In order to gain understanding about the behavior of a single BM subcircuit, we explored two transfer functions, the same approach as in [6] except that we used currents as
variables rather than voltages. In addition, the present BM design was different from that
work.
One transfer function, Gsgl (the subscript “sgl” represents the single isolated stage case),
defined as the relation between the resultant current through the BM impedance, Imem , and
the driving force on the fluid diffusive element, Iin0, can be expressed as:
Imem
Iin0
1
=
R + zm
Gsgl (s) =
=
τ1 τ2 s2
,
(b + 1) + (τ1 + τ2 )s + (R + 1)(τ1 τ2 )s2
(4.29)
where R represented the fluid mass as we discussed in the previous section. The transfer
function Gsgl (s) had a complex-zero-pair and a complex-pole-pair with the pole-pair corresponding to a peak at certain frequency. At even higher frequencies, Gsgl (s) becomes a
constant.
Another transfer function Hsgl (s), defined as the relation between the currents at the
116
two terminals of a single fluid diffusive element, Iin and Iin0, is described as:
Iin
Iin0
zm
=
R + zm
(b + 1) + (τ1 + τ2 )s + (τ1 τ2)s2
=
(b + 1) + (τ1 + τ2 )s + (R + 1)(τ1 τ2)s2
1 + τ1 + τ2 s + τ1 τ2 s2
b+1
b+1
=
+
τ
τ
1
2
1+
s + (1 + R) τ1 τ2 s2
b+1
b+1
Hsgl (s) =
(4.30)
Representing Hsgl (s) in terms of functional parameters, τ (time constant), Q (quality factor,
a measure of sharpness of frequency responses), and η, yields:
1+
Hsgl (s) =
1+
where τ =
τ
s + τ 2s2
Q
,
τ
s + (1 + η)τ 2s2
Q
(4.31)
p
p
τ1τ2 /(b + 1), Q = (b + 1)τ1 τ2 /(τ1 + τ2 ), and η = R. Using the same
relationships, we rewrote the transfer function Gsgl (s) as:
Gsgl(s) =
ητ 2s2
.
τs
)
R(1 +
2 2
Qs + τ s (1 + η)
(4.32)
The roots of the denominator of Hsgl(s) determined its complex-pole pair:
sp± =
p
1
[−1 ± 1 − 4Q2 (1 + η)].
2τ Q(1 + η)
(4.33)
while the roots of the numerator set its complex-zero pair:
sz± =
p
1
[−1 ± 1 − 4Q2 ].
2τ Q
117
(4.34)
As plotted in [6], the poles and zeros were close to the imaginary axis, which suggested a
pronounced peak and a pronounced valley in the magnitude of the transfer function Hsgl (s).
Since η is determined by the resistance R, which realization was described in Equation 4.7,
we expected that η > 1. As pointed out in [6], due to the positive value of η, the poles were
always closer to the real axis in the complex plane than were the zeros, so the peak in the
magnitude of the transfer function always occured at a lower frequency than did the valley.
As we can see from Equation 4.32, for the second-order BM circuit, its amplitude (or
gain) and phase responses was determined by the quality factor Q, when η was a constant
throughout the cochlea, and τ was chosen for a certain cochlear position. Increasing Q will
result in increased response gain.
Single-Stage Transfer Functions in a Loaded Network
Transfer functions of an isolated single BM-fluid section cannot represent its property
when it was loaded by a number of other sections in a network [6, 117]. Therefore, it was
beneficial to obtain the transfer functions when embedded in the network of BM sections
and fluid elements. Toward this end, the straightforward approach was to obtain an closedform expression of the transfer functions. It is, however, extremely difficult to achieve that
goal due to the fact that in a BM-fluid network, BM sections have exponentially increasing
time constants along the cochlear position, resulting in a non-uniform network. Therefore,
we combined semi-analytical solution and numerical simulation to obtain the frequency
response of the two transfer functions, and we checked the effect of loading on the single
BM-section function by comparing the transfer functions in the two cases.
We derived the semi-analytical solution to compare a single stage transfer function and
the combined-network stage transfer function, based on the single-stage shown in Fig-
118
Section i-1
Section i
Vfd
Iin(i-1)
Fluid
Vfd
R
BM
Section i+1
Iin(i)
Fluid
R
Iin(i+1)
BM zm(i+1)
zm(i)
Imem(i)
Imem(i+1)
Figure 4.3: Basilar Membrane-Fluid Interaction With Loading
A array of single basilar membrane (BM)-fluid forms a network. Transfer functions are altered by the network
loading.
ure 4.2 and the network setup shown in Figure 4.3.
According to the Kirchhoff’s Current Law (KCL), we have
Iin(i − 1) − Iin(i)
Iin(i) − Iin(i + 1)
= Imem (i) +
R
R
Iin(i − 1) − Iin(i)
Iin(i) Iin(i) − Iin(i + 1)
=⇒
=
+
.
R
zm (i)
R
(4.35)
This leads to
Hload(i) =
=
Iin(i)
Iin(i − 1)
zm (i)
.
R + (2 − Hload (i + 1)) zm (i)
(4.36)
where the subscript “load” denotes the loaded case. From Equation 4.36, we see further
119
that it is difficult to obtain a closed-form solution for the two transfer function because
zm (Equation 4.28) changes with cochlear position and it has a complex form. Therefore,
Equation 4.36 was solved numerically to obtain solution to Hload(i).
In seeking solutions for transfer functions at each cochlear position, we represented zm
and R in terms of biophysical parameters. The BM impedance zm was represented as:
zm =
Iin
Imem
=
S(x) + iωβ(x) + (iω)2 m(x)
,
(iω)2
(4.37)
where ω is the input signal’s angular frequency, and x is the distance from the stapes in the
cochlear model. As we can see, aside of its dependence on input frequency, zm varied along
the cochlear location x with varying stiffness S(x), damping β(x) and mass m(x). In our
case, the mass was constant across the cochlea. Also, the resistance of a small amount of
fluid was represented as
R = 2ρ(∆x)2 /H,
(4.38)
where ρ is the fluid density, H is the cochlear duct height, and ∆x is the discretization size
along the longitudinal axis. Using Equation 4.37 and Equation 4.38, the transfer functions
Hsgl (s) and Gsgl (s) were obtained easily in an isolated case based on the expressions in
Equation 4.31 and Equation 4.32.
Finding the transfer functions in the loaded network case was more complex. First
it was necessary to solve for the transfer functions at the last position (at the apex) of
the discretized cochlea, and then use the solution to solve for the transfer function at the
previous section, based on the relation described in Equation 4.36. Repeating the procedure
yielded the transfer functions at all cochlear positions. The parameter values used were the
same as those listed in Chapter 3.
120
Comparing the numerical simulation results in the loaded network with the isolated
condition revealed the effect of loading to the transfer functions of BM-fluid section. In
contrast to the responses of a single BM-fluid stage, the peak was reduced when stages were
combined (Figure 4.5A). Furthermore, the amplitude of Hload(i)’s frequency response did
not exhibit a pronounced peak before the valley in the loaded network (Figure 4.4A). The
result indicated that interactions between BM stages through the diffusive elements had a
destructive impact on the gain of BM responses. This destructive interference was very
likely due to the large phase difference between BM sections, with close-to-π phase difference causing cancelation of response amplitude. This destructive-interference phenomena
was evident with responses described in [7].
Adding ABC in the Loaded Network
It was instructive to take the loaded-network simulation further by adding the cochlear
amplifier mechanism into the system, and then investigating the effect of the active bidirectional coupling. Meanwhile, this task was formidable if the circuit model in Figure 4.3 was
used because the realization of ABC requires using the comprising lower-level variables
of the BM circuit to represent the BM impedance zm . Therefore, it was quite difficult to
formulate the transfer function as described in Equation 4.36. An equivalent approach we
took was to simulate our 1D mathematical model and obtain the value of transfer functions
defined by their definition.
When ABC was included in the simulation, the amplitude response of Hload (s) regained
a pronounced peak and a relatively large quality factor (Figure 4.6A and Figure 4.7A). This
suggested an important notion that the bidirectional coupling between BM sections due to
ABC counteracts the destructive interference among BM sections due to their coupling
through fluid. Therefore, ABC played a critical role in achieving desirable gain and quality
121
HHf L Amplitude HdBL
5
0
-5
0.5
1
5
10
Frequency HkHzL
0.5
1
5
10
Frequency HkHzL
A
HHf L Phase HΠ radsL
0
-0.5
B
Figure 4.4: Transfer Function H(f ) of a Single Basilar Membrane-Fluid Stage and in a
Loaded Network
Frequency responses of transfer function H(f) of a single basilar membrane (BM)-fluid stage (Hsgl (f)) and
in a loaded network (Hload(f)). Top Amplitude; Bottom Phase; Green Single BM-fluid stage; Red Loaded
network.
122
GHf L Amplitude HdBL
0
-20
-40
0.5
1
A
5
10
Frequency HkHzL
GHf L Phase HΠ radsL
1
0.5
0
0.5
B
1
5
10
Frequency HkHzL
Figure 4.5: Transfer Function G(f ) of a Single Basilar Membrane-Fluid Stage and in a
Loaded Network
Frequency responses of transfer function G(f) of a single basilar membrane (BM)-fluid stage (Gsgl(f)) and
in a loaded network (Gload(f)). Top Amplitude; Bottom Phase; Green Single BM-fluid stage; Red Loaded
network.
123
factor along the cochlea.
4.2.4 Adding Active Bidirectional Coupling
We have described our circuit design for the BM and fluid circuits and we have also investigated the characteristics of a single BM in isolated and loaded conditions. We were also
able to compute the transfer functions of the BM-fluid network with added cochlear amplifier mechanism ABC through solving the mathematical circuit equivalent We shall now
present the circuit design when including the active bidirectional coupling OHC forces in
the cochlea design (i.e., to include the FOHC term).
For reference, we rewrote the BM boundary condition with ABC as follows.
2ρφ̇ = S(x)δ(x) + β(x)δ̇(x) + M(x)δ̈(x) − FOHC ,
FOHC = αγS(x)T δ(x − d) − αS(x)T δ(x + d) ,
(4.39)
where T denotes saturation, and δ(x − d) and δ(x + d) represented the displacement of
adjacent upstream and downstream BM segments, respectively. Note that we took the
terms rfbS(x) and rff S(x) outside of the saturation operation T as an approximation and
for simplicity.
Let the gain factor rff = αγ denote the feedforward OHC motility factor and the gain
factor rfb = α denote the feedbackward OHC motility factor. Combining the two equations
124
HHf L Amplitude HdBL
0
-15
1
2
10
Frequency HkHzL
1
2
10
Frequency HkHzL
A
HHf L Phase HΠ radsL
0
B
-0.5
Figure 4.6: Transfer Function H(f ) with Loading in the Passive and an Active Case
Frequency responses of transfer function Hload(f). Top Amplitude; Bottom Phase; Red Passive case; Blue
Active case. The OHC motility factor used is 0.16 while the forward-to-backward ratio is 0.3 for the simulation.
125
GHf L Amplitude HdBL
0
-40
1
2
10
Frequency HkHzL
1
2
10
Frequency HkHzL
A
GHf L Phase HΠ radsL
0.5
B
0
-0.5
Figure 4.7: Transfer Function G(f ) with Loading in the Passive and an Active Case.
Frequency responses of transfer function Gload(f). Top Amplitude; Bottom Phase; Red Passive case; Blue
Active case. The OHC motility factor used is 0.16 while the forward-to-backward ratio is 0.3 for the simulation.
126
above and taking the first time-derivative yields:
˙
2ρφ̈ = S(x)δ̇(x) + β(x)δ̈(x) + M(x)δ̈(x)
+rfbS(x)Ṫ δ(x + d) − rff S(x)Ṫ δ(x − d) .
(4.40)
As in the passive design, we used Iin to model the quantity 2ρφ and Imem to represent
the BM velocity δ̇. Also, working in the frequency domain (Laplace domain) with s = jω
gave us:
Iins2 = S(x)Imem + β(x)Imems + M(x)Imem s2
Imem (x + d) Imem (x − d) s − rff S(x)T
s,
+rfbS(x)T
s
s
(4.41)
We based the synthesis of the active model, implementation of FOHC , on the outcome of
the passive design. By observing that in the passive design Equation 4.18, the state variable
Is was expressed in terms of Imem as
Is = −
Imem
.
s τ1
(4.42)
We then obtained expressions for Imem (x − d), received from the upstream BM segment,
and for Imem (x + d), received from the downstream BM segment, as follows.
Imem (x − d)
= −τ1f Isf ,
s
and
Imem (x + d)
= −τ1bIsb ,
s
(4.43)
where τ1f and τ1b represent the time constant, Isf and Isb the output currents, of the first
LPF at the upstream and downstream BM segment, respectively.
127
To reduce complexity in implementation, we used τ1 to approximate both τ1f and τ1b as
they have similar values due to the small longitudinal span between neighboring sections.
Recall that in the passive system, the BM stiffness S(x) is represented by τ1 , τ2 and the
gain b, S(x) = (b + 1)/(τ1 τ2). Thus, the FOHC term in Equation 4.41 was transformed to
the following:
FOHC = rfbS(x)T (−τ1Isb)s − rff S(x)T (−τ1Isf )s
= rfbS0 (x)T (−Isb)s − rff S0 (x)T (−Isf )s,
(4.44)
where S0 (x) = S(x)τ1 = (b + 1)/τ2 . Note that τ1 was taken out of T in order to simplify
the implementation.
Therefore, we have obtained the target equation for the active system synthesis:
Iins2 =
S(x)Imem + β(x)Imems + M(x)Imem s2
+rfbS0 (x)T (−Isb)s − rff S0 (x)T (−Isf )s.
(4.45)
Following similar synthesis procedures to that used in the passive design, we obtained
the synthesis result for an active design, which implemented the BM boundary condition
with active coupling, as follows.
τ1 Iss = −Iin + Io − Is,
τ2 Ios = Iin − bIs − Io + rfb (b + 1)T (−Isb) − rff (b + 1)T (−Isf ),
Imem = Iin + Is − Io .
(4.46)
Note that to include implementation of the cochlear amplifier mechanism ABC, we only
128
needed to add two current quantities into the input of the second LPF in each BM circuit
design. The two current signals were taken from the two adjacent neighbors of each BM
circuit, respectively. Specifically, Isf and Isb denoted the output current signals of the first
LPF, Is, in a BM circuit, fed forward and backward from its upstream (basalward) and
downstream (apicalward) BM circuits.
4.3 A Log-Domain Class AB Low-Pass Filter
4.3.1 Nodal Equations
We have synthesized the design for the cochlear fluid and the second-order BM section. We
used a single nMOS transistor as a physical model of a small amount of cochlear fluid. Next
we shall discuss the implementation of the BM design. We chose to realize the BM circuit
differentially. This results in Class AB operation which increases dynamic range, reduces
the effect of transistor mismatch, and lowers power consumption. Differential operation
also agreed with the way the biological cochlea operates—the vibration of BM is driven by
the pressure difference across it.
In a differential design, each signal was represented by the difference between its positive component and negative component. In addition to the constraint applied to differential
signals, the operation of a Class AB design also required additional common-mode constraint. Differential- and common-mode conditions combined together and determined the
two components of the output signal.
Following a bottom-up strategy, we will present the design of a differential LPF in detail
129
as a building block based on which we will develop the BM circuit. A current-mode LPF
can be described in the frequency domain as:
τ Iouts + Iout = Iin
(4.47)
where s = jω and τ is the time constant. Iin and Iout are the input and output current,
respectively. We wrote the differential-mode equation of the LPF as follows by representing
each signal using the difference between its two components (i.e., positive and negative).
+
−
+
−
+
−
τ (Iout
− Iout
)s + (Iout
− Iout
) = Iin
− Iin
.
(4.48)
Using the approach described in [4], we then enforced a common-mode constraint to additionally govern the two components of the output signal. This constraint is chosen as
follows:
+ −
+ −
τ Iout
Iouts + Iout
Iout = Iq2 ,
(4.49)
where Iq set the geometric mean of the positive and negative currents. This equation realized a dynamic common-mode constraint that had the same time constant as the differential
mode. At constant input (s = jω = 0), the following relation holds:
+ −
Iout
Iout = Iq2
(4.50)
Combining the common-mode constraint with the differential design equation, we de-
130
rived the nodal equations for the positive and negative paths.
+
Iq2
Iτ
dVout
+
−
+
= +
(Iin − Iin ) + ( + − Iout) ,
C
−
dt
Iout + Iout
Iout
−
Iq2
Iτ
dVout
−
+
−
= +
(Iin − Iin ) + ( − − Iout) .
C
−
dt
Iout + Iout
Iout
(4.51)
Here, C was the capacitance of the capacitors in each paths of the differential-mode LPF.
+
−
and Vout
were the node voltages of the capacitors; and the current flowing
Voltages Vout
+
+
through a MOS transistor gated by Vout
was the current signal Iout
, and same relation
−
−
and Iout
.
applies to Vout
4.3.2 Circuit Building Block
The nodal equations of the LPF (Equation 4.51) that were derived in the previous section
showed that the primitive computations required are multiplication and division. For that
implementation, we employed the log-antilog multiplier (Figure 4.8) as the building block
of the differential-mode LPF circuit and thereby the BM circuit.
The relation between the input current, Ia, and the output current, Ir, in the building
block (Figure 4.8) can be described as:
Ir =
IaIb
Ic
(4.52)
where Ia was the current flowing through the pMOS gated by Va (and sourced by the power
supply railVdd), Ib by Vb , and Ic by Vc . We tied the well (substrate) of the two pMOS
transistors gated by Vm to their sources, respectively, to eliminate the body effect 4.
4
Effect on the threshold of a MOSFET when the source to substrate voltage increases. Increase in Vsb
(Vsource − Vsubstrate ) results in increase in threshold voltage and decrease in channel current
131
Va
Ia
Vb
Vc
Ib
Multiplication
Vm
Ic
Vr
Ir
Division
Figure 4.8: Log-Antilog Multiplier
This circuit is the building block of a low-pass filter in the present design. Comprised of only seven transistors,
the circuit realizes multiplication and division of currents.
132
4.4 Log-Domain Class AB Circuit Implementation of the
Cochlea
In this section, we will present the transistor-level cochlear circuits, then the cochlear chip
architecture. We will first show the Log-domain Class AB circuit of a low-pass filter,
two of which will form the BM circuit. With additional circuit for realizing the cochlear
amplifier mechanism ABC, we shall present the schematic for the BM circuit in the active
case. Last, we will show the architecture of the entire cochlear chip which includes fluid
diffusive network, the BM circuit, and the coupling circuit for ABC.
4.4.1 BM Circuit
Using the building block (Figure 4.8), we obtained the circuit of the positive path of a Class
AB LPF (Figure 4.9A). The nodal equation set in Equation 4.51 suggests the transistor+
−
level LPF circuit, in which Vout
and Vout
denoted the voltage on plus- and minus- capacitor
nodes, respectively, and each of them gated a pMOS transistor to produce the corresponding
+
−
+
−
current signals, Iout
and Iout
. Vin+ and Iin
, Vin− and Iin
, were related in the similar way. The
bias Vq set the quiescent current Iq, which was usually close to zero, while Vτ determined
the current Iτ , which was related to the time constant by τ = CuT /κIτ (uT is the thermal
voltage, and κ is the subthreshold slope coefficient). Two of these subcircuits, connected
to push/pull with each other, formed a complete differential log-domain LPF.
The nodal equations for the two LPFs in the BM design (Equation 4.16) were derived
using the same approach as that for obtaining Equation 4.51. Based on the BM design
described in Equation 4.46, we put together the BM circuit using two Class AB LPFs and
133
+
circuitry that realized their interactions (Figure 4.9). The differential input signals Iin
and
−
represented the difference of velocity potentials across the BM, and the differential
Iout
+
−
and Imem
represented the BM velocity in the upward and downward
output signals Imem
+
−
directions, respectively. As can be seen, the final output Imem
-Imem
representing the BM
velocity, or Imem , was computed as the linear summation of three current signals, Iin, Is ,
and Io .
For realizing the ABC, each BM circuit sent out currents Is+ to its two immediate neighbors; each BM circuit also received IT+ and IT− from its neighbors, which were saturated
version of its neighbor’s Is+ and Is− , respectively. The bidirectional coupling currents, Is+
and Is− , were saturated simply by passing through a current-limiting transistor which was
inserted in series in each of the coupling current pathway (Figure 4.10).
The saturation of the input current through M1 was realized by connecting a current
limiting transistor M2 (Figure 4.10). When the input current, Iin, set by Vin , exceeded the
current level that Vsat sets, Vx increases which in turn reduced the current through M1. The
saturation current, denoted by Irsat, was described as
Irsat = T (Iin) =
Is Isat
Is + Isat
(4.53)
where Isat was set by the bias voltage, Vsat.
4.4.2 Spiral Ganglion Cell Circuit
In the cochlea, the mechanical vibration is eventually translated into neural activities; inner
hair cells (IHCs) and spiral ganglion cells are the agents for the translation. The output
of the biological cochlea is neurotransmitter release at the base of IHC, and ultimately
134
Iout-
Iin+
Vq
Iout+I out
Iin-
Iin-
C+
Half
LPF (+)
Iin+
Iin+
+
+
+
+
To Neighbors
Is-
Is+
b
b
+
- - +
Iin-
+
+
IT+
IT-
+
-
+
Io-
Io+
From Neighbors
+
+
+
+
LPF
Iout+
Iout-
Imem+
Imem-
BM
Figure 4.9: Class AB Low-Pass Filter and Basilar Membrane Circuit Design
A Low-pass filter (LPF) subcircuit. B LPF circuit formed by two LPF subcircuits. C Basilar membrane (BM)
circuit. It consists of two LPFs and connects to its neighbors through Is and IT .
135
Vsat
Vx
M1
M2
Saturated current HnAL
Vin
Vdd
20
10
0
0
50
A
100
Iin HnAL
150
200
B
Figure 4.10: Circuit Implementation of Saturation
A Circuit. B Mathematical simulation result showing saturation profile of the current that flows through the
pMOS transistor with its gate as Vin .
136
neurons’ electrical discharges, or spiral ganglion cell’s (SGC) spikes.
Several transduction processes occur from the BM’s motion to the spikes of the SGCs
[18]. The BM’s up and down motion causes the cochlear fluid in the sub-tectorial space to
move, causing viscous drag which deflects the cilia of inner hair cells (IHC). If the ciliary
deflection is in the radially outward direction (i.e., away from the modiolus), IHCs are excited which means that the transduction channels on the hairs open, allowing K+ to flow
into the IHCs, which in turn depolarizes the IHCs. Depolarization of IHCs causes voltagegated Ca2+ channels to open, and the resultant influx of Ca2+ causes neurotransmitter vesicle exocytosis. The neurotransmitter binds with glutamate receptors on the post-synaptic
membrane of spiral ganglion cells. The receptor potentials serve as generator potentials
that initiate a cascade of events that eventually produce action potentials at the SGCs.
SGCs in the cochlea fall into three categories based on their spontaneous firing rate;
there are high-, medium-, and low-spontaneous rate SGCs. In addition to spontaneous
rate, they also have different firing-rate profile with sound intensity (rate-intensity or RI)
functions [118].
To convert the analog current that represents the BM’s motion into digital pulses , or
spikes of SGCs, we built a circuit model to emulate the functions of SGCs. First, we
modeled the conversion between the BM’s velocity to the post-synaptic input current to the
SGCS. We used a current-splitter [114] to obtain a half-wave rectified current signal, which
represented the depolarizing K+ influx. Constrained by chip die size, we did not explicitly
model the IHC-SGC synapses in detail; instead, we used a low-pass filter to model the IHC
cell membrane’s filtering effect.
The circuit has three functional components: circuit to obtain full-wave BM velocity,
137
Full-Wave
BM Velocity
Imem+
Imem-
Current
Splitter
Vt
C
Vgain
SGC
Input
Vqs
Low-Pass Filter
Figure 4.11: Input to Spiral Ganglion Cells
Circuit that models the processes converting BM velocity into the driving current of spiral ganglion cells.
The outcome is a half-wave rectified current.
current splitter for generating half-wave currents, and a LPF. The circuit of full-wave BM
velocity was formed by taking the difference between the positive and negative components
of the BM velocity, Imem + and Imem − , which were half-wave rectified, and 180 degree out
of phase. Then this sinusoidal signal was split into a positive and a negative current, from
which we only used one of them (i.e., a half-wave current with near-zero baseline) as the
input for driving SGCs. Last, a LPF filters out the high-frequency component in the halfwave rectified input, emulating the effect of the membrane capacitance of IHCs.
In order to model the difference in their spontaneous rates, our silicon SGC modeled
have different gain of the input current (i.e., different bias Vgain ), and modeled the different
saturation levels by using different Vsat biases. What was in common among the high-,
medium-, and low-spontaneous rate SGC circuits are the types of ion channels and their
dynamics. Similar to the biology, our silicon SGC model consisted of a depolarizing fast
Na+ ion channel, a hyperpolarizing membrane-voltage-dependent K+ channel, a hyperpolarizing Ca2+ -concentration-dependent K+ channel, and a fast hyperpolarizing K+ channel
138
Input current
from IHC circuit
Vsat
Im
Neuron
membrane
capacitance
Hyperpolarizing
fast K+ current
Vth_Na
Ack
IReset
IK
IK_Ca
Vqa
VCa
CCa
Vm
Cm
INa
Req
Vth_K
VA
Hyperpolarizing
Ca2+-dependent K+ current
CK
Hyperpolarizing voltagedependent K+ current
Depolarizing fast
Na+ current
Figure 4.12: Silicon Spiral Ganglion Cells
Transistor-level circuit model of spiral ganglion cells. It consists of Na+ , K+ , and Ca2+ ion channels whose
dynamics together shape that of the spiral ganglion cell.
139
Figure 4.12 (Figure 4.12).
The neurons’ spikes were sent off chip through an asynchronous digital communication
protocol, address event representation (AER) [119, 120]. AER is an interface designed for
transmitting and receiving neuronal spike information between silicon neurons. Instead of
communicating using actual spikes of a neuron, each spike is represented by the neuron’s
unique address (i.e., coordinates) in a 2D array and the address is the token for communication. The form of digital pulses offers the advantage of robustness and high efficiency,
compared to an analog code.
Once a neuron’s membrane voltage Vm (Figure 4.12) reached its threshold, the neuron generated an action potential, or a spike, which then sent a request (Req) to the AER
transmitter circuitry. Upon the request was serviced, an acknowledgment (Ack) was received from the receiver (the receiver configured and mounted on the chip test PCB) and
the neuron were reset.
4.5 Chip Architecture
We fabricated a version of our design with 360 BM circuits and two 4680-element fluid
grids (360×13), with its architecture shown in Figure 4.13 and the chip die shown in Figure 4.14. This chip occupied 10.9 mm2 of silicon area in 0.25 µm CMOS technology.
Differential input signals were applied at the base while the two fluid grids were connected
at the apex through a fluid element, which represented the helicotrema. As we can see in
the close-up view (at the bottom of Figure 4.13)), BM circuits exchanged their currents realizing ABC, and the level of currents were controlled by a bias Vsat realizing the saturation
of ABC.
140
4.6 Summary
In this chapter, we described the process of the cochlear circuit design based on our mathematical cochlear model, including design synthesis and transistor-level implementation.
This silicon cochlea comprised the BM and the fluid circuits. The BM design was implemented by two interacting LPFs and the coupling to the cochlear fluid circuit, similar to
the biological cochlea in which the BM is driven by the pressure in the cochlear fluids,
and the BM’s motion in turn affects the motion of the fluid nearby. In order to achieving
simpler implementation, larger dynamic range, and low power consumption, we chosen to
implement the cochlear circuits in current mode with class AB operation. We synthesized
the passive and the active cochlea and implemented the design using current-mode circuit
building blocks. Inclusion of our proposed cochlear amplifier mechanism ABC yielded a
novel architecture of silicon cochlea design.
141
Differential
Audio Inputs
ISV
Diffusive grid
(Top)
Fluid
Base
Apex Second-order sections
BM
(BM)
Fluid
IST
IT +
IT Is+
Is-
T
T
T
T
Imem+
Iin+
Is+ Is+
Is- IsBM
IT + IT +
IT - IT -
Imem-
Iin-
Diffusive grid
(Bottom)
Active bidirectional
coupling
(T: Saturation)
IT +
IT Is+
Is-
T
T
T
T
6
Auditory nerve
Spiral ganglion cells
(Neural spike outputs)
Figure 4.13: Cochlear Chip Architecture
The cochlear chip comprises two diffusive-element grids that model the two cochlear fluid chambers, and 360
basilar membrane (BM) circuits that are embedded in between the two diffusive grids. Each BM segment
sends and receives currents from its two immediate neighbors. The current output of each section, representing BM’s velocity, drives six spiral ganglion cells that have three different spontaneous firing rates with the
ratio of 3:2:1.
142
Figure 4.14: Micrograph of Cochlear Chip Die
Different comprising elements of the cochlear chip die are labeled in the micrograph, including the input, the
basilar membrane, top/bottom fluid, and the auditory nerve (SGC: Spiral ganglion cell).
143
Chapter 5
Cochlear Chip Responses to Sound
In previous two chapters, we have elaborated our hypothesis about the cochlear amplifier
mechanism, active bidirectional coupling (ABC), in which we proposed that outer hair cell
(OHC) motile forces are delivered onto the basilar membrane through the tilted structure
within the organ of Corti, thereby giving rise to frequency-selective amplification and nonlinear cochlear responses. In this chapter, we present responses to sound stimuli in our
silicon cochlea that was described in detail in Chapter 4.
We first examined the effect of ABC on the basilar membrane’s (BM) motion in response to sound stimuli, thereby providing further verification of our hypothesis. We then
explored both the frequency responses and longitudinal responses of the silicon cochlea. To
demonstrate nonlinearity of the silicon cochlea, we performed experiments that involved
measuring BM’s responses with varying input intensities. Finally, we obtained spiking activities on the auditory nerve model, which represented the final output stage of the silicon
cochlea.
144
5.1 Chip Test Setup
Input signals of differential nature were required for our Class AB silicon cochlea. To stimulate the chip, we drove the top and bottom diffusive network simultaneously by applying
two complementary voltage signals at the beginning of the silicon cochlea, corresponding
to the base of the biological cochlea. A voltage signal of a node of the diffusive network
represented the logarithm of fluid velocity potential. Therefore, the current flowing through
a pMOS transistor that was gated by the node voltage emulates the fluid velocity potential.
By the same token, the currents flowing in and out of the two diffusive networks model the
complementary inward and outward motion (velocities) of the oval window and the round
window of the silicon cochlea.
In order to generate sinusoidal current input for the BM subcircuits, the voltage signals
applied were set to be the logarithmic of a half-wave rectified sinusoidal signal with certain
offset (i.e., DC level), based on the exponential I-V relationship of transistors operating in
subthreshold region. To this end, We obtained the value of κ through transistor characterization from the chip being tested and used this κ value in generating input voltage signals.
Further, in order to obtain a set of input current signals with log-linear increase in their
amplitude, a linear increase in the amplitude of the input voltage resulted in an exponential
increase in input current for the BM subcircuit, through which we obtained a linear increase
of current on a logarithmic scale.
Mimicking its biological counterpart, the chip’s outputs have two forms. One is the current output at each BM subcircuit representing the BM velocity, can be measured through
the voltage across a resistor through which the output current passes. The output current
goes through a current sense-amplifier in the output pad before it flows through the resistor. Given the class AB operation of the present design, we measured both current outputs
145
from the positive and negative paths and then took the difference between the two as the
complete output signal. The operation of taking the difference of two output signals is
conveniently performed off chip (i.e., through separate circuitry on the test printed circuit
board).
We included a serial-analog-multiplexer, or scanner, in the periphery of the chip to facilitate visualizing and measuring analog output signals at any desired chip position. The
scanner design we used in our cochlear chip was adapted from [121]. Considering the actual layout of the present silicon cochlea (i.e., columns of cochlear segments alternated in
downward and upward directions physically, forming a consecutive cochlea electrically),
we used two arrays of shift-registers—in downward and upward directions—connected
such that they formed a chain that enabled scanning the entire silicon cochlea sequentially
from the beginning of the silicon cochlea (corresponding to the base) to its end (corresponding to the apex). As the adjacent two columns of BM stages in the cochlear chip
were arranged in opposite directions, the shift registers in the two columns were arranged
in the similar manner to accommodate selection of BM stages in the chip core. By controlling the clock input of the shift registers, we were able to measure output signals of any
BM stage (i.e., subcircuit) in the silicon cochlea.
We directed digital-pulse outputs of the cochlear chip—the neuronal spikes on the silicon auditory nerve—to a PC for real-time visualization (including debugging). Through
the word-serial address-event representation (AER) interface [120], the spikes were represented in terms of their x and y addresses in the 2D array; then an on-board CPLD (complex
programmable logic device) and a USB (universal serial bus) controller were employed for
transmitting spike events together with time stamps to a PC program that we developed for
displaying the results.
146
5.2 BM Frequency Responses: Linear Domain
Although measurements of the human cochlea are unavailable, data of other mammalian
cochleas are abundant with which we can compare the chip measurements, at least qualitatively. In physiological measurements, pure tones were most often used for obtaining the
cochlea’s responses. The use of pure-tone stimuli provided a simple and straightforward
probe for the cochlea’s functions—including spectral analysis and active amplification. To
obtain frequency responses, we swept the frequencies of input signals and measured the
current outputs (from both positive and negative paths) of the BM subcircuit at certain
cochlear location that are selected by the scanner.
5.2.1 Effect of Active Bidirectional Coupling
We investigated the frequency responses for the passive and active cases by varying the
saturation level of outer hair cell (OHC) forces, and thus the extent of ABC between BM
stages. To that end, we varied the voltage bias Vsat that controlled the coupling current level
between BM stages.
Increasing Vsat decreased the coupling between neighboring BM stages, and vice versa,
as Vsat gated pMOS transistors. Specifically, the cochlear amplifier (i.e., ABC) was completely shut off by setting Vsat equal to or higher than the source of the saturation transistors;
that reduced the system to a passive cochlea with only electrical embodiments of the fluid
hydrodynamics, BM’s physical properties, and interaction between the fluid and the BM.
Decreasing Vsat increased coupling current level, introducing ABC into the system, thereby
giving rise to active cochlear responses.
147
At each saturation level, the input frequency was swept from 100 Hz to 30 kHz in
45 logarithmically spaced frequencies. At the BM Stage 100, we obtained a series of
frequency response curves (Figure 5.1), each of which had a peak—the corresponding frequency at which the stage vibrates maximally, known as the characteristic frequency (CF).
Increasing ABC changed the BM velocity amplitude responses but hardly affected the
phase response (accumulation or lag in the traveling wave). As can be seen, with decreasing
Vsat , the CF at which the peak occurs remains more or less the same (around 4 kHz).
The peak amplitude increased monotonically as a result of increasing strength of ABC
(through lowering the bias Vsat ). Plotted on an arbitrary dB scale (Figure 5.1), the peak
height values with increasing ABC level were listed as follows: 18.2, 18.5, 24.1, 26.2, 28.1,
31.6, 33.4, 34.1, and 34.3 dB. Note that the amplitude of the BM velocity were amplified
only over a restricted frequency region so that the responses became more sharply tuned. In
terms of the frequency tuning, the calculated Q10 values increased monotonically in general
from 0.45 to 1.14 with increasing ABC levels.
BM velocity phase responses relative to that of the input signal keep almost the same:
flat near the base and abrupt accumulation near the CF (Figure 5.1B). This result indicated
that a traveling wave was formed along the BM in the silicon cochlea.
5.2.2 Active Cochlear Frequency Responses
Under an active cochlea setting (with maximal saturation current level), we measured frequency responses of six linearly spaced BM stages (Figure 5.2). The six cochlear locations
were from Stage 30 to 230, spaced 40 segments apart. The CF that maximally excited the
first measured position (i.e., Stage 30) was 12.1 kHz. The remaining five CFs, from early to
148
BM Velocity Amplitude HdBL
40
30
20
Vsat
1.70 V
1.85 V
1.90 V
1.95 V
1.97 V
2.00 V
2.10 V
2.20 V
10
0
0.1 0.2
0.5 1
2
5
Frequency HkHzL
10
20
0.1 0.2
0.5 1
2
5
Frequency HkHzL
10
20
BM Velocity Phase HΠ radiansL
A
B
0
-1
-2
-3
-4
Figure 5.1: Frequency Responses in the Cochlear Chip with Varying Outer Hair Cell Forces
Measured frequency response of the basilar membrane velocity with increasing strength of outer hair cell
forces, thus varying ABC. A Amplitude. B Phase. Decreasing Vsat leads to increase in the saturation level,
which increases the strength of ABC. The amplitude of the BM velocity increases over a certain frequency
range and thus the tuning becomes sharper. The phase responses remains almost the same.
149
later stages, were 8.2 k, 1.7 k, 905, 366, and 218 Hz, respectively. Their tip-to-tail ratio values (amplitude difference between the response peak to the lowest response value) are 27,
18, 27, 17, 22, 26 dB, respectively. Q10s were also calculated as 1.4, 2.7, 2.2, 1.3, 1.4, 2.2,
respectively. The cutoff slopes were -21, -38, -54, -20, -20, -46 dB/Octave, respectively.
Measurements from the chinchilla cochlea were also plotted in Figure 5.2 for comparison (dashed lines). Q10 of the chinchilla cochlea was 2.55 at mid-sound intensity (computed from [47]) and the cutoff slope was about −85 dB/octave (computed from [47]). In
the chip, the phase accumulation at the CFs ranged from 0.56 to 2.67π radians, comparable to 1.67π radians in the mammalian cochlea ( [47]). Despite some irregularities with
the response shape (which were related to Q10 and cutoff slope) and peak height, the chip
responses were comparable to the biological data qualitatively.
We performed frequency-response measurements with the chip at several more cochlear
positions, from Stage 10 to 230, which were spaced by 20 stages, thereby we obtained the
frequency-position map in the chip. The frequency-position map depicted the tonotopic
(spatial) mapping of the spectral analysis that the cochlea performs. To obtain such a map,
we plotted the characteristic frequency for each BM stage we measured against its cochlear
position (Figure 5.3). The expression in Figure 5.3 is the regressed relationship between
the CF and cochlear position. It was evident that despite some deviation due to transistor
mismatch, the CFs decreased linearly on a logarithmic scale with the increasing cochlear
positions, thus distance from the beginning of the silicon cochlea.
150
BM Velocity Amplitude HdBL
Stage
30
70
110
150
190
230
40
30
20
10
0
-10
0.1 0.2
0.5
1
2
5
Frequency HkHzL
10
20
0.1 0.2
0.5
1
2
5 10
Frequency HkHzL
20
BM Velocity Phase HΠ radiansL
A
B
0
-2
-4
Figure 5.2: Frequency Responses in the Cochlear Chip Compared to Biology
Frequency responses of the basilar membrane (BM) velocity at different cochlear positions are obtained and
compared with physiological data measured in chinchilla cochlea [47]. A Amplitude. B Phase. These
linearly spaced BM stages, ranging from the beginning to the end of the silicon cochlea, have characteristic
frequencies that shift from high to low. In addition, the responses are comparable to biological measurement
in terms of shape, peak amplitude, and the phase accumulation.
151
Characteristic Frequency HKHzL
Log10 fHHzL = 4.26-0.0082 x
10
1
0.1
0
50
100
150
200
Cochlear Stage Number
250
Figure 5.3: Measured Frequency-Position Map in the Cochlear Chip
The expression in the figure describes the regressed function, indicating a linear relationship between the
characteristic frequency, f, and the cochlear position, x (x is in the unit of stage number).
152
5.3 Nonlinear Frequency Responses
We investigated the nonlinearity of the cochlear responses due to saturation of ABC. In an
active-cochlea parameter setting (high saturation level of ABC), we varied the input current
level and obtained the BM velocity frequency responses at the Stage 100 (Figure 5.4).
We increased the input voltage level linearly such that the corresponding current into the
BM increase exponentially; the input level (in dB) was then calculated based on the input
voltage and measured value of κ from the chip being tested. The dynamic range of input
current was limited on the high side by the threshold of transistors, and on the low side by
the noise floor of the output current.
BM Stage 100 responds to increasing input intensity distinctively at different frequencies. As expected in the log-scale plot, we observed the linearly increasing BM responses in
the lowest frequency region. In contrast, the BM responses around the CF did not increase
linearly with the input level as the near-CF region was where ABC took effect. Thus, saturation of OHC forces caused the compression, or compressive growth, with the increasing
input levels locally around the CF region. Figure 5.4B shows the phase responses of the
BM velocity, relative to that of the input signal.
The peak amplitude increases monotonically in a compressive way, which is evident
when we plot the peak amplitude against the input intensity (Figure 5.5). As can be seen,
the amplitude at the CF (peak) increase more linearly when input intensity is low while it
grows compressively when the input level is high, resulting in 24 dB compression between
the input and the output. In Figure 5.4, at lower frequencies, the output increases more
linearly than at the peak. The CFs at increasing input intensities hardly change while in the
biological cochlea the peak shifted a little basalward (to lower-frequency side) [32].
153
Peak Amplitude HdBL
60
40
Input Level
48 dB
40 dB
32 dB
24 dB
16 dB
8 dB
0 dB
20
0
0.1 0.2
0.5 1
2
5
Frequency HkHzL
10
20
0.1 0.2
0.5 1
2
5
Frequency HkHzL
10
20
A
Peak Phase HΠ radiansL
0
B
-2
-4
-6
-8
Figure 5.4: Nonlinear Compression in the Cochlear Chip
Chip data of BM velocity frequency responses at different input levels. A Amplitude. B Phase. The input
levels (dB) were listed in the figure legend: they were 8 dB spaced, ranging from 0 to 48 dB.
154
Peak Amplitude HdBL
25
20
15
10
5
0
0
10
20
30
Input Intensity HdBL
40
50
Figure 5.5: Saturating Input-Output Function in the Cochlear Chip
The basilar membrane velocity at the characteristic frequency (i.e., peak amplitude) saturated with the input
intensity.
The tuning of the responses became broader with increasing input level; Q10s were 1.8,
1.6, 2.2, 1.6, 1.4, 1.4, and 1.1, respectively (monotonically except for the 16 dB input, due
to some artifacts at lower input levels whose responses were noisier); the cutoff slopes were
−44, −29, −35, −24, −19, −16, and −13 dB/Octave, respectively; again, they became
shallower monotonically except for the response at 16 dB input. The phase plots at different
input level overlapped with one another, nearly independent of the input intensity. The
larger phase plateau at the lower input levels (0 and 8 dB) were due to the noisy responses
at the cutoff; as a result, the plateau occurred at −6π and -8π for the two lower input levels,
respectively, just one or two more cycles than that of other phase response curves.
155
5.4 Longitudinal Patterns
Longitudinal responses of the cochlea are the spatial patterns of BM vibrations in response
to input sound. We measured the longitudinal responses of BM velocity in the cochlear chip
by applying sinusoidal current input (input voltages were logarithm of sinusoidal signals).
At 500 Hz pure tone input, we measured the BM responses at every single stage and then
we obtained the smoothed response by taking a moving average over 10 stages (Figure 5.6).
We measured the silicon cochlea’s responses at 2 kHz input in the passive and active
case (Figure 5.7). In the active case, where the largest possible coupling gain between BM
stages was set, the cochlea showed larger response peak (about 14.6 dB) than that in the
passive case where there was no coupling made possible. In addition, the cutoff slope was
much steeper in the active case than in the passive case.
Stimulating the chip using three additional input frequencies independently—500 Hz,
1, 2, and 4 kHz—we obtained the BM velocity longitudinal responses (Figure 5.8). 4 kHz
input elicited a BM response peak at Stage 85 (defined as the characteristic place, or CP)
while 500 Hz sound traveled all the way to Stage 178 and peaked there. In addition, the
CPs for 1k and 2kHz were Stage 166 and 139, respectively. The tip-to-tail ratios of the
responses from 500 Hz to 4 kHz were 13, 14, 32, and 12 dB, respectively. Q10s were 1.1,
0.9, 1.2, and 0.9, respectively. The cutoff slopes were −70, −16, −27, and −21 dB/Octave,
respectively.
156
BM Velocity Amplitude HdBL
Input Freq. 500 Hz
Raw
Smoothed
20
10
0
-10
0
50
100
150
Cochlear Stage Number
200
Figure 5.6: Longitudinal Responses in the Cochlear Chip (I)
Measured BM longitudinal responses at 500 Hz input frequency. Gray: Raw data. Magenta: Smoothed data
BM Velocity Amplitude HdBL
(10-stage moving average).
30
Input freq: 2k Hz
Passive
Active
20
10
0
-10
0
50
100
150
Cochlear Stage Number
200
Figure 5.7: Longitudinal Responses in the Cochlear Chip (II)
Measured BM longitudinal responses at 2 kHz input frequency in the passive case and in an active case.
Curves are smoothed data (10-stage moving average).
157
BM Velocity Amplitude HdBL
Input Freq.
500 Hz
1 kHz
2 kHz
4 kHz
20
10
0
-10
0
50
100
150
Cochlear Stage Number
200
Figure 5.8: Longitudinal Responses in the Cochlear Chip (III)
Input frequency varies from 500 Hz to 4 kHz, resulting in response peaks at different cochlear positions
(10-stage moving average).
158
5.5 Silicon Auditory Nerve’s Spike Activities
The silicon auditory nerve in our cochlear chip converted the BM’s mechanical motion into
electrical discharge patterns, or spikes that encoded a sound wave’s frequency, intensity and
temporal information. The sound signal’s frequency was represented by neuronal spikes at
certain cochlear positions, or frequency channels; the temporal structure of sound signals
were replicated in terms of the temporal responses of the frequency channels. Sound intensity was coded by the firing rate of the spiral ganglion cells, whose axons formed the
auditory nerve.
Cochleargram
We measured real-time auditory nerve responses to a complex sound stimulus. The
complex sound comprised a chirp followed by a click. We constructed the chirp by sweeping frequency linearly on a logarithmic scale from 16 kHz to 200 Hz for 1.5 seconds. The
click contained 50 frequency components on a logarithmically linear scale. The spectrum
of the constructed sound signal was plotted in Figure 5.9.
We presented this complex sound to the silicon cochlea and obtained the spike responses on the silicon auditory nerve. There were two ways for generating the differential
input signals: one is to download signal data points into two function generators and to use
their output signals (out-of-phase half-wave rectified sinusoidal) as the input of the chip,
the other is to play the sound signals in a PC and use the sound card output to drive the
chip. We then obtained a cochleagram from the chip response (Figure 5.10), in which the
silicon cochlea responded to frequencies from high to low at cochlear positions from the
beginning (the base) to the end (the apex), and all of the frequency channels in the input
frequency range were activated by the click sound.
159
Frequency HHzL
10000
2000
200
0
0.5
1
1.5
Time H s L
Figure 5.9: Spectrum of A Complex Sound
In the spectrotemporal pattern of a complex sound stimulus (Frequency vs. time plot), the first component
was a 1.5-second chirp, with frequency sweeping from 16 kHz to 200 Hz logarithmically to match the logfrequency position map of the cochlea. Each of the all 50 frequency components lasted for the same number
of periods thus for a frequency-dependent duration. The second part of the complex sound was a click-like
sound for 0.1 second, which comprised all of the 50 frequencies.
160
In this cochleagram, we plotted the spike outputs of all the three types of spiral ganglion
cells (SGC) for cochlear channels ranging from 1 to 240, which amounted to spike activities
of 1440 (i.e., 240x6) frequency channels. The channel activities from Stage 240 to 360
(near the end of the silicon cochlea) were omitted in the cochleagram because the responses
outputs were not satisfactory under the bias setting used.
We could now appreciate one of the advantages of building analog VLSI model of
the cochlea, from the perspective of complex-sound processing. This mixed-mode silicon
cochlea responded to sound, simple or complex, in real time; in contrast, responses to the
complex sound discussed above would take thousand times more time to obtain in software simulation running on a PC. The real-time characteristic is desirable in engineering
applications such as sound/speech preprocessing in automatic speech recognition systems.
Rate-Intensity Function
Firing rate varying with input intensity is one of the coding approaches that the biological auditory nerve employs for representing intensity. We measured the average firing rate
at the characteristic place in the cochlea for a given sinusoidal input, and we obtained the
firing rates with increasing input intensity, a relation called the rate-intensity (RI) function.
We observed different RI functions for different SGC types, namely high-, medium, and
low-spontaneous rate (HSR/MSR/LSR) SGC (Figure 5.11). The input to the SGC cells is a
low-pass filtered version of a half-wave rectified signal that represents the upward velocity
of the BM.
In the chip, there were two major differences in the parameter setting among the three
types of SGCs. On one hand, HSR SGCs were assigned a higher gain bias voltage than
the MSR and LSR SGCs so that VgainH > VgainM > VgainL. Under this setting, HSR SGCs
161
Figure 5.10: Measured Cochleagram from the Cochlear Chip
The spectrotemporal structure of the sound stimulus is described in Figure 5.9. In response to the chirp, the
neuronal spiking activities shifted from high-frequency channel (i.e., near the beginning) to low-frequency
channels (i.e., near the end). In response to the click, almost all of the channels shown in the plot generated
spikes simultaneously.
162
obtained larger input current than MSR and LSR SGCs while MSR SGCs were stimulated
more than the LSR SGCs. By setting the neuron’s firing threshold at an appropriate level,
the three types of SGCs exhibited different firing rate, which can be interpreted as different
spontaneous rates. On the other hand, the saturation levels of the depolarizing input current
were set differently; the gate voltage of the pMOS transistor for the HSR was set to be
higher than the other two types of SGCs so that starting from certain input current level the
firing rate of the HSR SGCs saturated while the MSR and LSR SGCs did not show such
saturation profile.
Four different RI types have been reported in the chick cochlea nerve ( [118]), while
only three RI function profiles have been found in the guinea pig [122]. In our chip, we
only modeled the major three types of RI functions for simplifying the design—saturating
for HSR SGCs, sloping-up for MSR SGCs, and straight for LSR SGCs.
Comparing the RI function we obtained from the silicon cochlea with the physiological
data ( [118,122]), we found that our chip showed similar RI functions as that in the biological cochlea. As seen in Figure 5.11, HSR SGCs generated spikes even at the lowest input
intensities, while MSR and LSR neurons kept silent until the input intensity increased to a
certain level; LSR SGCs had an even higher threshold than the MSR SGCs. With the input
level increasing, the HSR SGCs’ firing rate saturated while the firing rate of LSR SGCs
continued to increase.
5.6 Nonideality in Chip Responses
We have shown in the previous sections that our silicon cochlea’s responses demonstrate
the characteristic features observed in the biological cochlea. Meanwhile, we also observed
163
Mean Firing Rate HspksL
150
HSR
MSR
LSR
100
50
0
16
24
32
40
48
Input Intensity HdB, arbitraryL
56
Figure 5.11: Rate-Intensity (RI) Functions Measured From the Cochlear Chip
The average firing rates of the silicon auditory nerve vary with the input intensity. The spike raw data of
silicon spiral ganglion cells (SGCs) were collected at Stage 110 for an 4 kHz pure tone input and the firing
rates were calculated. Different parameter settings (voltages) were used for generating distinct RI functions
for high-, medium-, and low-spontaneous rate (HSR/MSR/LSR) SGCs.
164
that the chip responses were different across the chip (e.g., in shape or amplitude), and/or
deviated somewhat from physiological measurements. This variation in responses can be
largely attributed to the variation in parameter setting (voltages) in the chip.
We shall explore the possible cause of the variation of chip parameters, thus suggesting
possible ways of reducing the nonideality with chip responses. We first describe the circuit
parameters that model the physical properties of the cochlea and discuss what an optimal
setting is. We then investigated the effect of transistor mismatch on chip responses due to
variations in the chip parameters.
Parameter Settings in the Cochlea Chip
We shall find out what the optimal bias voltage settings should be for achieving desired
responses that are quantitatively comparable with the physiological measurements or the
mathematical model responses. It is not reasonable to pursue precise matching thereby
exactly the same responses across the chip because that is not the case even in the biological cochlea. It is also not practical to find a set of parameters to make chip responses the
same as the mathematical model responses because the hardware and the software modeling work in different modeling paradigm—software simulation is performed under an
ideal condition without ant variations, while a VLSI system is unavoidably subject to nonideality in the physical medium that are introduced by the fabrication process. However, it
is instructive to scrutinize the effect of parameter settings and their variations because the
resultant knowledge will help the physical model yield more realistic responses.
We repeated the circuit analogs of the stiffness S, damping β, and mass M of the BM,
165
as follows.
b+1
,
τ1τ2
1
1
β =
+ ,
τ1 τ2
S =
M = 1.
(5.1)
Form the above expressions, we calculated the damping ratio as:
β
ξ = √
SM
τ1 + τ2
= p
(b + 1)τ1 τ2
(5.2)
The damping ratio value we chose in our mathematical simulation was 0.2, resulting
in a quality factor of 5 for this second-order system. In order to realize this damping ratio
in the chip by equalizing Equation 5.6 with 0.2 and solving the equation, the relationship
between the time constants τ1 and τ2 , and the gain factor b was calculated as:
p
τ1 = (0.02b − 0.98 + 0.02 (b + 1)(b − 99))τ2 ,
(5.3)
or
p
τ1 = (0.02b − 0.98 − 0.02 (b + 1)(b − 99))τ2 .
(5.4)
The relations above suggest that to obtain real-value time-constant solutions, the gain
factor b should be set to be equal to or greater than 99. In our circuit design, b is calculated
as:
b = eVb−Vdd
166
(5.5)
where voltages were in the unit of uT , the thermal voltage at room temperature. To this end,
the bias voltage Vb needs to be set somewhat higher than Vdd. Let us look at an example
of the setting to get a flavor of the relation between τ1 and τ2 , in which we made b = 100
meaning that Vb was about 0.12 V higher than Vdd . Then we obtained:
τ1 = 0.819τ2 ,
or
τ1 = 1.221τ2
(5.6)
In other words, only when this relation was satisfied across the chip (along the silicon
cochlea) can the desired damping ratio be realized. The larger b, the larger ratio between
τ1 and τ2 .
Note that τ1 and τ2 should increase exponentially along the cochlear position in the
chip. Also, a time constant τ is related to the time-constant current, namely Iτ as follows.
τ=
CuT
.
κIτ
(5.7)
We had two electrical variables for controlling the time constants, the capacitor C and the
current Iτ . In practice, it is convenient to have C fixed (corresponding to constant area for
capacitance in chip layout) and to decrease Iτ along the silicon cochlea. For each BM stage,
each of the two time-constant bias voltages was tied to a node on a polysilicon wire. These
nodes across the chip were linearly spaced, which in turn results in exponentially spaced
time-constant currents due to the exponential I-V (current-voltage) relation in transistors’
subthreshold-regime operation. A polysilicon wire with certain voltage potential difference
at its two ends were known as a tilt poly line.
With regard to keeping the ratio between τ1 and τ2 , we need to maintain the ratio at the
starting and ending terminals of the tilt poly lines. Assuming that the time constant ratio is
167
r, we have the relations as:
τ1 (x) = rτ2 (x),
1
Iτ 2,
r
uT
log(r).
=
κ
=⇒ Iτ 1 =
=⇒ Vτ 2 − Vτ 1
(5.8)
where we can see from that the two voltage tilt lines needed to have the same space constant.
From Equation 5.6 only, the values of r was determined by b and Vτ 1 can be set either
larger or smaller than Vτ 2 . In oder to help determine which τ of the second-order section
should be set higher, we needed to consider what setting will give rise to optimal responses.
Recall that the quality factor Q of the BM second-order section can be computed as
√
SM
Q =
β
√
b+1
p
= p
τ1/τ2 + τ2 /τ1
(5.9)
Note that this Q was not directly indicative of the value of Q10 in the cochlear chip responses because the tuning, or frequency selectivity, of the cochlea is the outcome of a
collective network behavior that comprised the loading effects among a number of secondorder sections. From Equation 5.6, we can see that when τ1 τ2 :
or when τ2 τ1 :
r
τ2
Q = (b + 1)
τ1
(5.10)
r
τ1
Q = (b + 1)
τ2
(5.11)
√
The highest Q can be obtained when τ1 = τ2 , which is
168
b + 1/2. Therefore, a larger value
of b will result in a higher Q.
However, another issue should also be considered—the parameter setting should be
optimal so that the chip’s responses become enhanced to the relatively large extent when
the active mechanism, ABC, is activated. As a reference, we repeated the second LPF that
includes the ABC design below.
τ2Io s = Iin − bIs − Io − rfb (b + 1)T (Isb) + rff (b + 1)T (Isf ).
(5.12)
As we know from the derivation above, the gain factor b has to be equal to or greater than
99, which adds at least two orders of magnitude gain for the current Is on the right side
of Equation 5.6. For that purpose, together with the subthreshold-regime current level in
mind, we want to have a relatively small value of Is relative to other current quantities, Io
for example. Therefore, we chose the latter case in Equation 5.6 for setting τ1 and τ2 such
that the effect of the last two terms, which implemented ABC, become relatively larger
once included. Of course, the effect of ABC can be additionally augmented by raising the
gain factors rfb and rff in Equation 5.6.
Variations in Parameters due to Transistor Mismatch
Device mismatch in integrated circuits refers to the variations in transistor’s properties and characteristics. It is essentially caused by random difference in dopant density in
the transistor channels. In practice, two identically designed transistors are never exactly
identical. Much research has characterized and modeled the device mismatch [115] and has
shown that the variation in the threshold voltages is one of the dominant sources underlying
the drain-source current and gate-source voltage mismatch [115]. The threshold mismatch,
∆VT , is due to the fluctuation in dopant density across the chip. These fluctuations exhibit
169
a Gaussian (or normal) distribution with a standard deviation, σ(∆VT ); it can be modeled
as:
AV
σ(∆VT ) = √ T
WL
(5.13)
where AVT is a proportionality constant that is technology-dependent, W is the channel
width and L is the channel length [115,123]. As can be seen from Equation 5.13, increasing
the channel area (either W or L or both) obviously decreases the standard deviation of
the variation in threshold voltage, but it is only employed to a very limited extent due to
the high cost in terms of chip die area. Due to this design tradeoff, transistor mismatch
inevitably contributes to variations in model parameters. Specifically, in our cochlear chip,
the variations in parameters τ1 , τ2, and b would result in variations in the implemented
stiffness S, damping β, and mass M.
In order to investigate the effect of parameter variations on the BM response, we simulated our mathematical model with additionally added log-normal distributed variations
to the model parameters (i.e., the BM’s stiffness and damping). Circuit analogs of these
parameters are scaled current variables (according to Equation 5.1 and the relationship between time constants and current variables described in Equation 5.7). Therefore, these
parameters with variances are lognormally distributed, to a first-order approximation, because their logarithms (i.e., voltage) are normally distributed.
According to [123], AVT in 0.25 µm technology is about 6 mV, which give the value
of σ(∆VT ) to be around 1.4% (based on Equation 5.13) for a 10 by 10 λ nMOS transistor
(mostly common in our chip) with its threshold voltage around 0.36 V. Therefore, in our
simulation under nonideal condition, we applied the standard deviation σ ranging from
0% (zero variation) to 2% to the logarithmic of mean parameter values, which has normal
distribution.
170
We found that model responses deviated from the ideal result, exhibiting smaller peak
amplitude and less sharp tuning and getting noisier with increasing σ (Figure 5.12). This
simulation provided just one example of the effect of parameter variations on model responses, thus we can gain understanding on the causes of the nonideal chip responses.
Multiple simulation can be performed further to obtain the distribution of some characteristics of the BM displacement response (e.g., peak amplitude, Q10, cutoff slope, etc.).
The effect of variation can be somewhat compensated by increasing the OHC motility
factor α in model simulations (Figure 5.13). This simulation example showed that increasing the OHC motility factor α raises the response peak but causes the cutoff slope (slope of
the response curve after the characteristic place) to become shallower than that when there
is no variation. In practice, we increase the source bias Vff and Vfb in the chip to increase
the OHC motility factor. The extent of this raise is limited as instability would occur if the
coupling among BM stages were too large. The non-steep cut-off slope was also observed
in our chip’s BM velocity responses (Figure 5.2) as a result of increasing OHC motility
factor.
In the biological cochlea, variations also exist with the physical properties of its comprising components. They do not, However, seem to deteriorate the cochlear behavior to a
severe extent. In other words, the biological cochlea tackles the variation problem somehow, likely through averaging responses locally as inner hair cells sense the nearby-fluid’s
velocity, which is a collective outcome of neighboring BM fibers’ motion. This suggests a
possible solution in the silicon cochlea design—to average BM responses locally and use
the smoothed BM responses as input signals to inner hair cell and the spiral ganglion cells.
171
BM Displacement
Amplitude HdBL
80
Σ H% of ΜL
60
0
0.5
1
2
40
20
0
-20
0
5
10
15
20
Distance from Stapes HmmL
25
Figure 5.12: Effect of Log-Normal Variations in Parameters on Model Responses
Basilar membrane (BM) displacement responses at 2 kHz pure tone input are obtained with Log-Normal
distributed variation added to original model parameters of fluid density ρ, BM stiffness S, damping β. Mass
M was kept without variation to match the circuit implementation scheme (constant mass). The values of
standard deviation σ used are 0% (original case), 0.5%, 1%, and 2% of mean (log) values. With increasing
variation, the BM displacement peak becomes reduced and the tuning becomes broader.
172
BM Displacement
Amplitude HdBL
80
OHC motility factor
Α
60
0.15 No var.
0.15 With var.
0.25 With var.
40
20
0
-20
0
5
10
15
20
Distance from Stapes HmmL
25
Figure 5.13: Effect of Log-Normal Variations and OHC Motility on Model Responses
Effect of Log-Normal variations on model responses can be remedied to a certain extent through increasing
the OHC motility factor α. The responses shown are examples with the standard deviation of the normaldistributed variation was set to be a relatively high value, 1% of the mean (log). When the OHC motility
factor α is increased from 0.15 to 0.25, basilar membrane displacement peak is raised but the cutoff slope
apical to the peak becomes shallower compared to the model response without variation.
173
5.7 Summary
Measured chip responses have demonstrated that inclusion of the OHC motility, simply
through the coupling between BM neighboring sections, enhances the basilar membrane
responses’ amplitude response and frequency selectivity, turning a passive cochlear model
into an active one. This active model produces cochlear responses that are qualitatively
comparable to physiological data. The cochlear chip consumed 52 mW power when ABC
was activated and the silicon auditory nerve’s spiking activities were being generated. The
overall performance of the present silicon cochlea is generally superior to existing designs,
in terms of response amplitude, sharpness of tuning and working frequency range.
Compared to the biological cochlea or the present mathematical cochlear model, the
silicon cochlea does not exhibit quantitatively comparable responses—its response peak
amplitude was smaller even when ABC was included, although its phase responses are
realistic. The BM response curves also existed irregular shapes across the chip. These undesirable features of chip responses are mainly caused by transistor mismatch, which gives
rise to parameter variations. Since the cochlear model took on a physical form, silicon
medium, to mimic the biological cochlea, it is subject to both advantages and constraints
that are imposed by the medium, the similar way as the biological cochlea has evolved. The
design, thereby performance, of a physical model relies on our understanding of the biological system; meanwhile, the final yield for functionality is also dependent on hardware
design techniques and implementation approaches. Therefore, performance optimization
is the next step in improving the silicon cochlea through reducing the negative effect of
device mismatch. This highlights an important issue for neuromorphic system design that
are compact, operates in subthreshold, and consumes low power—in addition to mimicking
the organization and function, neuromorphic systems should also learn from the nature’s
174
approaches for overcoming the physical constraints and combating the effect of variations.
What have we gained from building the silicon cochlea? Despite its less realistic responses compared to software simulation, a hardware cochlear model is superior to a mathematical model on several aspects. First, it runs in real time with electronically tunable
model parameters. This is especially attractive when processing complex sound such as
natural sound and speech, which usually require a great amount of computation resource
if simulated on a computer. Second, the low-power feature and compact design of analog
VLSI cochlear models promise a variety of applications. For example, it could work as the
acoustic front end of automatic speech recognition system on a mobile phone or a personal
digital assistant. It can also be tranformed into a fully-implantable cochlear implant if a
microphone can be embedded on the chip. Above all, through building the silicon cochlea,
we learned what it takes to build a system that emulates a biological system and what the
constraints are.
175
Chapter 6
Conclusions
The human cochlea transduces sound waves into the basilar membrane (BM) vibrations
and auditory nerve electrical discharge patterns. The cochlea decomposes complex sound
into its spectral components, representing them as vibrations at different locations along
the BM. This creates a tonotopic map of frequency along the cochlear partition. When
performing the spectral analysis, the cochlea achieves exquisite sensitivity and remarkable
frequency selectivity due to nonlinear active amplification of cochlear mechanics.
Although the outer hair cell (OHC) electromotility has been widely thought to amplify
the traveling wave [18], the exact manner in which OHCs enhance cochlear function remains unclear. In other words, understanding of how the cochlear amplifier (CA) gives
rise to the BM’s exquisite frequency sensitivity and selectivity is still lacking. Cochlear
modeling has been extensively employed to help deepen our understanding about how the
cochlea works. Cochlear models were based upon what has been known but also make
predictions on what still remains unknown and suggest further physiological experiments.
176
6.1 Summary of Present Work
The present effort was dedicated to explore a plausible CA mechanism that is possibly
based on the cochlear microanatomy. We hypothesized a CA mechanism in which OHC
motile forces actively couple the otherwise weakly coupled BMs transverse fibers, through
the tilts of Deiters cells and of their phalangeal processes (PhPs), as observed anatomically.
We implemented this cochlear amplifier mechanism, named active bidirectional coupling
(ABC), in both a mathematical model and an analog VLSI microchip to reproduce nonlinear active cochlear behavior.
The ABC mechanism captured structural features of the cochlear microanatomy and
thereby provided biologically plausible explanations to the outstanding question of how
the cochlear amplifier is realized. In the organ of Corti, the OHCs are tilted toward the
base of the cochlea while the phalangeal processes (PhPs) are tilted toward the apex. It is
assumed in our model that this tilted structural organization very likely provides an efficient means to transmit OHC motile forces onto the BM. Due to the tilted structure, OHC
forces are delivered in both forward and backward longitudinal directions, enhancing the
cochlea’s sensitivity and tuning. In addition, it is widely accepted that the saturating nature
of OHC forces imparts to the cochlea nonlinearity, which is also incorporated in the ABC
mechanism.
We formulated a two-dimensional nonlinear active model based on ABC. The numerical simulation results showed that activation of ABC boosted frequency-selective amplification, sharpened tuning, as well as increased sensitivity. Additional simulation demonstrated that the tilted structure was of primary importance in the cochlear amplifier because
including OHC motility without taking into account the tilts of OHC and PhP was not as
effective as ABC in terms of producing active amplification. Furthermore, including satu177
ration of OHC forces, as observed physiologically, produces compressive growth near the
characteristic place (CP) at high sound levels. And when two tones are present, the louder
one suppresses the other to a larger extent at higher suppressor intensity.
Analysis of the model revealed that the introduced OHC forces through the tilted structure leads to negative damping right before the peak over a restricted region in the cochlea.
Further analysis showed that the tilts work as an array of spatial filters, only amplifying
the traveling wave in the vicinity of the response peak where the shortening wavelength
becomes comparable to the OHC (and PhP) tilt distance.
Our cochlea model may be tested by measuring the wavelength of the traveling wave in
live (active) and dead (passive) cochleas. In the long-wave region, the model predicts a 9%
longer wavelength in the active case. In the short-wave region, the model predicts a 81%
shorter wavelength in the active case. Furthermore, the traveling wave peak amplitude was
displaced 3.2 mm apically in the active case; negative damping should occur between these
two peak locations.
Embodying a nonlinear active cochlear model in analog VLSI realizes cochlear processing in real time with low power consumption. We implement ABC in a silicon cochlea,
resulting in a novel design architecture that addressed the problems of existing designs. Our
cochlear chip comprises circuits that model the cochlear fluid, the BM, and the bidirectional
delivered OHC forces. The measured chip responses demonstrate that when ABC was activated, larger amplification and sharper tuning is achieved. Through limited currents that
represent ABC, the chip responses show nonlinear saturation at high sound levels. In addition, our chip also included silicon spiral ganglion cells, whose digital-pulse encoded
output emulates the spike patterns of auditory nerve.
178
The present cochlear modeling efforts and results have a great deal of implications in
the study of cochlear mechanics, clinical experiments, and sound-related engineering problems. Our mathematical and VLSI cochlear models will function as computational tools in
scientific exploration of the auditory system. With its basis in cochlear microanatomy, the
ABC-based cochlear model has clinical significance in understanding OHC-related hearing loss as well as in improving speech-preprocessing for cochlea implants. In addition,
the cochlear model, especially the silicon cochlea, provides a basis for building silicon audition that involves emulating the human auditory system. Last, the new architecture of the
silicon cochlea gives insight into the design of cochlea-like sound-processing microchips.
6.2 Future Work
The human auditory system has been recognized as the most efficient and noise-robust
speech recognition system [124]. Human is capable of perceiving and recognizing sound/speech
signals in rather noisy or reverberant environment with very low error rate while the stateof-the-art automatic speech recognition (ASR) system fails to perform well even with the
most advanced computation resource. Therefore, building models that morph the superior
capability of the auditory system in processing sound and speech, referred to as neuromorphic audition, would provide insightful hints or even solutions to the engineering problem
of building ASR as well as shed light on understanding of auditory processing mechanisms.
Building cochlear models, in mathematics and VLSI, are a starting point of building
neuromorphic auditory systems. The present silicon cochlea, inspired by the cochlear amplifier mechanism, can generate neuronal spikes that encode the input sound; its output
is ready to drive systems that model higher auditory processing centers to perform more
179
auditory tasks such as pitch perception and sound localization. Therefore, in addition to
improving the present cochlear model, future work along the track would include exploring practical applications using the silicon cochlea and developing neuromorphic auditory
models.
On the model improvement side, the present cochlear model can incorporate more
anatomical details of the cochlea, for example, the difference between the base and the
apex, in order to generate more realistic cochlear behavior. More simulation of cochlear
nonlinearity, such as distortion product otoacoustic emission, could further verify the notion that cochlear function is closely association with its structure and anatomy. Reducing
effect of transistor mismatch in the silicon cochlea would result in more physiologically
comparable behavior.
On the application side, an automatic speech recognition system could potentially benefit from the cochlea-like sound processing of the present silicon cochlea: it runs in real
time and it’s responses exhibit nonlinearity. Development of neuromorphic VLSI chips that
emulate further auditory processing will allow more complicated tasks because each processing center in the auditory hierarchy seems to extract different features (e.g., frequency,
timing, and location) of incoming sound. Building such models would be extremely challenging because it requires not only sufficient understanding of the auditory system, which
is still rather limited although our knowledge about its working is experiencing extension
through both experimental and theoretical approaches, but also advanced circuit and system
design techniques. Nevertheless, it is no doubt a rewarding task because once the real-time
low-power neuromorphic auditory system is built, it can be used to test our hypothesis
of the auditory mechanisms; it can be applied to preprocess sound and speech in attempt
to improve the performance of current ASRs, especially in acoustically-cluttered environment; Also, with appropriate silicon-neural interfaces, it can potentially substitute current
180
auditory prosthetic devices due to its advantages of low power consumption, compact size,
and real-time biomorphic signal processing.
181
Appendix A
List of Abbreviations
1D
One-dimensional
2D
Two-dimensional
2TS
Two-tone suppression
ABC
Active bidirectional coupling
AER
Address event representation
ASR
Automatic speech recognition
BM
Basilar membrane
CA
Cochlear amplifier
CF
Characteristic frequency
CMOS
Complementary metal-oxide semiconductor
CMRR
common-mode rejection ratio
CP
Characteristic place
dB
Decibel
DC (Re biology)
Deiters’ cell
DC (Re engineering)
Direct current
182
FD
Finite-difference
HSR
High spontaneous rate
IHC
Inner hair cell
IP
Inner pillar
IPC
Inner pillar cell
LG
Liouville-Green
LPF
Low-pass filter
LSR
Low spontaneous rate
MSR
Medium spontaneous rate
MOSFET
Metal-oxide-semiconductor field-effect transistor
OC
Organ of Corti
OHC
Outer hair cell
OP
Outer pillar
OPC
Outer pillar cell
PhP
Phalangeal process
RI
Rate-intensity
RL
Reticular laminar
SA
Semi-analytical
SAS
Semi-analytical solution
SGC
Spiral ganglion cell
SPICE
Simulation program with integrated circuit emphasis
VLSI
Very large scale integration
WKB
Wentzel-Kramers-Brillouin
WKBJ
Wentzel-Kramers-Brillouin-Jeffereys
183
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