Effect of Prolonged Contralateral Acoustic Stimulation on TEOAE Suppression Altelani van Zyl

Effect of Prolonged Contralateral Acoustic Stimulation on TEOAE Suppression Altelani van Zyl
Effect of Prolonged Contralateral Acoustic Stimulation on
TEOAE Suppression
By
Altelani van Zyl
In fulfilment of the requirements for the degree
M. Communication Pathology
In the
Department of Communication Pathology
Faculty of Humanities
University of Pretoria
Supervisor: Prof DCD Swanepoel
Co-Supervisor: Prof JW Hall III
November 2008
© University of Pretoria
Abstract
: The Effect of Prolonged Contralateral Acoustic
Title
Stimulation on TEOAE Suppression
Name
: Altelani van Zyl
Supervisor
: Dr. D Swanepoel
Co-supervisor : Dr. J Hall
Department
: Communication Pathology, University of Pretoria
Degree
: M Communication Pathology (Audiology)
Although the suppressive effect of the medial olivocochlear system (MOCS)
on peripheral auditory active mechanisms is well documented in humans, the
effect of efferent inhibition over prolonged periods of acoustic stimulation is
less well documented, especially as observed in suppression of transient
evoked otoacoustic emissions (TEOAE’s). The present study therefore
evaluated the relationship between the duration of contralateral acoustic
stimulation and the suppression of TEOAE’s in ten adults with normal hearing.
TEOAE recordings with linear clicks (60 dB sound pressure level) were
measured at four intervals during 15 minutes of continuous contralateral white
noise (45 dB sound pressure level), followed by two post-noise recordings. An
identical within-subject control condition was recorded without contralateral
noise. Experimental and control measurements were repeated three times, on
separate days. Results revealed significant and sustained TEOAE amplitude
reduction for the entire duration of contralateral stimulation. Suppression
increased across the duration of contralateral noise, but not sufficiently to be
statistically significant. After noise termination, TEOAE amplitudes increased
to values significantly above control recordings. The sustained suppression of
TEOAE’s indicates continuous efferent inhibition over time in normal adults,
with a significant increase in TEOAE amplitude after noise cessation possibly
indicating
increased
outer
hair
cell
responsiveness
after
prolonged
contralateral noise.
Keywords: Transient evoked otoacoustic emissions; Contralateral stimulation;
Medial olivocochlear efferent system; Prolonged stimulation
1
Table of Content
CHAPTER 1: ORIENTATION AND STATEMENT OF THE PROBLEM ....................6 1.1 INTRODUCTION ............................................................................................................................. 6 1.2 THE ORIGIN AND DEVELOPMENT OF OTOACOUSTIC EMISSION MEASUREMENTS ................ 6 1.3 OAE SUPPRESSION: A BRIEF OVERVIEW OF THE ONGOING DEVELOPMENT IN RESEARCH
AND ITS CLINICAL VALUE ....................................................................................................................... 9 1.4 PROBLEM STATEMENT ................................................................................................................ 13 1.5 OUTLINE OF THE THESIS ........................................................................................................... 14 1.6 CONCLUSION ..............................................................................................................................15 1.7 SUMMARY .................................................................................................................................... 15 CHAPTER 2: FUNCTIONING OF THE EFFERENT MEDIAL OLIVOCOCHLEAR
SYSTEM DURING PROLONGED STIMULATION. .................................................... 17 2.1 INTRODUCTION ........................................................................................................................... 17 2.2 ANATOMY OF AUDITORY PATHWAYS ......................................................................................... 19 2.2.1 Afferent pathways ...................................................................................................... 19 2.2.1 Efferent pathways....................................................................................................... 20 2.3 PHYSIOLOGY OF THE EFFERENT SYSTEM .................................................................................. 23 2.3.1 Neurotransmitters of the efferent system ....................................................... 23 2.3.2 Efferent Stimulation ................................................................................................... 24 2.4 THE ROLE AND CLINICAL RELEVANCE OF THE EFFERENT AUDITORY SYSTEM ....................... 26 2.5 OAE SUPPRESSION AS A MEASUREMENT OF EFFERENT FUNCTION ....................................... 32 2.5.1 Contralateral suppression ....................................................................................... 35 2.6 CONFOUNDING VARIABLES IN MEASURING CONTRALATERAL OAE SUPPRESSION ............. 41 2.7 DURATION OF CONTRALATERAL SUPPRESSION ....................................................................... 42 2.7.2 Duration of contralateral suppression: measured using OAE’s ............... 46 2.8 CONCLUSION ..............................................................................................................................50 2.9 SUMMARY .................................................................................................................................... 51 CHAPTER 3: RESEARCH METHODOLOGY ................................................................ 52 3.1 INTRODUCTION ........................................................................................................................... 52 3.2 RESEARCH AIMS ......................................................................................................................... 53 3.3 RESEARCH DESIGN .................................................................................................................... 54 3.4 RESEARCH SUBJECTS ................................................................................................................ 54 3.4.1 Population ...................................................................................................................... 54 3.4.2 Sampling technique ................................................................................................... 55 3.4.3 Criteria for subject selection .................................................................................. 55 3.4.4 Subject selection apparatus ................................................................................... 57 3.4.5 Subject selection procedures ................................................................................. 58 3.4.6 Description of the sample ....................................................................................... 61 3.5 PRELIMINARY STUDY.................................................................................................................. 61 3.5.1 Confirmation of subject selection criteria......................................................... 61 3.5.2 Determination of optimal stimulus and suppressor parameters ............ 62 3.5.3 Confirmation of data collection procedures ..................................................... 62 3.6 DATA COLLECTION ..................................................................................................................... 63 3.6.1 Apparatus ....................................................................................................................... 63 3.6.2 Data Collection Protocols ......................................................................................... 63 3.6.3 Data Collection Procedures ..................................................................................... 65 3.6.4 The Criteria for acceptable TEOAE amplitudes measured with
contralateral white noise. ...................................................................................................... 68 Data organization and calculation of suppression amplitude ................................. 69 3.6.5 Analysis ........................................................................................................................... 69 3.7 ETHICAL CONSIDERATIONS ...................................................................................................... 72 3.7.1 Respect for the privacy of research participants........................................... 72 2
3.7.2 Informed consent ....................................................................................................... 72 3.7.3 Beneficence and non- malfeasance .................................................................... 73 3.8 VALIDITY AND RELIABILITY OF RESEARCH METHODOLOGY .................................................... 73 3.9 SUMMARY .................................................................................................................................... 74 CHAPTER 4: RESULTS .................................................................................................... 76 4.1 SUB-AIM 1: TEOAE AMPLITUDES OVER TIME FOR CONTROL AND EXPERIMENTAL
CONDITIONS ......................................................................................................................................... 76 4.1.1 Consistency of noise levels in control and experimental conditions
over time ....................................................................................................................................... 77 4.1.2 TEOAE amplitudes for control and experimental conditions over time79 4.2 SUB-AIM 2: RELATIONSHIP BETWEEN THE DURATION OF CAS AND TEOAE
SUPPRESSION ....................................................................................................................................... 87 4.2.1 Overall suppression ................................................................................................... 89 4.2.2 Suppression at half-octave frequency bands.................................................. 90 4.2.3 Frequency suppression differences over time ................................................ 92 4.3 CONCLUSION ..............................................................................................................................93 4.4 SUMMARY .................................................................................................................................... 93 CHAPTER 5: DISCUSSION ............................................................................................ 95 5.1 INTRODUCTION ........................................................................................................................... 95 5.2 SUB-AIM 1: TEOAE AMPLITUDES OVER TIME FOR CONTROL AND EXPERIMENTAL
CONDITIONS ......................................................................................................................................... 96 5.2.1 Consistency of noise levels within and between conditions ..................... 96 5.2.2 TEOAE amplitude reduction as a function of CAS ......................................... 97 5.2.3 TEOAE amplitude characteristics after noise termination ......................... 98 5.3 SUB-AIM 2: RELATIONSHIP BETWEEN THE DURATION OF CAS AND TEOAE SUPPRESSION
99 5.3.1 Suppression as a function of CAS duration ................................................... 100 5.3.2 Suppression across half-octave frequencies ................................................. 101 5.4 SUSTAINED SUPPRESSION OVER PROLONGED STIMULATION DURATIONS ......................... 102 5.5 CONCLUSION ............................................................................................................................ 104 5.6 SUMMARY .................................................................................................................................. 104 CHAPTER 6: CONCLUSION AND RECOMMENDATIONS .................................... 106 6.1 6.2 6.3 6.4 CONCLUSION ............................................................................................................................ 106 IMPLICATIONS OF STUDY ......................................................................................................... 108 RECOMMENDATIONS FOR FURTHER RESEARCH ..................................................................... 113 FINAL CONCLUSION .................................................................................................................. 114 REFERENCES ................................................................................................................... 115 APPENDIXES ................................................................................................................... 138 3
List of Tables
Table 2.1
Table 2.2
Table 2.3
Table 3.1
Table
Table
Table
Table
3.2
3.3
3.4
4.1
Table 4.2
Table 6.1
Table 6.2
Difference between Type I and Type II afferent
fibres……........................................................................
The difference between LOC and MOC efferent
subsystems…………….……………………………………………………………….
Summary of existing studies to date, investigating the
duration of contralateral suppression.................................
Subject selection apparatus..............................................
Subject selection testing procedures..................................
Test protocol for TEOAE stimuli parameters........................
Test protocol for TEOAE suppressor parameters..................
Descriptive analysis of overall and half-octave frequency
noise levels as calculated for both conditions…....................
Descriptive analysis of TEOAE amplitudes in control and
experimental condition…..................................................
Critical evaluation of test method variables as applied in
current study…...............................................................
Recommendations on improving test method and new
research areas to be investigated......................................
19
21
48
57
59
63
64
76
79
108
111
List of Figures
Figure 2.1
Figure 2.2
Figure 2.3
Figure 3.1
Figure 3.2
Figure 4.1
Figure 4.2
Figure 4.3
Figure 4.4
Figure 4.5
Figure 4.6
Figure 4.7
Concepts and theories discussed in chapter 2……….............…
Slow and fast effects on CAP amplitude when the OCB is
electrically stimulated………………………………………………………………
Relative DPOAE amplitude over 32 min without and with
CBBN applied for 20 min…………………………….……………………………
Schematic representation of the test procedure in the
controlled and experimental conditions…………………….…………...
Analyses of the differences amongst the mean suppression
amplitudes in the various time intervals………….…………………….
Mean overall OAE amplitudes over time across conditions…...
Mean OAE amplitudes at half-octave frequency intervals for
control and experimental conditions over time....................
Mean amplitude differences between control and
experimental conditions in the post-noise period…………………..
Mean TEOAE (overall & half-octave frequencies) suppression
as a function of CAS ………………………………………………………………..
Overall TEOAE suppression as a function of contralateral
noise duration (Mean ± SD) …………………………………….…………….
TEOAE suppression as a function of CAS duration at halfoctave frequencies (Mean ± SD)…………………………………………….
Mean suppression across frequency bands as a function of
CAS duration…………………………………………………………………………...
17
43
47
66
70
80
82
84
86
87
89
90
5
Chapter 1: Orientation and Statement of the Problem
1.1 Introduction
Since David Kemp first described otoacoustic emissions (OAE’s) in 1978, an
extensive body of research has been produced, studying the many aspects of
OAE’s and their relation to auditory functioning. These studies have led to
many new applications of this procedure towards more specific diagnoses of
pathologies related to audition. A more recent area of interest that has started
to enter clinical practice is the measurement and quantification of contralateral
suppression of OAE’s.
The purpose of this chapter is to present a brief overview of the ongoing
development of OAE measurements with specific focus on contralateral
suppression of OAE’s, its clinical applications, advances and continued
interests of research. It provides a brief explanation of the effect of
contralateral acoustic stimulation (CAS) on the suppression of OAE’s and the
rationale for investigating the time course of this effect during continual CAS.
This overview clearly indicates the need for investigations of the effect of
prolonged excitation of the efferent system.
1.2 The Origin and Development of Otoacoustic Emission
Measurements
The discovery of active cochlear mechanisms, in the form of OAE’s (Kemp,
1978) contradicted established cochlear theories of the day. It was widely
accepted that the cochlea was mechanically passive and functionally linear.
These findings supposed that travelling waves along the cochlea moved
without attenuation, that they could be reflected and reversed by non-linear
processes and thereafter could reverberate along with distortions inside the
cochlea for a notable period of time (Kemp, 1978).
Kemp discovered that sound could be recovered from the cochlea by means
of an ear canal microphone following stimulation by either tones or clicks.
6
These low intensity sounds, referred to as “evoked acoustic emissions”
(Kemp, 1978) were noticed for several milliseconds in the ear canal and were
called “Kemps Echoes”. They provided an indication of the cochlear response
to acoustic stimulation. These echoes are today commonly known as
otoacoustic emissions (Glattke & Kujawa, 1991).
It is generally believed that OAE’s are the by-product of the amplification of
sound in the cochlea (Kemp, 1978). These by-products of the preneural
mechanisms of the cochlear amplifier are particularly linked to the normal
functioning of the outer haircells (OHC) (Brownell and Kachar, 1985; Brownell,
1990). Of all the cells of the organ of Corti, only the OHC have been shown to
produce active mechanical movements and may, therefore, generate sound
(Folenkov, Belyatseva, Kurc, Mastroianni & Kachar, 1998). Thus OAE’s are
sounds generated by the motility of OHC in normal cochleae, either in
response to acoustic stimulation or spontaneously.
Different types of
emissions can be distinguished by the relationship between the type of
stimulation and the latency of the response after the stimulus onset.
Traditionally, OAE’s have been classified into two types, namely Spontaneous
OAE’s (SOAE’s) that are emitted from the ear in the absence of stimulation,
and Evoked OAE’s (EOAE’s) that can be observed in response to a stimulus
applied to the ear. SOAE’s are believed to be caused by the active processes
that occur in the cochlea (Pujol et al., 1994).The clinical value of SOAE’s is
restricted by their low prevalence in normal ears. These emissions occur in
only 72% of healthy ears at frequencies that vary greatly among subjects
(Talmadge, Long, Murphy & Tubis, 1993). While an absence of SOAE’s does
not imply outer hair cell dysfunction, their presence is evidence for the
presence of an “active” element in the cochlea (Kim, 1986) and is a sign of
normal cochlear function (Bright & Glattke, 1986).
The second type of OAE, EOAE’s, can be subdivided into three subtypes.
Stimulus Frequency OAE’s (SFOAE’s) are evoked by constant pure tone
stimulation at low intensity levels and are normally swept gradually across a
region of frequencies. More commonly researched and clinically applied,
7
however, are the Transient Evoked OAE’s (TEOAE’s) and Distortion Product
OAE’s (DPOAE’s). TEOAE’s are low-level sounds emitted by the ear in
response to brief stimuli, such as broadband clicks or tone bursts that can be
measured with a low-noise, sensitive microphone in the external ear canal
(Kemp, 1978). They have a high prevalence of nearly 100% in people with
completely normal cochlear functioning (Bonfils, Uziel & Pujol, 1988b;
Johnson & Elberling 1982; Kemp, 1978; Norton & Leely, 1987). DPOAE’s are
created by two slightly different pure tones closely spaced in frequency that
activate the cochlea in the same region of the basilar membrane. The tow
primary tones interact on the basilar membrane and create a family of
distortion products that have a mathematical relationship to the primary tone
frequencies. DPOAE’s, like TEOAE’s, can be measured in almost 100% of
ears with normal hearing and normal middle ear function, and are stable
within a given ear over time (Lonsbury-Martin et al., 1990a).
TEOAE’s and DPOAE’s are recognized to be very sensitive, clinically feasible
measures of outer hair cell functioning and form an integral part of the basic
test battery for evaluations of auditory functioning, therefore being commonly
used in clinical practices.
Although they are not tests of hearing, they
complement the audiogram and provide sensitive measures of OHC integrity.
Their role in the early identification and diagnosis of OHC dysfunction in
paediatric and adult populations has become increasingly important and are
applied in various forms of clinical application. Neonatal hearing screening
(Bonfils et al., 1998) the assessment of suspected functional hearing loss
(Musiek,
Bornstein & Rintelmann, 1995), the monitoring of ototoxicity
(Stavroulaki,
Vossinakis,
Dinopoulou,
Doudounakis,
Adamopoulos
&
Apostolopoulos, 2002), the diagnosis of tinnitus (Ceranic, Prasher & Luxon,
1995, 1998) and the differentiation between cochlear and retrocochlear
dysfunction are among some of the clinical applications of OAE testing.
8
1.3 OAE suppression: a brief overview of the ongoing development in
research and its clinical value
Apart from the clinical applications, advances in the field of OAE’s are an area
of continuing research interest. Recently numerous studies have been
devoted to the suppression of OAE’s by CAS (Collet, 1993; Veuillet, Collet &
Morgon, 1992; Collet, Veuillet, Bene & Morgon, 1992; Berlin, Hood, Cecola,
Jackson & Szabo (1993); Norman & Thornton, 1993; Morlet, Collet, Salle &
Morgon, 1993; Froehlich, Collet & Morgon, 1993; Chery-Croze, Moulin &
Collet, 1993; Moulin, Collet & Duclaux, 1993; Morlet, Collet, Salle & Morgon,
1993; Berlin, Hood, Hurley & Wen, 1994; Pujal, 1994; Collet & Grandori, 1994;
Lind, 1994; Thornton, 1994; Graham & Hazel 1994; Prasher, Ryan & Luxon,
1994; Veuillet, Duverby-Bertholon & Collet, 1996; Maison, Micheyle & Collet,
1999; Hood et al., 1999). Contralateral suppression of OAE’s is the
phenomenon whereby the presentation of a sound ipsilateral or contralateral
to a normal functioning ear from which OAE’s are being recorded, reduces or
suppresses the amplitude of the OAE (Berlin, Hood, Cecola, Jackson, &
Szabo, 1993a; Berlin, Hood, Hurley, Wen & Kemp 1995b; Berlin, Hood,
Hurley & Wen, 1994; Collet, Kemp & Veuillet, 1990b; Collet et al., 1992; Ryan,
Kemp & Hichcliffe, 1991; Veuillet, Collet & Duclaux, 1991). This effect is
attributed
to
alteration
of
cochlear
micromechanics
by
the
medial
olivocochlear bundle (MOCB), activated by acoustic stimulation of the
contralateral ear (Maison, Micheyl & Collet, 1995).
The MOCB innervates the organ of Corti and OHC’s via efferent pathways
(Rassmussen, 1946). This efferent or descending auditory system/reflex
mediates sound-induced suppression of OAE’s. Thus a reduction in the
amplitude of OAE responses, in the presence of an acoustical signal in the
contralateral ear, provides a non-invasive, objective approach for assessing
MOCB efferent feedback activity in humans (Giraud, Collet, Chery-Croze,
Magnan & Chays, 1995).
OAE’s are the only objective and non-invasive method for the evaluation of
the functional integrity of the medial efferent system and, therefore, for the
9
evaluation of the structures lying along its course, at least up to the level of
the inferior colliculi (VIII nerve, cerebellopontine angle and pons). Although
contralateral acoustic suppression of OAE’s is not yet completely understood
and not widely used in clinical practice, some important information about the
functioning of the medial efferent system and cochlear hair cells can be
obtained from the presence, absence and amount of suppression.
Although data is rather limited in the literature, there is preliminary evidence
that the efferent test could be useful in the diagnosis of pontine lesions either
extrinsic (acoustic neuromas, meningiomas, congenital cholesteatomas) or
intrinsic (multiple sclerosis, ischemic infarcts, tumours). Prasher et al., (1994)
conducted a study in 18 patients suffering cerebellopontine angle (CPA)
tumours and 11 patients with intrinsic pontine lesions. According to the
results, 15 of the 18 patients with CPA tumours demonstrated abnormal
TEOAE suppression ipsilateral to the lesion. The suppression was abnormal
in all patients suffering intrinsic pontine lesions. The author performed the
TEOAE suppression test in a group of 11 patients with CPA tumours (6 with
acoustic neuroma, 1 congenital cholesteatoma, 3 meningioma, 1 lipoma) and
a second group comprised of 21 patients suffering intrinsic pontine lesions
(10 with multiple sclerosis, 7 ischemic infarct, 1 pontine haemorrhage and 3
tumours). A third group of 20 young healthy, normal hearing volunteers
served as the control group for the TEOAE suppression test. Normal
suppression in sound pressure level (≥1 dB SPL) was demonstrated in 18 out
of the 20 controls (false positive rate 6.7%). All patients with CPA tumours
showed abnormal suppression (<1 dB SPL), either ipsilaterally to the lesion
or bilaterally (sensitivity 100%). Bilateral abnormal suppression was found
whenever pressure was exerted on the pons due to the size of the tumour.
Abnormal suppression was recorded in 17 out of the 21 patients with intrinsic
pontine lesions (sensitivity 81%).
Auditory neuropathy is a clinical entity that has attracted the interest of
audiologists and researchers in auditory function. It is characterized by
sensorineural hearing loss in pure tone audiometry, speech discrimination
difficulty, absence of acoustic reflexes, normal OAE’s and absent or severely
10
abnormal auditory brainstem responses (ABR’s) without any radiologically
evident retrocochlear lesion. The age of patients ranges from infancy to
adulthood and it could present as a neuropathy of the VIII nerve alone or,
most frequently, as a part of hereditary sensori-motor neuropathies (i.e.
Charcot-Marie-Tooth syndrome, Friedreich’s Ataxia syndrome) (Doyle,
Sininger & Starr, 1998; Starr, Sininger, Hood & Berlin, 1996). Studies have
demonstrated that patients with auditory neuropathy have absent efferent
suppression of TEOAE’s with binaural, contralateral or ipsilateral noise but
usually have normal
otoacoustic emissions (Berlin et al., 1993a; Hood,
Berlin, Bordelon & Rose, 2003; Lalaki 2003; Abdala, Sininger & Starr 2000).
In consideration, evidence exists that the assessment of the medial
olivocochlear system by recording OAE’s under CAS in a suspected lesion of
the CNS could contribute to neuro-otological topographic - or site of lesion
diagnostics. It could be performed to complement ABR’s in cases with mean
hearing thresholds worse than 60 dB HL, where the ABR test is of limited
sensitivity (provided that TEOAE’s could be recorded, due to the
retrocochlear nature of the hearing loss).
It is important to know that efferent suppression of OAE’s is difficult to study
in patients with greater than mild cochlear hearing losses because emissions
are absent when hearing thresholds exceed 30-40 dB HL. Liang, Liu & Lui,
(1997), measured contralateral suppression of TEOAE’s with broadband
noise in 24 ears with cochlear hearing losses. They reported that TEOAE
amplitude and suppression of emissions were significantly reduced in
patients with cochlear hearing losses in comparison with normal ears.
Cases of tinnitus have also been linked to the efferent system and a possible
link between the efferent system and the generation of tinnitus has been
suggested by several authors. Veuillet, Collet and Duclaux (1991) observed a
smaller suppression effect in the ear ipsilateral to the tinnitus in a patient with
unilateral tinnitus. Chery-Croze, Collet and Morgon (1993) reported on 16
patients with bilateral tinnitus and 20 patients with unilateral tinnitus where
suppression was measured using contralateral suppression with evoked
otoacoustic emissions (EOAE’s). Little suppression was observed in 10 of the
11
20 patients with unilateral and bilateral tinnitus. The majority of patients with
unilateral tinnitus showed a decrease in the amount of suppression or an
enhancement of emission amplitude on the side ipsilateral to the tinnitus,
whereas a few patients showed an increase in emission amplitude under the
suppression
condition.
Suppression
of
DPOAE’s
indicated
medial
olivocochlear (MOC) dysfunction in the frequency range of the tinnitus.
Ceranic, Prasher, Raglan & Luxon (1998) studied efferent suppression in
patients with tinnitus following head injury and difficulties listening in
background noise despite normal peripheral hearing sensitivity. They also
observed a reduction in suppression in the patients with tinnitus when
compared with a control group of patients without tinnitus. In 19 subjects with
unilateral tinnitus, Rita and de Azevedo (2005) found that the overall TEOAE
response levels were significantly higher in the ear with no tinnitus and the
medial olivocochlear system (MOCS) as measured with TEOAE suppression
was significantly less efficient in the ear with tinnitus.
Berlin, Goforth-Barter, Hood and Bordelon (1999) reported that hyperacusis
patients show abnormally large amounts of efferent suppression. They
observed an increase in efferent suppression in a group of three patients with
hyperacusis, two adults and one child, who complained that ordinary sounds
were perceived as loud and frequently intolerable. These results suggested
that efferent suppression may be a good tool to identify certain types of
hyperacusis objectively.
Other studies in the field of contralateral suppression have focused on
speech perception and the detection of sound in noise (Kumar & Vanaja,
2004; Micheyl et al., 1995). In addition to this, several studies have provided
evidence suggesting that activation of the medial efferents serves a
protective function against high-level auditory stimuli in the mammalian
auditory periphery (Canlon, 1996; Subramanian, Henderson & Spongr,
1993).
12
1.4 Problem Statement
It is well documented that hearing deficits may result from exposures to
relatively intense acoustic stimulation and, in addition, it has been shown that
exposure to high intensity sounds results in various structural changes in the
cochlea(Saunders, Dear & Schneider, 1985). Previous studies have shown
that damage to, or abnormalities in, the efferent auditory system degrades
perception of signals in noise (Muchnik et al., 2004) and may even make the
cochlea more susceptible to damage from exposure to noise (Kujawa &
Liberman, 1997; Maison & Liberman, 2000). Prasher et al. (1998) noted
significant reductions in efferent suppression of TEOAE’s with contralateral
noise stimulation of up to one hour.
Industrial workers are commonly exposed to high noise levels for long
durations. It is important to know if the protective function of the efferent
system remains stable even when the cochlea is exposed to noise for
prolonged durations. The fatiguing characteristics of sensory cells and
auditory afferent neurons are well researched, but these characteristics are
not as well-documented in efferent neurons. Sliwinska and Kotylo (2002)
compared OAE suppression in subjects with normal hearing and subjects with
occupational exposure to noise of up to five years. Their results showed that
the amount of suppression was significantly decreased in the exposed group
compared to non-exposed subjects (Sliwinska & Kotylo, 2002). This reduction
in amplitude was ascribed to the damaged efferent auditory neurons in
individuals exposed to noise. If this is the case, the protective characteristics
of the efferent auditory neurons are expected to adapt or weaken over time,
making the OHC’s more susceptible to acoustic trauma and permanent
damage. Another explanation for the noise induced hearing loss may be that
the have a weak or poorly functioning MOCS to begin with and therefore
might be susceptible to noise damage as a result of MOCS status.
From the existing literature it is clear that there is a need to study the
relationship between outer hair cell integrity and the extent of efferent
13
inhibition. An investigation of the effects of prolonged excitation of the efferent
pathway in order to monitor changes in the amount of OAE suppression over
a predetermined time of contralateral stimulation will provide better
understanding of whether the mechanisms underlying these protective effects
persist over longer periods of noise exposure. Thus the purpose of the
present study is to evaluate the relationship between the duration of
contralateral acoustic stimulus and suppression of evoked otoacoustic
emissions.
1.5 Outline of the Thesis
Chapter One: Orientation and Statement of the problem
This chapter provides a brief overview of the development of OAE
measurements with a specific focus on the development of research in
contralateral suppression of OAE’s. It formulates the need for knowledge
regarding the effect of prolonged excitation of the efferent system and
delineates the purpose of this study, which is to further investigate the initial
time course of the suppressing effect during continual CAS.
Chapter two: Literature review
Chapter two primarily focuses on TEOAEs and suppression of TEOAES. It
provides an in depth overview of the anatomy (the general afferent and
efferent innvervation fiber distribution) and physiology of the olivocochlear
bundle, the feedback loop that involves OHC’s, and the differenential
characteristics of ipsilateral, bilateral and contralateral suppression of OAE’s
with specific focus on the literature on the effect of prolonged contralateral
excitation on OHC’s and the implications of this knowledge.
Chapter Three: Research Methodology
Chapter three will be a discussion of the methodology for data collection,
preparation and analysis, apparatus, subjects, the research design and the
procedures chosen for optimal contralateral suppression of TEOAE’s.
14
Chapter Four: Results
Chapter four will present results of all experiments: Suppression of the overall
TEOAE response and responses at each half-octave frequency over a
predetermined period of separate TEOAE measurements.
Chapter Five: Discussion and Interpretation of Results
Chapter five will discuss and interpret all findings in terms of significance as
well as readiness for broad clinical use. A few interesting findings will also be
discussed.
Chapter Six: Summary, Evaluation of the study and Conclusion
The last chapter will evaluate this study in terms of validity, reliability and
limitations and make recommendations for future research.
References
Only references mentioned in this thesis appear alphabetically in the
reference list.
1.6 Conclusion
Auditory efferent nerve fibres have an inhibitory influence on the auditory
periphery, which in turn may serve as a protective reflex against acoustic
overstimulation. Contralateral suppression of OAE’s is known to be an
objective, non-invasive clinical test for the exploration of the non-linear
micromechanics of OHC’s and the clinical evaluation of the descending
efferent bundle in humans. Thus OAE’s can be used to explore the duration of
this protective reflex in order to determine whether it remains consistent over
a long duration of acoustic stimulation, or adapts and weakens over time,
making the OHC’s more susceptible to acoustic injury.
1.7 Summary
This chapter provides a brief outline on the progress and advances in OAE
measurements and in particular, suppression of OAE’s. It briefly explains the
15
effect of CAS on suppression of OAE’s and why a study of the duration of
suppression during constant CAS will provide more information on the
function of medial olivocochlear efferents and their ability, or lack thereof, to
provide a sustained effect on OHC functioning.
16
Chapter 2: Functioning of the efferent medial
olivocochlear system during prolonged stimulation.
2.1 Introduction
Efferent control of the OHC’s and the cochlear efferent neurons has been
investigated using studies of cochlear micromechanics, cochlear and eighth
nerve electrophysiology and OAE’s (Berlin, Hood, Hurley & Wen, 1996).
Studies have used OAE recordings from subjects involving ipsilateral or
contralateral competing stimuli, or artificial electrical stimulations of neurons in
the efferent system in the case of animal subjects, to investigate the efferent
influence on the OHC’s and on cochlear efferent neurons. Through this, the
anatomy and physiology of the olivocochlear bundle and its efferent control
over the auditory periphery could be understood more thoroughly.
The purpose of this chapter is to describe the anatomy and physiology of the
olivocochlear bundle, the feedback loop that involves OHC’s and IHC
afferents, the different characteristics of ipsilateral, bilateral and contralateral
suppression of OAE’s with a specific focus on the effect of prolonged
contralateral excitation on OHC’s and the implications of this knowledge. The
existing literature provides evidence for ruling out the limitations on
suppression duration studies, which emphasizes the need for further
investigations, guidance in formulating the research methodology and
assistance in the interpretation of the results.
The content of this chapter is organized in such a manner that the theoretical
basis of the efferent system and its function are explained before elaborating
on the use of OAE’s to investigate certain properties of the efferent system
and feedback loop, in particular the duration of the efferent effect on OAE’s.
The structure of concepts and theories clarified in this chapter is illustrated in
figure 2.1 as a flow chart.
17
AUDITORY
PATHWAYS
AFFERENT
EFFERENT
PHYSIOLOGY OF THE EFFERENT
Neurotransmitters of the efferent system
Efferent Stimulation
The role and clinical relevance of the efferent auditory
Suppression as measurement of efferent function
CONTRALATERAL SUPPRESSION
SOAE’s
DPOAE’
Types of OAE used to measure
contralateral suppression
TEOAE’s
The acoustic reflex
Confounding variables in measuring
contralateral OAE Suppression
Duration of contralateral suppression
Acoustic crossover
Duration studies in animals
(Cochlear Action Potential &
Ensemble Background Activity)
Duration studies in humans
(OAE’s, TEOAE’s & DPOAE’s)
Figure 2.1 Concepts and theories discussed in chapter 2
It is well known that efferent suppression of OAE’s is mediated by the
olivocochlear bundle (OCB) (Berlin et al., 1996; Liberman, Puria & Guinan,
18
1996). Thus it is critical to discuss the neuroanatomy and physiology of this
efferent system to better understand the mechanism and consequences of
OAE suppression.
2.2 Anatomy of auditory pathways
The cochlear branch of cranial nerve VIII (vestibulocochlear nerve) is known
as the auditory nerve. It contains afferent and efferent fibres. Auditory afferent
fibres are mostly responsible for carrying incoming electrochemical signals
that are transduced by sensory cells (hair cells) to the brainstem and auditory
cortex. Efferent fibres in turn send information from the auditory cortex
descending to the cochlea, forming the olivocochlear bundle, where they
synapse with dendrites of ascending afferent fibres or directly with outer hair
cells. Evidence confirms that the descending or efferent, auditory system
plays a role in outer hair cell physiology and, therefore, influences OAE
measures. It is difficult, however, to discuss the efferent system of the cochlea
in isolation, since they are so closely integrated with the sensory cells. The
following discussion will, therefore, explain both the afferent and efferent
auditory pathways.
2.2.1 Afferent pathways
The innervations of OHC and IHC include both efferent and afferent
connections. The afferent fibres leave the inner ear through the internal
auditory canal (or meatus) located on the posterior surface of the petrous part
of the temporal bone. They enter the brainstem at the level of the
cerebellopontine angle (CPA) and terminate in the cochlear nucleus complex
(CNC). The innervations of OHC and IHC include both afferent and efferent
connection. Amongst a total of 30,000 eighth cranial nerve fibres in the
human, two clear types of afferent neurons can be identified (Kiang, Rho,
Northrop, Liberman & Ryugo, 1982), namely Type I and Type II afferent fibres.
Table 2.1 delineate the differences between these two afferent fibres.
Table 2.1 Difference between Type I and Type II afferent fibres
Type I afferent fibres
Type II afferent fibres
19
Also known
as
• Radial afferents
• Outer spiral afferents
Percentage
of total
afferent
fibres
• 88%
(Nandol et al., 1990)
• 12 %
(Nandol et al., 1990)
Synapse
with
• Inner hair cells
(Nandol et al., 1990)
• Outer hair cells
(Nandol et al., 1990)
Type of cell
bodies
• Bipolar cell bodies
(Kiang et al., 1982)
• Monopolar or pseudomonopolar
(Kiang et al., 1982; Brown et al., 1988)
Amount of
afferents
connecting
with hair
cells
• Form direct connections with
inner hair cells, about 20 per
hair cell (Kiang et al., 1982)
• Each outer hair cell may receive
processes from up to 20 afferent fibres
(Kiang et al., 1982)
Neural
pathway
• Sends large myelinated axons
to the cochlear nucleus in the
brain stem (Brown et al., 1998)
• Sends small unmyelinated axons to
neurons around the periphery of the
cochlear nucleus (Brown et al., 1998)
Function
• Fibres convey sensory
information from the cochlea to
auditory regions of the central
nervous system
• Difficult to document (Brown et al.,
1998). Possible function of carrying
information about the mechanical state
of the cochlear duct. It is thought that
these neurons possibly do not respond
to sound (Robertson et al., 1999)
2.2.1 Efferent pathways
Delineation of the efferent auditory pathways, and specifically the crossed and
uncrossed olivocochlear bundles (OCB), dates back to the mid 1940’s
(Rasmussen, 1945; 1960). Since then auditory physiologists have developed
a specific interest in the OCB pathways and its function. Efferent innervation
of the cochlea in mammals is provided by the OCB. Efferent fibres transmit
impulses from the brain to the cochlea. These fibres arise from neurons
whose cell bodies are located in the brain stem, mostly on the side opposite
from the ear that they innervate. Once the efferent fibres reach the cochlea,
they branch out to form a large number of nerve endings. Two distinct
populations of efferents have been identified in the cochlea (Warr & Guinan,
1979). The two efferent divisions differ with respect to a number of
morphological features, including the pattern of development, the size of their
cell bodies, brainstem locus of origin, the preferred lateralization of projection
20
to the periphery and the postsynaptic targets within the auditory periphery
(Sahley et al., 1997a).
Hence the efferent olivocochlear system is divided into two subsystems,
namely the lateral olivocochlear (LOC) system and the medial olivocochlear
(MOC) system (Guinan, Warr & Norris, 1983). The pathways of both
subsystems originate from the superior olivary complex (SOC), where their
axons extend through the reticular formation (Warr, 1992) and join to form the
olivocochlear bundle (OCB) close to the floor of the fourth ventricle
(Rasmussen, 1947; Gacek, 1961). The OCB is made up of both fibers from
LOC (63%) and from MOC neurons (37%) (Aschoff & Ostwald, 1987; Nakai &
Igarashi, 1974; Warr, 1992). Table 2.2 summarizes the difference between
LOC and MOC efferent subsystems.
21
Table 2.2 The difference between LOC and MOC efferent subsystems
LOC efferents
MOC efferents
Arises from
• Lateral superior olivary (LSO) nucleus complex in the upper pons
• The majority of MOC neurons are located in the medial
periolivary region surrounding the medial superior olivary (MSO)
Type of axons
• Unmyelinated
• Myelinated
Innervates
• IHC’s (89%-91%) of the ipsilateral cochlea (Warr, 1992). These
efferents do not synapse directly at the basal surface of the IHC’s
but at specialized postsynaptic regions on afferent type I dendrites
(Liberman, 1980; Pujol & Lenoir 1986)
• MOC fibres are unidirectional (Brown, 1987) and synapse with
the base of the outer hair cells (OHC’s) of the organ of Corti
(Warr, 1975; Warr et al., 1986).
• They innervate both cochleae (but mostly in the contralateral
cochlea) and synapse with the OHC’s
Possible
Function
• The way the lateral efferents synapse with dendrites of the auditory
ganglion neurons clearly points to a postsynaptic regulation of the
IHC-auditory nerve synapses. Because lateral olivocochlear axons
project to inner radial (type I) afferent fibres that communicate with
IHC’s, they do not directly influence hair cell activity (Spangler &
Warr, 1991; Warr, 1992). However, they may affect neural activity
resulting from IHC stimulation by virtue of their synapses with the
inner radial fibres (Chen & Bobbin, 1997).
• MOC fibres play a significant role in altering and modulating the
cochlear micromechanics that are discussed in depth elsewhere
in this chapter. The MOC synaptic terminals at the hair cell body
include innervation with a portion of the cistern structure of the
OHC that is thought to enable the OHC to change length (Lim,
1986).
Tonotopic
organization
• LOC neurons have the same tonotopic organization of the LSO
neurons on the ipsilateral side and it is known that the LSO is the
only nucleus in the SOC that receives a complete ipsilateral
frequency representation from the ventral cochlear nucleus (Warr,
1992).
• Like the LOC fibres, the MOC fibres also have tonotopic
organization by connecting areas with similar characteristic
frequencies (Warr,1992).
22
As mentioned in table 2.2, the MOC efferent pathway is mainly involved in
OHC physiology. Thus the focus will be on the physiology of the efferent
system. The physiology of the afferent system, which synapses with IHC, will
not be discussed further in this chapter.
2.3 Physiology of the efferent system
Even though the physiologic role of each efferent system is not yet completely
understood, much can be deduced by taking into account what is now quite
clear about the function of each type of hair cell and the feedback loop of the
efferent systems. To clearly comprehend the physiology of the efferent system
in terms of efferent stimulation, it is critical to understand some neurochemical
characteristics of the efferent system.
2.3.1 Neurotransmitters of the efferent system
Neurochemically, both the lateral and the medial efferent system have been
found to be cholinergic, using acetylcholine as their neurotransmitter. The
axodendritic
synapses
of
the
lateral
efferents
consist
of
several
neurotransmitters, namely acetylcholine (ACh), dopamine and gama-aminobutyric-acid (GABA) and neuropeptides such as calcitonine gene related
peptide (CGRP), dynorphins and enkephalins.
2.3.1.1
The
Neurotransmitter of the lateral efferent neurons
lateral
efferent
neurotransmitters
neurons
depending
can
on
synthesize
different
and
release
physiological
different
conditions.
Enkephalins are negatively coupled to adenylate cyclase activity (Eybalin,
Pujol & Bockaert, 1987a). Lateral efferents have an inhibitory function and
may be related to the release of metaenkephalin during noise exposure
(Eybalin, Rebillard, Jarry & Cupo, 1987b), or the increase in the perilymphatic
enkephalin in noise-stimulated animals (Drescher, Drescher & Medina, 1983).
It is thought that the enkephalinergic lateral efferent is responsible for the
firing of auditory nerves or in protecting against abnormal spontaneous firing
23
(tinnitus). The release of dopamine in noisy conditions has been observed
(Vincent-Torres et al., 1993), indicating a protective effect against noiseinduced toxicity. It also assists in the repair of auditory dendrites and their
synapses with OHC’s (Pujol, Zajic, Dulon, Rapheal, Altschuler & Schacht,
1991; Pujol et al., 1993).
2.3.1.2 Neurotransmitter of the medial efferent neurons
The axo-dendritic synapses between the medial efferents and outer hair cells
are known to contain ACh, CGRP and GABA. Although GABA and CGRP
may play some role, most of the protective effects of the system seem to be
ACh-dependent. When applying ACh at efferent synaptic terminals, it mimics
the effects of electrical stimulation of the olivocochlear bundle (Bobbin &
Konishi, 1971). It reduces the compound action potential and alters cochlear
micromechanics (Kujawa, Glattke, Fallon & Bobbin, 1992). Neuromodulation
is regulated by the presence and release of ACh, and the presence of
synthetic and degradative enzymes (Altschuler, Kachar, Rubia, Parakkal &
Flex, 1985). ACh has a rapid synaptic effect, whereas neuroactive peptides
show a more slow and sustained action (Musiek & Hoffman, 1990).
2.3.2 Efferent Stimulation
Most of the earlier studies defining the physiology of the efferents of the
LOC and MOC pathways was performed on animals. Although much is
understood about the neuroanatomy of the LOC efferents, the influence of the
LOC on the auditory system still remains unclear. The possibility of a
postsynaptic control over type I afferents of the IHC’s has been suggested
(Liberman, 1980). Sahley et al. (1997) proposed a model of lateral efferent
action wherein LOC efferents release ACh, which causes hyperpolarization of
the type I radial afferents. Chen et al. (1997) suggested that the LOC efferents
release afferent neurotransmitters that depolarize the type I fibres. The
enkephalins and dopamine from the lateral efferents were thought to protect
the auditory nerve dendrites from acoustic trauma damage and excitotoxity
(Pujol, 1994). Much more is understood about the physiology of the MOC
24
efferent functioning, however, and this section will focus more on the function
of this system.
To better understand the effect of efferent auditory stimulation, and ultimately
its effect on OAE’s, it is necessary to understand what happens when the
OCB is stimulated (electrically or acoustically) and the response of OHC’s to
olivocochlear (OC) stimulation. Galambos (1956) first described the effect of
electrical stimulation on efferent fibres that project to the cochlea. He reported
a reduction of auditory nerve responses to acoustic stimulation on concurrent
electrical activation of these fibres by comparing the compound action
potentials (N1) stimulated acoustically with a click to those elicited under
similar conditions in the presence of recurring electrical shocks to efferent
fibres. Reduction in N1 amplitude was observed in the presence of electrical
stimulation and thus signified an inhibitory role of the OCB (Galambos, 1956).
It is now clear that olivocochlear stimulation of the inner ear results in two
different efferent effects, namely slow and fast effects of OHC innervation.
OHC’s move when electrically stimulated by shortening when depolarized and
lengthening when hyperpolarized (Brownell, 1983). This OHC movement is
extremely fast effect and is known as the fast effect of OHC innervation
(Reuter & Liberman, 1995). The change in electromotility is ascribed to
changes in voltage across the OHC membrane (Santos-Sacchi, 1991;
Kalinec, Khanna, Ulfedahl & Teich, 1992) and appears to be produced by
molecular “motors” along the length of the cell (Dallos, Evans & Hallworth,
1991). Single hair cell studies have observed nonlinearities in the
electromotility of OHC’s (Evans, 1990; Santos-Sacchi, 1993). These
nonlinearities originate in the electromotile response inherent in the
transducer channel that provides the voltage changes that drive motility
(Santos-Sacchi, 1993). Slow motility effects of OHC efferent innervation can
be described as gradual changes in length that occur over the course of
several seconds (Ohnishi et al., 1992). Zenner et al., (1989) suggested that
activation of the efferents at the bases of the OHC’s might produce this
response and that molecular mechanisms different from those of fast motility
25
may produce this slow effect (Zenner, 1988). Both the slow and fast effects of
OHC innervation are discussed more thoroughly later in this chapter.
Acoustical stimulation of OHC’s via the travelling wave to the cochlea is
believed to ultimately depolarize and hyperpolarize the ionic current in OHC’s
by way of K+ and CA2+ regulation (Ashmore, 1988). Cell depolarization
results in OHC contraction and an enhancement of the upward movement of
the basilar membrane (Evans & Dallos, 1993). Hyperpolarization results in an
increase of OHC length and decrease in width that restores the cell to its
resting length (Ashmore, 1987). It is believed that medial efferent induced
hyperpolarization counteracts the amplifying effect of OHC activity (Szikai et
al., 1993). Stimulation of MOC efferents results in a release of ACh at OHC
synapses that is responsible for K+ efflux from the OHC’s, thus
hyperpolarizing them (Ashmore, 1998). Hyperpolarization of OHC’s therefore
causes the cells to lengthen (Evans & Dallos, 1993) and reduce the gain that
could result from OHC electromotility. It is known that medial efferent
stimulation also has an inhibitory effect on IHC’s. It reduces IHC sensitivity
and broadens IHC tuning (Brown, Nuttal & Masta, 1983).
2.4 The role and clinical relevance of the efferent auditory system
As described above, the MOCS has an inhibitory effect on the OHC’s, but this
effect depends on the stimulus conditions and in some cases actually
modulates the OHC electromotility. The result can be described as either
inhibitory or enhancing. In quiet backgrounds, MOC activation by noise or
electrical stimulation at the midline results in the suppression of N1 action
potential and TOAE’s (Galambos, 1956; Collet, Gartner, Moulin & Morgon,
1990a). In noisy listening environments, MOC activation decreases
physiological thresholds and increases the response amplitudes to transient
signals (Nieder & Nieder, 1970; Winslow & Sachs, 1987, 1988; Dolan &
Nattall, 1998; Kawase, Delguette & Liberman, 1993; Kawase & Liberman,
1993). Whether suppressive or enhancing, it is now generally presumed that
the MOC acts by reducing the motility of the OHC’s (i.e. it acts as a cochlear
26
amplifier). In situations where the effects are enhancing, the result is an
inhibition of the OHC response to the concurrent, sustained masking noise,
resulting in an unmasking of the response to the transient target stimulus
(Winslow & Sachs, 1987, 1988; Kawase et al., 1993; Kawase & Liberman,
1993).
Although the physiological function of the efferent auditory system is well
documented, its specific biological role remains uncertain. In view of the
preferential innervation of the OHC’s by MOC fibres, it has been
hypothesized that the stimulation of the medial efferents alters IHC sensitivity
indirectly by altering the micromechanical properties of the OHC’s. It is well
established that the length, tension and the stiffness of the OHC’s are under
the control of the MOC bundle. The MOC bundle enhances the auditory
sensitivity, especially for low-level stimuli at 30 to 40 dB sensation level (SL)
(Brownell 1990; Guinan 1986; Kim 1986).
There is also increasing evidence suggesting that the MOCS enhances
frequency
resolving
capacity
(Micheyl
&
Collet,
1996)
and
vowel
discrimination, especially in noisy background environments (Muchnik et al.,
2004; Sahley , Nodar & Musiek, 1997c). The presentation of contralateral
noise has been found to enhance speech-in-noise intelligibility in subjects
with normal hearing. This improvement was minimal in patients with deefferented ears (Giraud, Collet, Chery-Croze, Magnan & Chays, 1995) A
relationship has been found between the improvement in perceptual
performance on speech-in-noise intelligibility tasks (brought about by
contralateral noise) and the effectiveness of the MOCS feedback (as
assessed by strength of contralateral suppression of TOAE’s) (Giraud, Collet
& Chery-Croze, 1997; Micheyl, Morlet, Giraud, Collet & Morgon, 1995). In
addition to this, a correlation has been established between contralateral
suppression of OAE’s and detection-in-noise thresholds (Micheyl & Collet,
1995). These correlations suggested that normal-hearing subjects with the
strongest improvement in speech-in-noise intelligibility with contralateral
noise were those with the most robust MOCS feedback (Micheyl & Collet,
1996). It has been suggested that this improvement in speech intelligibility in
27
noise, when the MOCS is activated, is the result of suppression in the
response of fibres to continuous noise, which in turn become more
responsive to transient stimuli such as speech (Kawase, Delgutte, &
Liberman, 1993; Kawase & Liberman, 1993). Tolbert et al. (1982) suggested
that the olivocochlear bundle (OCB) optimizes the detection of interaural
intensity differences for higher frequency signals by increasing, within the
cochlea, the interaural difference, reaching the lateral superior olive (LSO).
Thus it is supposed that a better comprehension of the medial efferent
system and its pharmacological manipulation may be beneficial for subjects
struggling with speech discrimination difficulties in noisy environment, despite
normal pure tone audiometric thresholds.
A number of research reports have suggested that medial efferent stimulation
also serves as a protective function against high levels of acoustic stimulation
(Canlon, 1996; Subramanian et al., 1993; Liberman, 1991). The long-standing
observation that electrical stimulation of the olivocochlear (OC) efferents to
the OHC’s raises acoustic thresholds in the cochlea (Galambos, 1956) has led
to speculation that activation of this pathway might protect the ear from
acoustic overstimulation (Thrahiotis & Elliot, 1969). Studies in animals have
shown that the crossed olivocochlear efferent system can reduce the cochlear
neural desensitization caused by loud sounds. Several authors in the past
have indicated that a hearing loss produced by loud sound in one ear of a
guinea pig can be reduced by simultaneously presenting a non-traumatising
sound to the other ear (Cody and Johnstone, 1982; Rajan and Johnstone,
1983, 1989). According to these authors, it was highly unlikely that this effect
was due to the action of middle ear muscles, since their experiments were
carried out in paralysed animals and at high frequencies where contraction of
the middle ear muscle has very little effect (Moller, 1962). This reducing effect
was
also
abolished
by
systemic
administration
of
strychnine,
a
pharmacological blocker of the olivocochlear pathway, (Cody and Johnstone,
1982; Rajan and Johnstone, 1983), or by lesioning these neurons at the floor
of the fourth ventricle (Rajan and Johnstone, 1989). These findings led
authors to believe that a “contralateral protective effect” may be mediated
28
within the cochlea by the MOCS of efferent neurons, which cross the brain at
the floor of the fourth ventricle.
It is thought that forces generated by the OHC’s act to partially cancel friction
within the organ of Corti, and that these forces are generated by an electromechanical transduction process which relies on the receptor current through
the OHC’s, and that any disruption of the mechano-electrical transduction
process producing these receptor currents should reduce vibration and
produce elevation of neural thresholds. Patuzzi, Yates and Johnstone (1989b)
studied the link between disruption of the mechano-electrical transduction of
OHC’s and the elevation of neural thresholds and found that the amount of
noise-induced hearing loss is highly correlated with the amount of disruption
of OHC mechano-electrical transduction. These authors suggest that it is
possible that the efferent neurons of the MOCS act to protect the cochlea from
acoustic trauma by reducing the disruption of the mechano-electrical
transduction at the apex of the OHC’s. Based on these observations, Patuzzi
and Thompson (1991) monitored the influence of simultaneous contralateral
sound on the changes in the neural response and low-frequency microphonic
response produced by acoustic trauma in the first turn on the guinea pig
cochlea. They reported that the MOCS may operate to protect the ipsilateral
ear by reducing the inactivation of these channels. This was indicated by a
smaller decrease in the low-frequency microphonic by reducing the loss of
electrical drive to the active process within the cochlea caused by acoustic
trauma (Patuzzi & Thompson, 1991).
Rajan (1991) found that activation of the olivocochlear (OC) efferent system in
anaesthetised animals minimized the acute and temporary threshold shifts
(TTS’s) seen with hazardous noise exposure. The role of olivocochlear
activation in protecting the ear in animals or humans from the damaging
effects of acoustic overexposure is much less understood, although some
research has been done in attempt to explain the protective role of the OC
efferent system (Handrock and Zeisberg, 1989; Liberman and Gao, 1995;
Zeng et al., 1997a). These studies all found that chronically de-efferented
animals can show greater permanent threshold shifts (PTS’s) than identically
29
exposed animals without sectioning of the efferents. This protective role of
the OC bundle has led authors to speculate that the MOCS may also play a
role in the reduction of threshold shifts seen with noise exposure. This
protective effect has been referred to as “conditioning” or “toughening” of the
ear (Canlon 1996; Subramaniam, Henderson & Henselman, 1996). The
toughening effect of the ear has been studied by conditioning animals with a
daily exposure to moderate-level, non-damaging acoustic stimuli for several
days and then exposing them to a traumatic acoustic stimulus of a shorter
duration (Canlon, 1996). Canlon (1996) found that when the animals were
conditioned before the traumatic acoustic exposure, less severe PTS’s could
be observed than in the animals that were not conditioned previously.
A form of protection is also demonstrable in a very different paradigm, the
repeated-exposure paradigm, in which animals are exposed to a mildly
traumatic acoustic stimulus on a daily basis. In the repeated-exposure
paradigm, protection is seen as a daily decrease in the acute threshold shifts
measured immediately after each day’s noise exposure. As the daily
exposures continue, the animals develop a slowly growing residual threshold,
as seen from the deterioration of thresholds measured before each daily
exposure, which ultimately becomes a PTS (Boettcher, Sprongr & Salvi,
1992).
Thus the protection measured in the repeated-exposure paradigm
appears to be a compound threshold shift, consisting of relatively large TTS
and smaller accumulating PTS. Boettcher et al. (1992) suggested that this
slow progression in PTS may involve slowly progressing damage to the
stereocilia on IHC’s and/or OHC’s, causing a decrease in ion fluxes during
daily noise exposure and decreasing the TTS each day.
Zeng et al. (1997) explored the possible OC system’s role in protection by
using a combination of the repeated-exposure and condition/trauma
approaches. The animals were exposed daily to a mildly traumatic stimulus,
and DPOAE’s were measured before and after each exposure. Then, after the
last of these daily exposures, the animals were exposed to the same stimulus
at a much higher SPL, and the final PTS was measured several days later.
The OCB was cut in the inferior vestibular nerve of one group of these
30
animals, but there were only three successful de-efferentations, of which only
two completed the whole protocol. Zeng et al. (1997) observed that, in the
repeated-exposure, control animals showed reducing compound threshold
shifts (CTS’s) (i.e., protection). In contrast, the three de-efferented animals
showed less reduction of CTS (less protection), but protection was not
abolished, even though the de-efferentation was essentially complete. As for
the final PTS after the high-level traumatic exposure, the two de-efferented
animals that completed the protocol showed significantly higher PTS’s than
the control animals with the same noise exposure. These studies have all
provided promising evidence suggesting that activation of the MOCS serves a
protective function in the mammalian auditory periphery against high-level
auditory stimuli and that de-efferented animals are more vulnerable to
acoustic injury, regardless of their noise-exposure history.
To add support to this theory, Maison and Liberman (2000) examined intersubject variability of vulnerability to acoustic injury in relation to differences in
olivocochlear reflex strength. They used DPOAE suppression to measure
MOC reflex strength in normal hearing awake guinea pigs two days before
exposing them to 109 dB SPL noise for four hours. They then measured
compound action potentials one week after the exposure, allowing recovery
from temporary noise-induced hearing loss (Maison & Liberman; 2000). From
their results, they suggested that the strength of the MOC reflex is a major
contributor to the differences in vulnerability to acoustic overexposure.
The discovery of slow effects of OC activation on the inner ear (Sridhar,
Liberman, Brown & Sewell, 1995) led researchers to believe that there might
be a direct relationship between this slow effect and the efferent protection
from acoustic injury. Pharmacological and physiological evidence suggests
that both the slow and fast effects that involve conductance changes in the
OHC’s are affected by the interaction of acetylcholine with the same receptor
on the OHC’s, and both are mediated by the MOCS (Sridhar et al., 1995). The
classical fast effects of OC activation are known with an onset and offset of
suppression in the order of 50 to 100 ms (Wiederhold & Kiang; 1970). Slow
effects of OC activation (slow suppression of cochlear response) appears with
31
a time constant of 30 to 70s and can last for 1 to 2 minutes after the
termination of the OC activation. There are two differences between fast and
slow effects. Slow effects are largest for cochlear regions tuned to 14 kHz
and are minimal below 10 kHz, whereas fast effects peak from 6 to 10 kHz.
Slow effects are maximal for OC stimulation of 1 to 2 minutes and are virtually
extinguished when duration exceeds 4 minutes, whereas fast effects remain
essentially undiminished by OC stimulation in excess of 10 minutes (Sridhar
et al., 1995).
Reiter and Liberman (1995) examined the effects of exposure frequency and
duration on the OC-mediated protection from acoustic exposure in guinea pigs
in order to examine the relation between this protective phenomenon and the
slow effect of OC stimulation. From their results they observed that TTS
protection from brainstem electrical stimulation was only demonstrable for
exposure frequencies above 8 kHz and for exposure durations of less than 2
minutes, which proved that cochlear protection arises from the slow effects of
OC stimulation rather than from the classic fast effect of OC action on the
auditory periphery (Reiter & Liberman, 1995). If cochlear protection is mainly
contributed to by the slow rather than the fast effect of OC stimulation,
protection from acoustic overexposures would be predicted for exposure
duration of 1 to 2 minutes and not for longer exposure durations of more than
4 minutes.
2.5 OAE suppression as a measurement of efferent function
Because descending medial efferent fibres preferentially terminate on OHC’s,
the prevailing view is that the micromechanical properties of the OHC’s are
under direct control of efferent innervation. Since OAE’s are thought to reflect
these dynamic properties, it has been hypothesized that activating the medial
efferents would produce alterations to cochlear micromechanics and, hence,
to OAE’s. Indeed, there is now a great body of evidence that auditory
stimulation, presented ipsilaterally, bilaterally or contralaterally (Warr, Guinan
& White,1986; Warr & Guinan, 1978; Peul & Rebillard, 1990; Liberman, 1989
32
Kujawa, Glattke, Fallon & Bobbin, 1993; Kujawa, Glattke, Fallon & Bobbin,
1994; Tavartkiladge et al., 1993;1997; Wilson, 1980; Liberman et al., 1996;
Berlin, Hood, Hurley, Kemp & Wen,1995a;1995b) results in the reduction of
the amplitude of both spontaneous and evoked OAE’s (TEOAE’s and
DPOAE’s) (Ryan et al., 1991; Collet et al., 1990). This phenomenon is called
suppression of OAE’s and there is evidence that it is mediated through the
medial efferent system (Kujawa, Glattke, Fallon & Bobbin, 1992; Veuillet et
al., 1991; Warren & Liberman 1989). Thus, it has been suggested that the
suppression of OAE’s could serve as an objective, non-invasive clinical test
for the exploration of the non-linear micromechanics of OHC’s and the clinical
neurological evaluation of the auditory brainstem in general, and descending
efferent bundle specifically.
As mentioned before OAE’s, can be suppressed when auditory stimuli are
applied ipsilaterally, contralaterally or bilaterally. In relation to contralateral
stimulation, ipsilateral masking can result in more pronounced suppression of
evoked OAE’s (Kemp & Chum, 1980; Tavartkiladge, Frolenkov, Kruglov &
Artamasov, 1994; Wilson, 1980). The mechanisms underlying this effect seem
to be twofold. One perspective is that the suppression results from
intracochlear masking processes, whereas from another perspective it
appears to be mediated through the olivocochlear system. There are two
approaches for assessing the effect of ipsilateral suppressors on OAE’s. The
first approach, known as the ipsilateral simultaneous masking paradigm, uses
a suppressor of one or more tones that is presented simultaneously with an
OAE-evoking stimulus, at nearby frequencies. The simultaneous masking
experiments are recorded with custom-designed acoustic probes, consisting
of a microphone and two electroacoustic transducers, which provide the same
ear with the suppressor signal and the recording-evoked OAE. With the
second approach, the ipsilateral forward masking paradigm, a suppressor
signal (ranging from a click to a relatively extended duration noise band), is
presented to an ear prior to the presentation of an OAE–evoking stimulus
(Tavartkiladge et al., 1994; Berlin et al., 1995).
33
Contralateral suppression is more commonly used in both clinical and
experimental projects than ipsilateral suppression. The reason for this is that
ipsilateral suppression measurements require special equipment (probe) and,
as stated by the authors, suppression of TEOAE’s could not be attributed only
to the MOC bundle but also to intracochlear processes.
The mechanism underlining binaural suppression is similar to that of
contralateral suppression. In addition, the full effect of the MOCS is brought to
bear on the OAE elicited by bilateral acoustic stimulation. The approach
typically used for bilateral suppression of TEOAE’s uses a short-duration burst
of noise that precedes a click stimulus, with a duration that is sufficient to elicit
an efferent response. The typical latency for eliciting MOC effects is
approximately 100 ms from the onset of an acoustic stimulus (Liberman et al.,
1996). Berlin et al. (1995b) studied the effect of binaural noise on TEOAE’s.
They used a technique of presenting one ear with linear clicks that were
preceded by binaural, ipsilateral, or contralateral noise bursts with a duration
of 408 ms. In their experiment they found that the greatest amount of
suppression was measured using binaural noise busts with the click train
onset no later than 5 ms after the noise burst ended, and no suppression was
observed when the time period between the end of the noise and the click
onset was 1000 ms or longer.
Ipsilateral noise in the same time frame
showed less suppression than noise presented bilaterally and the least
amount of suppression was observed with contralateral noise bursts (Berlin et
al., 1995b)
Though the use of bilateral suppressors elicits more OAE suppression than
contralateral suppressors and has been proved to measure the effect of the
medial efferents on OHC’s (acoustic stimulation of MOCS) effectively, the
specific techniques used in measuring bilateral suppression with TEOAE’s
(binaural noise bursts with a click onset 5 ms after the noise burst ceases) can
not be used when measuring the time course of the medial efferent effects
with the use of prolonged continual noise stimulation. Binaural stimulation also
has the same limitation as ipsilateral stimulation, namely intracochlear
processes which contribute to the suppressive response. Thus this chapter
34
will focus more on the different characteristics of contralateral suppression of
OAE’s.
2.5.1 Contralateral suppression
Since the fundamental paper by Buno (1978), first described auditory nerve
activity influenced by contralateral sound stimulation, contralateral auditory
stimulation and the effect on peripheral auditory responses have been
extensively studied. Through the use of animal (Buno, 1978) and human
studies (Folsom & Owsley, 1987), it became evident that contralateral auditory
stimulation alters the afferent nerve fibre response of the opposite ear. More
recently, researchers discovered changes in OAE’s of humans (Mott, Norton,
Neely & Warr, 1989) and in animals (Puel & Rebillard, 1990; Kujawa et al.,
1992). In numerous studies of the effect of contralateral stimuli on various
parameters of OAE’s in human and in animals, it was evident that the
stimulation of the ear opposite the one in which emissions are being
measured reduces the amplitude of the OAE (Berlin et al., 1993a; Berlin,
Hood, Wen, et al., 1993b; 1994; Collet et al., 1990b; Collet, Veuillet, Bene &
Morgon, 1992; Harrison & Burns, 1993; Kujawa et al., 1993; Peul et al., 1990;
Ryan et al., 1991; Veuillet et al., 1991). Contralateral suppression of OAE’s
was thought to be an ideal tool for studying the effect of contralateral auditory
stimulation, because the medial olivocochlear bundle synapses directly with
OHC’s of the organ of Corti and OHC’s are directly involved in the generation
of OAE’s (Collet, 1993; Giraud, Collet, Chery-Croze, Magnan & Chays, 1995).
2.5.1.1 Types of OAE used to measure contralateral suppression
Three types of OAE’s have been used to study the effect of contralateral
auditory stimulation, namely SOAE’s (Mott, Norton, Neely & Warr, 1989;
Moulin, 1993; Harrison et al., 1993; Irby, 1998), TEOAE’s with linear clicks
(Collet et al., 1990; 1992a, 1993; Veuillet et al., 1991; Veuillet, Collet &
Morgon,1992), non-linear clicks (Veuillet et al., 1991: Berlin et al., 1993a, b),
tone pips (Berlin et al., 1993b) and DPOAE’s (Moulin, Collet & Morgon, 1992;
Moulin, Collet & Duclaux, 1993; Chery-Croze et al., 1993).
35
Contralateral suppression of SOAE’s
Harrison et al. (1993), Irby (1998) and Mott et al. (1989) found that introducing
acoustic stimulation to the contralateral ear alters the frequency and amplitude
of SOAE’s. It has been reported that an upward shift of SOAE frequency can
be observed in the presence of a contralateral tonal stimulus (Mott et al.,
1989; Harrison and Burns, 1993; Irby, 1998). When Mott et al. (1989) used
contralateral stimulation with continuous pure tones of various frequencies
and sound pressure levels (SPL’s), they observed an upward shift in SOAE
frequency, with the greatest frequency shift when the suppressor tone was
one-half to three-eights of an octave below the SOAE frequency.
SOAE
amplitudes increased, decreased or remained unchanged in the presence of
contralateral tones that elicited these frequency shifts (Mott et al., 1989). The
researchers noted that no suppression effects could be recorded with
contralateral stimuli below 60 dB SPL and that the suppression remained
stable for up to 4 minutes of contralateral stimulation (Mott et al., 1989). With
the onset of the contralateral stimulus, an abrupt shift in SOAE frequency
could be observed, followed by a gradual decrease over the stimulus duration
and returning to the pre-stimulus SOAE frequency when the contralateral
stimulus ceased (Harrison & Burns, 1993; Irby, 1998). No consistent
relationship between the frequency of the contralateral stimulus and frequency
shifts of SOAE’s could be found (Irby, 1998). The SOAE amplitude effects of
contralateral stimulation proved to be variable (Harrison & Burns, 1993; Irby,
1998). Although contralateral effects on SOAE’s have been observed to vary
between subjects, they are repeatable and reliable (Harrison and Burns, 1993;
Irby, 1998; Mott et al., 1989).
Contralateral suppression of DPOAE’s
Brown (1988) was the first to describe the effects of continuous contralateral
low-level auditory stimulation on DPOAE’s and suggested that an efferent
effect may exist. Since then other researchers also investigated the effect of
contralateral sound stimulation on DPOAE’s and supported Brown’s (1998)
hypothesis that the suppressive effect of contralateral sound stimulation of
DPOAE’s is mediated by the medial efferent system (Peul & Rebillard, 1990).
The suppression of DPOAE’s is now clearly understood to be controlled by
36
the medial efferents (Chery-Croze et al., 1993; Moulin et al., 1993). The
overall decrease in DPOAE amplitude in the presence of a contralateral
acoustic stimulus is reported to be between 1 and 4 dB (Chery-Croze et al.,
1993; Moulin et al., 1993).
Broadband and narrowband noise have been found to be effective
contralateral stimuli in the suppression of DPOAE’s. Narrowband noise with a
centre frequency of the noise band close to that of the DPOAE is most
effective in suppressing DPOAE’s, especially when the DPOAE’S are in a
range of 1 to 2 kHz (Chery-Croze et al., 1993). Chery-Croze et al., (1993)
observed that a 3 kHz noise band showed effective suppression of DPOAE’s
in the area of 3 KHz. Contralateral noise band stimuli with centre frequencies
between 250 and 750 Hz were found to be less effective in suppression
(Chery-Croze et al., 1993). Broadband noise appears to have the greatest
effect on DPOAE’s between 1 and 3 kHz (Moulin et al., 1993; Santaolalla
Montoya et al., 1997; Williams & Brown, 1997).
An inverse relation between DPOAE amplitude and the level of contralateral
stimulus can be seen (Peul & Rebillard, 1990). Peul and Rebillard (1990)
reported that DP amplitudes decreased as the SPL’s of contralateral stimuli
increased. This increase in contralateral level showed a greater suppressive
effect when the DP primary levels were 60 dB SPL (Peul & Rebillard, 1990).
When a contralateral broadband noise of 80 dB SPL was used to suppress
DPOAE’s from 35 to 65 dB primary levels, it was noted that suppression was
most effective with primary levels of 55 dB SPL and the effectiveness thereof
decreased after 55 dB SPL, to negligible suppression at primary levels of 70
dB and greater (Puria, Guinian & Liberman,1996).
Contralateral suppression of TEOAE’s
Like SOAE’s and DPOAE’s, certain properties of TEOAE’s are altered in the
presence of contralateral auditory stimulation (CAS). The main effect of
contralateral stimuli on TEOAE’s, is the attenuation of the TEOAE amplitude
of about 1 to 4 dB (Berlin et al., 1993b; 1994; Collet et al., 1990b; 1992; Ryan
et al., 1991; Veuillet et al., 1991). Alterations in phase shifts can also be
37
observed in TEOAE’s in the presence of CAS (Veuillet et al., 1991). The
suppression of TEOAE’s in normal hearing adults shows great intra-individual
variability, but, according to several studies, 1 dB SPL is considered to be the
cut-off point for normal suppression(Prasher et al., 1994; Collet, 1993;
Micheyl & Collet, 1995). Considering 1 dB SPL as the lowest “normal” level,
the method shows a false positive rate of 6% in normal hearing subjects
(Prasher et al, 1994).
Researchers have studied the effectiveness of different types of contralateral
stimuli, such as clicks, pure tones, narrowband noise and white noise, in
suppressing TEOAE’s (Berlin et al., 1993b; Norman & Thornton, 1993). Only
low-frequency pure tones have enough energy to elicit contralateral TEOAE
suppression (Berlin et al. 1993b). This may be because low frequency puretones stimulate a larger area on the basilar membrane than higher
frequencies
and
activate
more
efferents.
Using
narrowband
noise,
suppression has been observed at hearing levels (HL’s) as low as 20 dB
(Berlin et al., 1993b). White noise, consisting of energy from 20 to 20,000 Hz
(the frequency response range of the human ear), stimulates the whole
contralateral cochlear partition and activates the largest number of MOC
efferents, thus making it the most effective stimulus in suppressing TEOAE’s
(Berlin et al., 1993b; Norman and Thornton, 1993).
Several authors have found an inverse relationship between the level of the
contralateral stimulus and the amount of TEOAE amplitude reduction (Collet
et al., 1990b; Ryan et al., 1991; Berlin et al., 1993b). These findings
concluded that the TEOAE amplitude decreases as the level of contralateral
stimuli increased, regardless of the type of contralateral stimuli. Collet et al.
(1990) used contralateral white noise stimuli with intensities ranging from 0 to
80 dB SPL and found that the TEOAE amplitude decreases are observed
from as low as 30 dB SPL. As the contralateral stimulus level increased in
intensity, a decrease in TEOAE amplitude was observed. Berlin et al. (1993b)
used narrowband stimuli at SPL’s of up to 80 dB and found a relationship
between contralateral stimulus level and TEOAE amplitudes. Norman and
Thornton (1993) measured emission with 75 dB SPL nonlinear clicks and
38
narrowband noises with intensity levels ranging from 40 dB to 60 dB sensation
level (SL). It was reported that decreases in emission amplitude only became
significant when the intensity of the narrowband noise exceeded 40 dB SL.
Ryan et al. (1991) used recordings with and without broadband noise (BBN)
from 0 dB to 70 dB SL and noticed a reduction in TEOAE amplitude from as
low as 20 dB SL, but better perceived from 50 dB SL BBN. However, when
the contralateral noise level was held constant and the stimulus level was
varied, the amount of suppression was reported to be relatively constant
(Veuillet et al., 1991)
These results were inconsistent with other findings of greater suppression of
auditory nerve responses by lower intensity electrical or acoustic stimuli, with
either saturation or lesser effect at higher intensity levels (e.g. Nieder &
Nieder, 1970; Gifford & Guinan, 1987; Warren & Liberman, 1989). In order to
address the discrepancies between previous reports of intensity effects on
suppression of emissions versus auditory nerve responses, Hood, Berlin,
Hurley, Cecola & Bell (1996), examined suppression of TEOAE’s in human
subjects whilst systematically varying both the emission-eliciting stimulus and
the suppressor noise over a wide range of click and suppressor noise levels.
This was done to determine the appropriate click and noise levels of TEOAE
suppression studies. In their study the authors reported that greater amplitude
suppression for emissions was found with lower intensity level clicks when the
intensity of the contralateral noise was at or near 60 dB SPL (Hood, Berlin,
Hurley, Cecola & Bell, 1996a). The sound pressure level of the click used to
evoke TEOAE’s can also affect the amount of suppression resulting from a
contralateral stimulus. Click stimulus intensities of below 65 dB SPL have
been found to be the most effective when recording TEOAE suppression
amplitudes with contralateral white noise stimuli (Hood et al., 1996a). Hood et
al. (1996a) suggested using 55 or 60 dB peak SPL with the overall intensity
level of the noise set at, or 5 dB higher than, the click intensity. Regardless of
the intensity selected for measuring decreases in TEOAE amplitude, it is
imperative to avoid using high click intensities (e.g. 70 dB SPL), in order to
minimize the risk of major participation of the middle ear muscle reflexes, as
discussed elsewhere in this chapter.
39
Studies by Berlin et al., (1993) revealed that narrowband noise with different
frequency centres all have the same effect on the TEOAE spectrum This
implies that contralateral suppression of TEOAE’s is not frequency specific.
Although the contralateral suppression effect of TEOAE’s does not seem to be
tuned, the greatest effects occur between 1000 and 4000 Hz in the TEOAE
spectrum (Berlin et al., 1993b; Collet et al., 1990b; Norman & Thornton,
1993). This may be due to greater density of MOC efferent innervation of
OHC’s in the area of the cochlea that responds to this frequency range, which
implies that more efferent control may exist in this area (Guinan, Warr &
Norris, 1983; Liberman & Guinan 1998; Warr et al., 1986). The second reason
why this frequency range seems to create a greater contralateral suppression
effect may be because a greater efferent response may be generated from
the area of the cochlea that has the greatest sensitivity, which is the cochlear
portion between 1000 and 4000 Hz (Fletcher and Munson, 1933).
Normally nonlinear click stimuli are used to evoke emissions when measuring
TEOAE’s. Clicks are presented in sets of four with the first three at the same
SPL and phase and the last click, 180 degrees out of phase with the
preceding clicks and at an SPL 10 dB greater than the first three clicks. The
reason for the use of this mode is because the nonlinear stimulus has the
advantage of largely eliminating the stimulus artifact in the recording(Berlin et
al., 1993). The last click of the linear mode evokes a larger response than the
first three, resulting in some growth in the emission between the third and the
fourth clicks. This effect of growth can affect suppression in the presence of
contralateral stimuli(Berlin et al., 1993). Another disadvantage with nonlinear
clicks is the higher level of residual noise (Moleti et al, 2002) which leads to
lower response reproducibility and a lower signal-to-noise ratio (SNR). Hoth et
al. (2007) investigated TEOAE’s at stimulus levels ranging from 83 dB SPL
down to individual response thresholds, using linear and nonlinear recording
methods. They found that, when using stimulus levels above 70 dB SPL, the
TEOAEs recorded in linear mode were contaminated with stimulus artefacts.
They suggested that when lower stimulus levels are used, the linear mode
proves to be better suited for signal detection due to inherent lower noise
40
levels. When using linear clicks at lower stimulus levels (65 dB peak SPL or
less)
in
the
evaluation
of
contralateral
suppression
effects,
fewer
contaminated responses were generated (Berlin et al., 1993b).
2.6 Confounding variables in measuring contralateral OAE Suppression
Considering that OAE’s are measured in the ear opposite to the stimulus, it is
imperative to acknowledge the confounding variables, namely the acoustic
reflex and transcranial acoustic crossover, both of which may influence
suppression measurement.
Elicitation of the middle ear acoustic reflex
The acoustic reflex can also be described as an efferent feedback loop that
may affect the response of the auditory system by reducing energy
transmission through the middle ear (Borg, 1973). Several researchers were
initially concerned that the acoustic reflex may be responsible for the
reduction in OAE amplitude resulting from contralateral stimulation (Berlin et
al., 1993; Veuillet et al., 1991; Harrison & Burns, 1993). The acoustic reflex
results in a reduction in the transmission power of the ossicular chain and may
be responsible for the attenuation of OAE amplitude observed as a result of
contralateral stimulation, because OAE’s travel backwards through the middle
ear. Numerous studies have proven the possibility of the acoustic reflex
playing a significant role in contralateral suppression of OAE’s to be doubtful.
The stimuli necessary to produce OAE suppression are presented at sound
pressure levels below the level required to elicit acoustic reflexes (Berlin et al.,
1993b; Collet et al., 1990; Hood et al., 1996a; Norman & Thornton, 1993;
Veuillet et al., 1991). In addition to this, several researchers have observed
contralateral suppression of OAE’s in subjects with paralyzed or severed
stapedius muscles (Giraud et al., 1995; Veuillet et al., 1991). Guinan et al.
(2003) used stimulus SFOAEs in humans to distinguish medial efferent from
acoustic reflex effects by relying on group-delay differences in their respective
latencies. The results of these investigators indicated that efferent effects
were mixed in terms of MOC efferent and acoustic reflex contributions and
that the acoustic reflex dominated the effects for MOC noise elicitors of 55 dB
41
SPL or above. If CAS of 55dB SPL or above will result in acoustic reflex
elicitation, lower CAS intensities may be more useful in the investigation of the
MOC efferent effects.
Acoustic crossover
Another confounding problem that has concerned researchers was that
contralateral suppression of OAE’s may be due in part to masking from the
acoustic crossover. However, many animal studies have provided evidence
that the contralateral suppression effect on OAE’s results from OCB activation
and is not the result of acoustic crossover (Peul & Rebillard, 1990; Kujawa et
al., 1993). Acoustic crossover has also been investigated using unilateral
totally deaf patients where the EOAE’s were recorded in the healthy ear with
contralateral stimulation of the deaf ear. No effects on the ipsilateral OAE
were found with CAS of up to 80 dB SPL intensity white noise, thus ruling out
any crossover influences (Collet, 1993). Velonovsky (1998) found no
significant acoustic crossover for contralateral noise up to 85 dB. In addition to
this, it has been shown that insert earphones can also reduce the influence of
acoustic crossover. If standard ear phones such as TDH-39 with MX 41-AR
cushions are used, crossover by bone conduction or even partly by air
conduction could conceivably take place at levels as low as 40 dB HL. When
insert earphones are used, the amount of interaural attenuation increases to
80 to 82dB HL thus minimizing leakage of air-conducted noise to the opposite
ear due to the seal in the ear canal, but more importantly there is less
opportunity for bone-conducted
sound transmission due to material
differences and surface area contact difference between supra-aural and
insert earphones.
2.7 Duration of contralateral suppression
Amongst the previously described factors that influence the OAE suppression
effects when stimulated contralaterally, one can include contralateral stimulus
duration effects. Before describing these characteristic in OAE’s, it is
necessary to review existing literature on the duration of MOCS efferent
42
effects when the MOCS are electrically or acoustically stimulated for different
durations.
Since Wiederhold and Kiang (1970) reported that the decrease in the
discharge rate of auditory neurons under electrical stimulation of the crossed
olivocochlear bundle persisted for the duration of the delivery of the electrical
shocks (350 seconds), some authors developed an interest in the duration
characteristics of the olivocochlear efferent effects, whether this suppression
effect has the same temporal characteristics when the OCB is stimulated
acoustically, and if this effect is prone to response adaptation over time.
Attempts had been made in the past to study the duration characteristics of
the MOCS efferent effects (suppression) in animals and humans using
different recording methods, ranging from cochlear compound action potential
(CAP) measurements (Puria, Guinan & Liberman, 1996), ensemble
background activity (EBA) measurements of the VIII nerve from an electrode
implanted on the round window (Da Costa, Chibios, Erre, Blanchet, De
Suauvage & Aran, 1997) and EOAE’s (DPOAE and TEOAE) (Giraud, Collet
& Chery-Croze, 1997; Moulin & Carrier, 1998) while delivering constant CAS.
These methods were used to gain insight into the response adaptation
properties of olivocochlear neurons during ongoing stimuli of long duration.
2.7.1 Duration
of
contralateral
suppression:
measured
with
electrophysiological recordings (electrical stimulation)
Puria et al. (1996) studied the suppression of cochlear CAP during 12
seconds of contralateral broadband noise in anaesthetized cats and found
that suppression reached a constant, steady state after two seconds of the
onset of contralateral noise. When the contralateral noise was turned off, the
suppression disappeared in less than 0.62 seconds (Puria et al., 1996). In
contrast to this supposed steady state effect, it has been suggested that
suppression increases during the first 60 to 80 seconds after contralateral
broadband noise onset and then decreases, as measured during 500 seconds
of CAS in guinea pigs (Kujawa, Glattke & Fallon, 1993). Studies of duration
properties were further influenced by the discovery of two different effects,
namely the fast and slow effects on the MOCS when electrically stimulated
43
and measured on the CAP (Shridar et al., 1995). These fast and slow effects
of OC stimulation in guinea pigs are illustrated in figure 2.2.
Figure 2.2 Slow and fast effects on CAP amplitude when the OCB is
electically stimulated. Figure taken from Shridhar et al. (1995)
As illustrated in figure 2.2, the CAP amplitude was reduced to less than 50%
of the control values (‘fast’ effect onset) almost immediately after the electrical
stimulation onset. From there on the CAP amplitude continued to decline
slowly by a further 20% (‘slow’ effect onset). The CAP amplitude remained
depressed for up to two minutes after the cessation of electrical stimulation.
The fast effect of OCB stimulation reached its maximum within 100 ms (fast
effect offset) of efferent electrical stimulation onset and persisted during the
efferent electrical stimulation. At the efferent electrical stimulation offset, the
fast effect was extinguished within 100 ms, whereas the slow effect did not
return to preshock control values until almost 100 seconds later (slow effect
offset) (Shridhar, Liberman, Brown & Sewel, 1995).
Da Costa et al. (1997) further studied these slow and fast components of OC
stimulation by measuring ensemble background activity (EBA) of the VIII
nerve from electrodes at the round window of guinea pigs and comparing EBA
measurements with and without a contralateral low-level broadband noise.
They observed a rapid decrease in EBA value with a latency of less than ten
milliseconds after the onset of the contralateral noise stimulation (fast effect
44
onset). At the offset of contralateral noise stimulation, EBA rapidly returned to
the control values at a similar latency. With longer contralateral broadband
noise stimulation (one minute or longer), EBA presented, after the fast
decrease, an additional slower decrease (slow effect onset) and remained
constant for more than two hours (steady state effect) of contralateral
stimulation. At the offset of contralateral noise stimulation, EBA returned to
control values with fast and slow phases (Da Costa et al., 1997). If these
findings are true and an efferent suppressive effect is considered to persist for
the entire duration of electrical and acoustical OC stimulation, even over
prolonged stimulation (Da Costa et al., 1997), it may mean that the MOC
neurons do not adapt to ongoing stimuli, but exert their effect throughout
stimulation.
Adaptation is known as a neuropsychological process whereby neurons are
able to produce small responses to constant ongoing stimuli and larger
responses to transient stimuli. It is generally accepted that most sensory
neurons adapt their responses to constant stimuli. Several researchers have
found that auditory neurons adapt to ongoing stimulation with tone bursts
(Chimento & Schreiner 1991; Delgutte 1980; Javel, 1996; Kiang, Watanabe,
Thomas & Clark, 1965; Muller & Robertson, 1991; Nomoto, Suga & Katsuki,
1964; Rhode & Smith, 1985; Smith, 1979; Smith & Zwislocki, 1975;
Westerman & Smith, 1984). Long-term adaptation over several minutes in
nerve fibres may be the result from synaptic processes which involve
depletion of neurotransmitters at the hair cell/nerve fibre synapse (Javel,
1996; Furakawa, Hayashida & Matsuura, 1978; Norris et al., 1977).
Although the adaptation characteristics of the auditory nerve are well
established, Brown (2001) found that they differ in several ways from the
adaptation characteristics of the MOC neurons. He noted that there was
almost no adaptation in the single MOC neuron responses for more than
several seconds of sound stimulation in guinea pigs. Even though the auditory
nerve fibres provide input to the MOC reflex, the difference in adaptation was
ascribed to compensation for the decline in auditory nerve input by elements
within the MOC reflex at more central locations, or as a result of changes at
45
the level of the MOC neurons themselves (Brown, 2001). In addition to this,
Brown (2001) sometimes observed suppression and a slow recovery after the
acoustic stimulation ended. In his report he suggested the investigation of
MOC adaptation to longer stimulation durations (longer than ten seconds),
although he doubted that there would be any change in the MOC firing rate
(Brown, 2001).
2.7.2 Duration of contralateral suppression: measured using OAE’s
Though several studies describe the duration characteristics of the MOCS
efferent effects arising from continual stimulation (electric or acoustic) in
animals, only a few focused on these properties in humans (Giraud et al.,
1997; Moulin and Carrier, 1998). These studies investigated efferent effect
duration by examining the influence of CAS duration on suppression of
EOAE’s, which can be considered as a good physiological indicator of efferent
activation. The following section will provide a literature overview of the few
attempts to describe the effect of contralateral stimulus duration on the
amplitude of TEOAE’s and DPOAE’s
Giraud et al. (1997) used stimulus durations ranging from ten to 180 seconds
prior to the onset of TEOAE recordings, and continued throughout the
recording time of 60 seconds. The authors used broadband noises of various
durations on twenty human subjects. TEOAE’s were recorded in responses to
a 63 dB SPL stimulus in the presence and absence of a 35 dB SL (sensation
level) noise, preceding the onset of TEOAE recordings by a variable time (10,
20, 40, 80 or 180 seconds). The study concluded that, within four minutes of
low-level continuous acoustic stimulation, there is no appearance of significant
efferent fatigue, because suppression remained constant throughout the
measurement. The focus was placed on shorter durations of one to four
minutes of CAS duration, because click-evoked TOAE clinical protocols
usually do not require more than four minutes on average. However, these
short durations of contralateral stimulation are possibly not long enough to
cause efferent fatigue.
Giraud et al. (1997) observed a slight tendency
towards reduction of suppression after three minutes of CAS, which may
46
suggest that amplitude reductions due to OAE suppression may be
observable in TEOAE’s with increasing duration of contralateral broadband
noise beyond three minutes. Since these findings no other attempts have
been made in using TEOAE’s to explore the duration of contralateral
suppression.
Duration
characteristics
of
contralateral
stimulation
have
also
been
investigated using DPOAE’s. Moulin and Carrier (1998) studied the time
course of the medial olivocochlear efferent effect on DPOAE’s in 20 normal
hearing humans. 2f1-f2 DPOAE’s were recorded in the one ear of each
subject, while a 40 dB SL contralateral broadband noise was applied to the
other ear with DPOAE primary levels set at 45 dB SPL and 50 dB SPL
respectively for f1 and f2. F2 was fixed at frequency 1501 Hz. This made it
possible to measure DPOAE’s every minute during continuous contralateral
stimulation, because the responses could be collected in a rapid manner.
Moulin and Carrier (1998) recorded DPOAE’s for two minutes without
contralateral broadband noise, followed by 20 minutes with contralateral
broadband noise and ten minutes without contralateral broadband noise.
Figure 2.3 displays the relative amplitude of DPOAE’s before, during and after
the contralateral broadband noise (CBBN)
47
Figure 2.3 Relative DPOAE amplitude over 32 min without (black dots)
and with (triangles) CBBN applied for 20 min (from 3-22min) as
measured by Moulin and Carrier, (1998)
Moulin and Carrier (1998) found that the suppressive response on DPOAE
amplitude with contralateral stimulus offset, increased for more than one
minute, then reached a constant effect of no change in suppression
amplitudes for the duration of 20 minutes and lasted more than two minutes
after the contralateral stimulation was ended. The persistence of the effect
after the stimulation ceased, indicated that the efferent effect can outlast the
CBBN by more than one minute.
48
Table 2.3 Summary of existing studies to date, investigating the duration
of contralateral suppression
Study (Year)
Subjects
Stimulus
duration
Measurement
with
MOCS
suppressor
Duration of
efferent effect
Wiederhold
and Kiang
(1970)
• Animal
• 350 s
• Electrical
stimulation of
crossed
olivocochlear
bundle
• Electrical
stimulation
• Entire duration
of stimulus
5 min 48 s
Puria et al.
(1996)
• Animal
• 12 s
• Cochlear CAP
• Contralateral
broadband
noise
• Entire duration
of stimulus
12 s
Kujawa,
Glattke &
Fallon
(1993)
Shridar et
al. (1995)
• Animal
• 500 s
• Cochlear CAP
• Contralateral
broadband
noise
• Increases
during the first
60 – 80 s
• Animal
• 50 s
• Cochlear CAP
(Slow and fast
effects)
• Electrical
stimulation
• Entire duration
of stimulus
50 s
Da Costa et
al. (1997)
• Animal
• 2h
• Ensemble
background
activity (EBA)
• Contralateral
broadband
noise
• Entire duration
of stimulus
2h
Giraud et al.
(1997)
• Human
• 4 min
• TEOAE’s
• Contralateral
broadband
noise
• Entire duration
of stimulus
4 min (Slight
tendency
towards
reduction after 3
min)
Moulin and
Carrier
(1998)
• Human
• 20 min
• DPOAE’s
• Contralateral
broadband
noise
• Entire duration
of stimulus
20 min
Collectively, the results of the majority of these studies lead to the impression
that suppression that results from MOCS stimulation (electrical or acoustic),
exerts its effect for the entire duration of efferent CAS. Most of these studies
focused on shorter stimulus durations of seconds to a few minutes and were
possibly not long enough to cause efferent fatigue or sensory adaptation. To
add to this, many were conducted only on animal subjects, due to the invasive
nature of measurements. Minimal research has been done describing
prolonged contralateral acoustic stimulation on OAE suppression in humans.
The generalization that the MOCS is capable of sustaining the suppressive
response on OAE amplitudes for prolonged durations and the assumption that
49
contralateral suppression of OAE’s is not affected by fatigue, based on limited
findings, can be questioned. It may prove to be valuable to use different
measuring techniques to examine the same duration properties. TEOAE’s,
measured as a function of prolonged contralateral stimulation (more than four
minutes) could support or contradict previous findings of suppression to
prolonged durations of CAS.
Knowledge of the duration characteristics of contralateral suppression would
be useful in the prediction of exposure durations for which MOC protection is
most effective and the durations for which the MOCS plays a role in adjusting
the dynamic range of the cochlea and in reducing the effects of noise
masking.
Not only will this study attempt to explain the relationship between OHC
integrity and the extent of efferent inhibition in a attempt to understand the
medial olivocochlear feedback loop more thoroughly, it will also seek possible
explanations for why the amount of suppression is significantly decreased in
individuals exposed to noise for long durations, even though they show
normal hearing thresholds (Sliwinska & Kotylo, 2002). Can the damage to
efferent auditory neurons in these individuals be ascribed to neural
adaptation, weakening the protective function of the medial efferent system
and making the OHC’s more susceptible to noise damage? Or does the
MOCS produce a sustained effect in the auditory periphery, providing a
protective and speech-in-noise-enhancing role during ongoing stimuli of long
durations?
2.8 Conclusion
It has been suggested that the medial efferent system is involved in protection
against acoustic overstimulation (Canlon 1996; Subramanian et al., 1993;
Liberman, 1991) or in different auditory perception properties, such as
speech-in-noise intelligibility (Muchnik et al., 2004; Sahley et al., 1997c). The
suppressive effect of the MOCS on the peripheral auditory system is still
under active exploration and very little has been documented about the
50
duration of its effect on efferents in humans. An overview of different studies
of MOCS efferent stimulation, in particular OAE suppression, revealed limited
research describing the effect of prolonged contralateral stimulation. Since
little literature exists describing prolonged (e.g. >4 minutes) acoustic
stimulation effects on OAE amplitude in humans, it would be interesting to
investigate these properties with broadband click TEOAE’s.
2.9 Summary
The purpose of this chapter was to explain the anatomy and physiology of
auditory pathways, to serve as a platform for understanding the efferent
feedback
loop
and
its
clinical
relevance
and
value
to
auditory
neurophysiologists and audiologists. This chapter also served to identify and
discuss all the different aspects of CAS in suppression of OAE’s. This aids in
identifying an optimal set of parameters for the measurement of TEOAE
suppression, which was applied in the testing procedures in the following
chapter. These parameters were also adjusted to suit the purpose of the
study, in an attempt to describe the effect on TEOAE’s of a prolonged duration
of contralateral stimulation. The need for research on the duration properties
of contralateral suppression of TEOAE’s was emphasized, because there is
currently a lack of available studies describing these properties in humans.
51
Chapter 3: Research Methodology
3.1 Introduction
One interesting viewpoint on the essence of research methodology was given
by Leedy (1993) “The process of research is largely circular in configuration: It
begins with a problem; it ends with that problem solved.
Between crude
prehistoric attempts to resolve problems and the refinements of modern
research methodology the road has not always been smooth, nor has the
researcher’s zeal remained unimpeded.” According to Leedy (2003) research
originates with a question or problem, it requires clear articulation of a goal
and a specific plan for proceeding, it divides the principal problem into more
manageable sub-problems and it is guided by the specific research problem.
The researcher accepts certain critical assumptions that require the collection
and interpretation of data in an attempt to resolve the problem that initiated
the research.
The problem inspiring this research project has already been extensively
stated in Chapters 1 and 2. In short, many researchers studied the different
contralateral stimulus characteristics that have an impact on OAE suppression
amplitudes in an attempt to understand the effects of the medial olivocochlear
efferent system on OHC’s. Few of them focused on longer stimulus durations
of several minutes and whether these neurons adapt to ongoing stimuli
(Giraud et al., 1997; Brown, 2001; Moulin & Carrier, 1998). This study
attempted to explain the relationship between OHC integrity and the extent of
efferent inhibition in an attempt to understand the medial olivocochlear
feedback loop more thoroughly. By investigating TEOAE suppression as a
result of prolonged CAS, the researcher was able to understand the extent of
prolonged MOCS inhibition on OHC’s. Is the MOCS capable of a sustained
suppressive response on TEOAE amplitudes over several minutes of
contralateral noise or does TEOAE amplitudes increase as a result of efferent
adaptation to ongoing stimuli?
52
The aim of this chapter is to describe the research aims, methods and
apparatus that were used in order to describe the relationship between the
prolonged CAS and the amount of efferent suppression measured with
TEOAE’s.
3.2 Research Aims
One essential step in the process of creating a clearly articulated primary aim,
generated from the specific research problem, is to divide the principal aim or
problem into more manageable sub-aims. By addressing these sub-aims, the
researcher can more easily address the main problem. The research study
aims were divided into a primary aim and underlying sub-aims.
Primary Aim
To investigate TEOAE suppression as a function of prolonged CAS
Sub-Aims
•
To compare recordings of TEOAE amplitudes obtained over an
identical time period divided into identical intervals in a controlled
condition (20 minutes without CAS) and in an experimental condition
(16 minutes with and four minutes without CAS).
•
To describe the relationship between the duration of CAS and TEOAE
suppression amplitude
53
3.3 Research Design
This study follows an experimental design that is quantitative in nature.
Experimental
research
involves
formulating
a
hypothesis,
modifying
something in a situation and comparing the outcomes with and without the
modification (Neuman, 1997). OAE amplitudes were measured in normalhearing subjects at predetermined time intervals in the absence (controlled
condition) and presence (experimental condition) of contralateral acoustic
noise (CAS). The amplitudes found in both conditions were then compared to
investigate the possible effects of prolonged CAS. According to Leedy &
Ormrod (2005) “quantitative research is used to answer questions about
relationships among measured variables with the purpose of explaining,
predicting and controlling phenomena”. The study is focused, with known
variables, using predetermined methods and standardized instruments to
collect numeric data (OAE amplitudes) that represent a larger sample. A few
participants who could best shed light on the phenomenon under investigation
were selected, using a purposive sampling method. This sampling technique
was selected because subjects had to adhere to specific criteria of normal
hearing. Deductive reasoning, beginning with aims and sub-aims, was used
to draw logical conclusions. Objectivity in data analysis was retained by
conducting predetermined statistical procedures and using objective criteria to
evaluate the outcomes of those procedures (Leedy & Ormrod, 2005). In this
case, OAE amplitude measurements were used as objective measurements
(in the sense that they require no behavioural response from the subject).
3.4 Research Subjects
In this section the population, sampling technique, subject selection criteria
and subject selection apparatus and procedures are discussed.
3.4.1 Population
Ten normal-hearing subjects between the ages of 20 and 29 were recruited
for TEOAE suppression measurements.
54
3.4.2 Sampling technique
The selection process can be described as convenience sampling (Maxwell &
Satake, 2006), as the sample consisted mainly of students and other people
living in and around Pretoria. This technique was followed because subjects
had to adhere to certain selection criteria to ensure assure the opportunity to
record TEOAE suppression in a healthy human ear.
3.4.3 Criteria for subject selection
Subjects were selected according to set criteria as discussed below. Should
the criteria not have been met, the results of the study would have been
negatively impacted upon.
Normal hearing acuity
According to Kemp (1978) otoacoustic emissions are absent in ears with a
sensorineural hearing loss greater than 30 dB HL. For this reason it was
critical for the subjects to have normal hearing ability. Clark (1981) described
normal hearing as having hearing sensitivity between 0 and 15 dB HL across
250 to 8000 Hz. Thus subjects underwent pure tone testing to verify their
hearing ability. It has been found that some subjects may exhibit abnormal
OAE’s, despite having despite having normal pure tone thresholds. In a study
by Attias, Bresloff and Furman (1996), it was found that in some cases,
subjects with normal pure tone thresholds of 0 dB HL exhibit abnormal
otoacoustic emissions, due to noise exposure. The physiological effects of
noise exposure can clearly be seen long before the actual hearing loss
occurs. This is also true for ototoxic medication (Danhauer, 1997). A short
interview collecting information regarding hearing history such as exposure to
noise or ototoxic medication was conducted in order to exclude these subjects
from the research project. Subjects who reported being exposed to ototoxic
medications or to hazardous sound environments and did not show normal
hearing sensitivity (thresholds between 0-15 dB) were referred for a diagnostic
hearing assessment at the University of Pretoria and were excluded from the
study.
55
Normal external and middle ear functioning
Because OAE’s are transmitted from the cochlea to the ear canal via the
middle ear, the transmission properties of the middle ear directly influences
OAE characteristics [Margolis, 1999 (cited in Robinette & Glattke, 2000)]. An
otoscopic examination of the external auditory meatus was conducted on all
subjects. If any foreign objects, impacted cerumen, growths, abnormal tissue
or redness of the tympanic membrane were observed, the subjects were
excluded from the study and referred for further medical intervention.
The second criterion was that all the subjects who were selected to participate
in the study had to have normal middle ear functioning. Otoacoustic emissions
can be recorded only in subjects with normal middle ear function. Only a very
small amount of energy is released by the cochlea to be transmitted
backwards through the oval window and ossicular chain to vibrate the
tympanic membrane.
Normal middle ear function is crucial to this
transmission process (Norton, 1993; Osterhammel, Nielsen & Rasmussen,
1993; Zhang & Abbas, 1997). To ensure that all subjects who participated in
the study had normal middle ear functioning, immittance measures was
performed. Only subjects with Type A tympanograms with a middle ear
compliance of 1.68 to 1.75 ml, volume of 0.65 to 1.57 ml and middle ear
pressure of +50 to -50 daPa were included in the study. The subjects who had
abnormal tympanograms or any external abnormalities were referred for
further medical intervention.
No broadband acoustic reflexes below 70dB SPL
Numerous studies have established that normal ART’s (acoustic reflex
thresholds) range from about 85 to 100 dB SPL for pure tone stimuli and are
roughly 20 dB lower for BBN (Gelfand, 1984) Low threshold broadband
acoustic reflexes are problematic, in that they may trigger the contraction of
the contralateral stapedius muscle in some subjects. CAS can be
contaminated and the effect on OAE’s invalidated by contraction of the
56
stapedial muscle and the resulting alteration of middle ear transmission.
Acoustic reflex-induced changes in middle ear transmission can greatly affect
the inward/outward propagation of energy in OAE measurement and totally
obscure the relatively slight true efferent effects. Thus it is imperative to use
an adequately low suppressor intensity level to minimize the effect of acoustic
reflex involvement. This problem can be overcome, as in other studies, by first
determining the contralateral acoustic reflex threshold for subjects (Williams,
Brookes & Prasher, 1994). In comparing the threshold of this contralateral
acoustic reflex with the intended maximum amount of contralateral noise the
researcher would be able to determine whether the contralateral acoustic
reflex would negatively impact upon the OAE elicited in the presence of
contralateral broadband noise. For this reason all subjects had to undergo a
contralateral acoustic reflex test elicited at a minimum intensity of 70 dB. If
subjects did not fit these predetermined criteria they were not allowed to
participate in this study.
Age (20-29 years)
Age appears to affect contralateral suppression of OAE’s. Several authors
have revealed that efferent suppression effects decline with age (Castor,
Veuillet, Morgon & Collet, 1994; Hood, Hurley, Goforth, Bordelon & Berlin,
1997; Parhasarathy, 2001). According to Parhasarathy (2001) age does not
play a significant role in the amount of amplitude measured with OAE’s
between the ages of 20 and 29. For this reason, subjects between these ages
were selected to ensure that age-related factors did not have any impact on
research results. Another reason was that this population are more likely to
have normal hearing ability, because they have not had the potential for more
years of noise exposure as compared to older participants would.
3.4.4 Subject selection apparatus
Table 3.1 contains the subject selection apparatus that was used to ensure
that subjects met the criteria (as discussed in section 3.4.3) and could be
included in the study.
57
Table 3.1 Subject selection apparatus
Selection apparatus
Apparatus
Rationale
Short Interview
• Appendix A, reviewing
the aspects that were
addressed in the short
interview
• Collection of information
regarding hearing status, such
as the amount of noise or
ototoxic exposure and
complaints of middle ear
problems
Otoscopic Examination
• Heine Mini 2000
• Visual inspection of the external
ear to identify possible
abnormalities
Pure Tone Audiometry
• GSI 61 Clinical
Audiometer with TDH
39 – supra-aural
earphones.
Test was performed in
a soundproof booth
• To determine hearing thresholds
for audiometric test frequencies
at octave intervals from 250 Hz
to 8000 Hz
Tympanometry
• GSI Tympstar, with a
226 Hz probe tone
• To determine if middle ear
functioning was normal
Contralateral Acoustic
Reflexes
• GSI Tympstar, with
BBN signal
• To determine the ART and
whether stapedius reflexes
would have an effect on TEOAE
suppression
3.4.5 Subject selection procedures
The procedure in which subjects were selected started with a brief interview,
followed by an otoscopic examination of the external meatus, immitance
testing, acoustic reflex measurements and pure tone audiometry bilaterally.
Case history and personal information
A short interview was performed prior to testing to obtain a limited case history
and some biographical details. The research project was also discussed with
the subject briefly and any questions were answered. The purpose of the case
history was firstly to obtain sufficient biographical detail to open a new subject
file and to determine the subject’s age and gender. Secondly, information
regarding hearing status factors, such as exposure to noise or ototoxic
medication and a history of middle ear problems, was obtained. Appendix A
58
reviews
the
aspects
that
were
addressed
in
the
short
interview.
Subject selection testing prosedures
Table3.2 provides a summary of the testing procedures that were followed for
the selection of participants. These procedures were followed when selecting
subjects to ensure that all subjects adhered to the selection criteria discussed
in 3.4.3.
59
Table 3.2 Subject selection testing procedures
Test
Instructions
Test Procedure
References
Pass criteria
Otoscopic
Examination
•
The subjects were instructed to
remain still. An explanation of the
procedure was given.
•
A speculum was selected
according to the size of the
entrance of the ear canal.
Observation of the pinna, external
ear meatus and tympanic
membrane.
• Hall III &
Chandler
(1994)
•
Normal amounts of cerumen with
no occlusion of the ear canal.
Observation of pearl-colour
tympanic membrane with visible
light reflex.
Pure Tone
Audiometry
•
The subjects were informed that
different tones of varying
intensities would be presented
and that they should react to all
audible tones by pressing the
button
•
The stimulus was presented at 30
dB HL. Lower intensity tones in
intervals of 10 dB were presented
until the subject reacted 50% of
the time
• Clark (1981)
•
Hearing sensitivity of 15 dB or
less at octave intervals between
125 Hz and 8000 Hz.
Tympanometry
•
Subjects were seated and
instructed not to talk or swallow
•
A tight seal was obtained by
choosing the appropriate size
probe. The compliance of the
tympanic membrane, volume and
pressure of middle ear were
measured
• Block &
Wiley (1994)
•
Compliance: 0.3-1.9 ml
Volume: 0.65-1.75 ml
Pressure: +100 daPa to -100
daPa. Type A tympanograms
Contralateral
acoustic reflexes
(CAR)
•
The subjects were asked to
remain still. The procedure was
explained before hand
•
A contralateral BBN signal was
measured in each ear. The ART
was obtained by decreasing the
stimulus intensity in 5 dB SPL
steps
• Williams et
al.(1994)
•
Contralateral BBN reflex
threshold of ≤ 70 dB HL
60
3.4.6 Description of the sample
Ten adults (5 females and 5 males, ranging in age from 20 to 26 years) were
evaluated in this study. All subjects met the following inclusion criteria based
on the subject interview: (1) negative family history of hearing loss, (2) no
history of ototoxic drug use, (3) no history of excessive noise exposure and (4)
no history of middle ear disease. All subjects presented with no external ears
abnormalities on otoscopic inspection, with external auditory canals free from
obstruction. Individual thresholds were less than 10 dB HL for frequencies
0.25 to 8 kHz and normal results were obtained for their immitance
audiometry. All subjects had contralateral broadband acoustic reflex
thresholds above 70 dB SPL and perception thresholds for contralateral white
noise of less than 10 dB, excluding middle ear responses to experimental
TEOAE’s conducted. All subjects had normal, measurable TEOAE’s. These
tests were conducted bilaterally on each subject.
3.5 Preliminary Study
The preliminary study was done to determine whether any changes to the
series of tests were necessary and what amount of time would be needed to
complete all the tests. The reasons for the preliminary study were: firstly, to
confirm subject selection criteria and secondly, to determine which stimulus
parameters and procedures to use in the measurements of TEOAE
suppression.
3.5.1 Confirmation of subject selection criteria
A large part of the determination of subject selection criteria was based on an
extensive overview of related literature. The subjects on whom the preliminary
study was performed were two students of the University of Pretoria. They
had no history of being exposed to noise or ototoxic medications and no
family history of hearing loss. During otoscopic examination normal external
ear canals and tympanic membranes could be observed bilaterally. Normal
Type A tympanograms that met the criterion were obtained. During pure tone
testing, normal hearing thresholds were obtained.
61
When contralateral broadband acoustic reflexes were measured one subject
had reflex thresholds at stimulus levels as low as 70 dB SPL. This correlates
with the finding of Wiley, Oviatt and Block (1987) in which he described BBN
reflex thresholds being almost 20 dB lower than pure tone acoustic reflex
thresholds. The criteria were changed in order to exclude subjects with BBN
reflexes at thresholds < 65 dB SPL from further data collection. A second
series of tests was conducted to determine optimal stimulus and suppressor
parameters and the best procedure to follow to provide an opportunity to
collect measurements.
3.5.2 Determination of optimal stimulus and suppressor parameters
Most of the stimulus parameters for this study were derived from an in-depth
literature study. Parameters such as the loudness levels of the TEOAE
stimulus and the optimal contralateral suppressor parameters were selected
according to recommendations made in previous studies. The rationale for
these stimulus and suppressor parameters is described in 3.4.2 “Data
collection protocols”. After the completion of the second series of tests, it was
evident that the contralaterally suppressed TEOAE’s were successfully
elicited within the selected parameters, enabling the comparison of the
responses with other findings. During the TEOAE recordings, the amount of
contralateral suppression found was between 1 and 3 dB, which correlates
with published norms (Morlet, Collet, Salle, Morgon, 1993; Veuillet et al.,
1991; Collet et al, 1990). It was thus not necessary to make further
modifications.
3.5.3 Confirmation of data collection procedures
The data collection procedure was developed in such a way as to investigate
the relationship between the duration of CAS and the amplitude of
suppression of TEOAE’s. The selected procedures are described in 3.4.3
“Data collection procedures”. After the preliminary study, it was decided to
include an additional post-noise TEOAE measurement, three minutes after the
contralateral suppressor was ceased, in order to investigate a longer time
62
course of possible persistence of a suppression effect after prolonged
contralateral noise stimulation ceased.
3.6
Data Collection
The following procedures and instruments were used during the collection of
all data:
3.6.1 Apparatus
TEOAE’s were recorded and analysed using the Otodynamics V6 Analyzer
hardware and software (Otodynamics Ltd.) The probe was calibrated for a
quiet room before testing and all measurements took place in a soundproof
booth. The masking of the ear contralateral to the one in which the OAE’s
were evoked, was done using a GSI 61 clinical Audiometer. Insert earphones
were used to increase the amount of interaural attenuation and prevent
acoustic crossover to the test ear (Ryan et al., 1991).
3.6.2 Data Collection Protocols
There are many stimulus parameters that must be specified to enable the
repetition of this research project. The following section summarizes and
discusses the protocols that were used when measuring TEOAE recordings
and the contralateral stimulus used to suppress these recordings. The
“stimulus” refers to the signal used to elicit the OAE and the “suppressor”
refers to the signal presented to the contralateral ear to elicit the suppression
effect. The TEOAE protocol, summarized in table 3.3, includes the test
parameters that are most effective when using CAS in suppressing TEOAE’s.
The stimulus parameters are discussed according to the specific settings and
ranges selected for TEOAE recordings and the rationale supporting this
selection.
63
Table 3.3 Test protocol for TEOAE stimuli parameters
TEOAE
stimulus
Settings and
Range
Rationale
References
parameters
Stimulus type
Stimulus
polarity
Stimulus
intensity
• Broadband 80 µs
clicks
• The use of broadband
clicks ensures that the
components of the
TEOAE across a broad
frequency range can be
elicited
• Berlin et al.
(1993b)
• Linear stimulus
with a set of four
clicks of the same
phase and SPL
• A constant stimulus
polarity can be used,
because lower click
intensity levels are used
in the evaluation of
contralateral suppression
effects
• Berlin et al.
(1993)
• 60 dB SPL
• A contralateral BBN
stimulus is most effective
in suppressing TEOAE’s
when the intensity of the
click stimulus used to
evoke an emission is
between 55 and 65 dB
SPL
• Hood et al.
(1996)
• Kemp (1978)
• Veuillet et al.
(1996)
• Veuillet et
al.(1991)
Click repetition
rate
• 50/s
• Significant reduction in
amplitude occurs as the
stimulus rate increases
from 50/s up to over
1000/s
• Granade and
Collet
(1995,1997)
Stimulus
presentation
ear
• right
• OAE’s show more
suppression in the right
ear when compared with
the left. The MOCS
appears to be more
efficient in the right ear
than in the left ear
• Kumar & Vanaja
(2004)
The suppressor parameters are summarized in table 3.4.
64
Table 3.4 Test protocol for TEOAE suppressor parameters
Suppressor
parameters
Settings and
Range
Rationale
Suppressor
type
• White noise
• Suppressors with
broadband spectrums
are more effective than
narrowband noise or
tonal suppressors
Suppressor
frequency
bandwidth
• 100 to 4000 Hz
• The GSI 61 Clinical
Diagnostic Audiometer
produces a White
(Broadband) noise
spectrum of between 100
Hz to 4000 Hz ± 5dB.
Suppressor
intensity
• 45 dB SL
• Sensation level
intensities of 30-50 dB
SL are low enough to
prevent any crossover
and contraction of the
contralateral stapedius
muscle.
References
• Berlin et al.
(1993b)
• Berlin et al.
(1994); Ryan et
al. (1991)
3.6.3 Data Collection Procedures
OAE measurements were performed directly after the subject selection
procedure. Subjects were instructed to sit, not to talk and to remain as still as
possible. Subjects were allowed to read as long as they kept their heads as
still as possible. A new file was initiated for the subject. An appropriately
sized probe was selected by examining the size of the subject’s external
auditory meatus and was inserted into the right external meatus such that an
airtight seal was obtained. A foam tip probe, connected to the audiometer
stimulus transducer, was selected according to the size of the ear canal and
inserted in the left ear, ensuring that there was no cords noise that can
interfere with the OAE recordings. The probe tip was inserted deeply into the
ear canal to insure an airtight pressure seal instead of flush with the ear canal
opening. Before and during TEOAE measurements, stimulus stability was
monitored. Stimulus ringing was prevented by adjusting the probe in the ear, if
necessary.
65
TEOAE recordings were measured in two conditions:
a. Controlled condition: OAE measurements at specific intervals over a
period of 20 min without CAS (white noise) to establish a baseline.
b. Experimental condition: OAE measurements at specific intervals over a
period of 16 min with CAS (white noise) and 4 min without contralateral noise.
TEOAE amplitudes were first recorded in the control condition followed by the
same recordings in the experimental condition. Both conditions were
subdivided into individual OAE amplitude recordings with specific time
intervals of five minutes between each recording. In the experimental
condition a continuous white noise was introduced in the contralateral ear for
a period of 16 minutes while four OAE measurements were recorded in the
ipsilateral ear. OAE amplitudes were measured directly after the CAS was
introduced, a second time after CAS for five minutes, a third time after ten
minutes of CAS, and a fourth time, after CAS for 15 minutes. A schematic
representation of the research procedures is illustrated in figure 3.1.
Thereafter, a post-noise period of one minute and three minutes (without
CAS) were allowed, followed by OAE amplitude measurements after each
quiet period respectively to determine if the suppression effect persisted after
prolonged exposure to CAS ceased and if this effect still existed after three
minutes post-stimulation. The controlled condition followed the same pattern,
only without the presence of CAS. Measurements of both conditions were
repeated three times, each on separate days, in all the subjects, to increase
the reliability of the results.
66
1st OAE
recording
Control
2nd OAE
recording
1
4min
1
5min
4min
nd
1 OAE
recording
4min
5min
4th OAE
recording
4min
4min
5min
1
1
Post-noise
recording
1
1
Post-noise
recording
1min after CAS
4th OAE
recording
3 OAE
recording
1
1
Post-noise
recording
1
5min
rd
2 OAE
recording
1
1
5min
st
Experimental
condition
3rd OAE
recording
4min
1
1
1
1
Post-noise
recording
3min after CAS
1
5min
Duration of CAS
1min OAE recording
Without CAS
With CAS
Figure 3.1 Schematic representation of the test procedure in the controlled and experimental conditions
The red areas represent the single OAE recordings that were measured without contralateral noise (white areas) in the controlled condition and
without contralateral noise (grey areas) in the experimental condition. A post-noise period of one minute without noise was given after the fourth
recording to determine if the suppression effect persisted after the noise had ceased.
67
3.6.4 The Criteria for acceptable TEOAE amplitudes measured with
contralateral white noise.
In order to set criteria to differentiate the presence from the absence of an
acceptable TEOAE response, several factors related to the stimulus and
recording parameters should be considered. TEOAE response measures
include the overall correlation of the two waveforms obtained from time
averaging (reproducibility), the overall level of the response in relation to noise
in the measurement, the reproducibility within specific frequency bands and
the stimulus stability. In earlier TEOAE studies, the analysis of TOAE
responses was almost limited to a description of overall reproducibility (in %)
and overall amplitude level (in dB SPL). These response measures do not
take advantage of the frequency-specific information available in TEOAE’s.
Thus it is important to not only investigate the whole reproducibility, but also
the band reproducibility from 1.0 to 4 KHz. In general the TEOAE’s may be
considered as always present when reproducibility is 60% or greater or the
response level minus the level of the noise is 2.4 dB or greater (Welzl-Muller
and Stephan, 1994). Several investigators have determined that percentage
reproducibility values from 50% to 70% would separate normal from impaired
ears (Gorga, Neely, Bergman, Beauchaine, Kamisnski, Peters & Jesteadt,
1993; Prieve, Gorga, Schimidt, Neely, Peters, Schultes & Jesteadt, 1993).
Thus a signal to noise ratio of ≥ 6 dB SPL and reproducibility greater than
70% was required to accept the TEOAE as being present (significant) in any
frequency band. Only those ears which had significant TEOAE responses
meeting these creteria at half octave frequencies between 1000 Hz and 4000
Hz were considered for further analysis. Responses to stimulus sweeps in
which the intensity of ambient noise exceed 48.8 dB peak SPL 9 (default
settings on the equipment) were rejected . The stimulus stability was
monitored throughout the sampling duration and only responses generated
with stimuli at a stability of 80% and greater were included in the data
analysis. If TEOAE date did not meet the criteria (e.g. a stimulus stability of
less than 80% or reproducibility less than 70 %), it was excluded from
statistical analysis, by not including the data to the averaging of the three
separate trials and using only the remaining two trial recordings.
68
Data organization and calculation of suppression amplitude
TEOAE amplitude values for both controlled and experimental conditions were
extracted from the specified time interval recordings and tabulated into
Microsoft Excel 2007 spreadsheets for further analysis. TEOAE overall
amplitudes as well as the amplitude at each half-octave frequency (1.0 to 4
kHz) following averaging of 260 sweeps were noted and included in the
spreadsheets. Thereafter, the suppression amplitude was derived by
subtracting the three trials of emission amplitudes with contralateral noise
(controlled condition) from the three trails of emission amplitudes without
contralateral broadband noise (experimental condition) at each specified time
recording. This was done for each individual.
3.6.5 Analysis
The mean contralateral broadband acoustic reflex threshold for the sample
was 85.3 dB SPL (SD = 5.3) Individual subjective thresholds for detection for
contralateral white noise were measured using insert earphones for each
subject and found to be equal to or less than 10 dB SPL with a mean
threshold of 5.2 dB SL (SD = 5.4). The mean OAE amplitudes of the control
condition stayed relatively stable over the course of measurement, with no
statistically significant differences (p>0.05) between the mean amplitude of
the different time intervals recordings. TEOAE amplitude was reduced with
contralateral stimulation at 45 dB SL compared to that found without
contralateral stimulation.
The analysis process consisted of a series calculation of descriptive values for
amplitude and noise floors for each time recording in both the control and
experimental
condition
separately,
including
the
post-noise
periods.
Comparisons of these values were done to evaluate consistency and/or
change over time and between conditions. Thereafter suppression values (at
each time recording) were derived from subtracting the with-noise
(experimental condition) TEOAE amplitude from the without-trace in the
corresponding time recording of the controlled condition. Suppression values
between different time recordings were compared to assess the change or
69
sustainability over the duration of CAS. All calculations were done for the
overall TEOAE response and filtered frequencies (1kHz, 1.5kHz, 2kHz, 3kHz
and 4kHz). Suppression for half-octave frequencies over time was then
compared for corresponding time recordings between frequencies. The
analysis procedures are discussed according to sub-aims.
Sub-aim 1: To compare replicated recordings of TEOAE amplitudes
obtained over an identical time period divided into identical intervals in
the
control
condition
and
in
the
experimental
condition
The Otodynamic V6 provided both a TEOAE signal (amplitude in dB SPL) and
noise level in the form of an overall response and half-octave frequencies
(1kHz, 1.5kHz, 2kHz, 3kHz and 4kHz) for each TEOAE recording. The data
collection procedure was repeated over three different days for each subject
and the mean TEOAE amplitude and noise level for all specified recordings
were therefore calculated for each subject over the three trials. Descriptive
analysis (means, standard deviations and ranges) of both the signal (TEOAE
amplitude) and noise levels were done for the overall response and filtered
frequencies measurements at each time recording in both conditions.
Thereafter two different comparisons were made. Firstly, comparisons were
made between the different recordings (amplitude and noise levels) for each
condition. This was done by using the Wilcoxon matched-pair signed rank
test. These comparisons provided information about the stability of noise
levels in both conditions and the stability of TEOAE amplitudes in the control
condition in order to insure validation of consistent noise floor levels across
trials. Statistical comparisons also provided information about the changes in
ipsilateral TEOAE amplitudes over time with contralateral stimuli and the postnoise period (experimental condition). Secondly, comparisons were made
between corresponding recordings of conditions (also with the Wilcoxon tests
of significance). This was done to evaluate the significance of the reducing
effect of contralateral stimuli on TEOAE amplitudes and after the cessation of
the stimuli. TEOAE amplitudes in both conditions were used to calculate
suppression values.
70
Sub-aim 2: To describe the relationship between the duration of CAS
and TEOAE suppression amplitude
After suppression values were calculated, (for overall response and filtered
frequencies) descriptive analyses for TEAOE suppression at each specified
time recording were done. Thereafter, amplitude comparisons between
recordings over time were made using the Wilcoxen test of significance.
These comparisons were done for the overall response and half-octave
frequencies. Differences between amplitudes of the first OAE recording and
the OAE recording after five minutes, the OAE recording after five minutes
and recording after ten minutes, the OAE recording after ten minutes and
recording after 15 minutes and finally the initial OAE recording and recording
after 15 minutes, were calculated to determine whether there were any
significant differences amongst these time intervals. Figure 3.2 illustrates the
comparisons between different time intervals of OAE recordings that were
done.
Figure 3.2 Analyses of the differences amongst the mean suppression
amplitudes in the various time intervals.
This provided information about change or sustainability of suppression
amplitudes as a function of contralateral noise duration. Additionally, TEOAE
suppression amplitudes amongst half-octave frequencies for each specified
71
time recording were compared (Wilcoxen signed rank test) to evaluate
differences between frequencies.
3.7 Ethical Considerations
Responsible ethical conduct in research is fundamental to the quality of
science in any discipline, and ultimately to the advancement of knowledge
(Ingham, 2003). The following ethical aspects were therefore taken into
account in the planning of this research project:
3.7.1 Respect for the privacy of research participants
In order to ensure the privacy of all the participants (Huysamen, 1994; Hegde,
2003), no individuals were named in the research report. This was achieved
by assigning a code to each subject, and noting it in the name section on the
questionnaire (Appendix A). No names were used during the study and all
personal information remained confidential. Data was processed, analysed
and discussed according to these codes to make the best possible effort to
maintain confidentiality. This was clearly explained in the informed consent
letter.
3.7.2 Informed consent
According to Hegde (2003), informed consent is a crucial ethical principle,
which consists of the following components – the participants should fully
comprehend the nature of the study and research procedures that will be
undergone and they should be given the choice to participate or not.
Consequently, the present study acquired written informed consent from each
participant by their signing the appropriate form (attached as Appendix B).
This form was signed by each participant after they had read a letter
explaining the goal of the study and what was expected of them. The letter
provided a brief description of the nature of the study, including the rationale
and time it would take for each measurement. It clearly stated that
participation was strictly voluntary and participants could withdraw from the
72
study at any time. The letter guaranteed participants of the confidentiality of
their personal particulars.
3.7.3 Beneficence and non- malfeasance
None of the test procedures caused physical, social or emotional harm to the
participants. The potential inconvenience of participating in the study (the time
and effort required to undergo the test) was indicated in the letter of consent.
The letter also explained that the information gathered would provide useful
data to the field of Audiology through publication of the results upon
conclusion of the study. The letter also informed subjects that all data
obtained in the study would be stored for a minimum of fifteen years for
record-keeping purposes. An offer was made to provide detailed information
about the study and individual results of the test procedure upon its
completion. However, there were no incentives or rewards (financial or other)
offered for participation in the study.
3.8
Validity and reliability of research methodology
To measure the validity of the present study in terms of instruments and data
collection procedures, the researcher had to re-evaluate the chosen approach
and evaluate the extent to which the apparatus and procedures measured
what they were supposed to measure (Leedy & Ormrod, 2005). The main
objective for the selection of subjects was to ensure that all had normal
hearing, which implied that subjects had no symptoms associated with
hearing loss, the absence of any ear pathology, and had normal cochlear
function. The validity of the subject selection apparatus (measuring normal
hearing) was affirmed by a multi-method (Campbell & Fiske, 1959) approach.
Various measuring methods (both subjective and objective) were included, all
with the intention to screen subjects with normal hearing. The use of objective
measurements in the subject selection protocol further validates findings,
because it does not require subjects to participate actively, thus excluding
subject
input
bias.
Subject
selection
criteria
ensured
a
somewhat
homogenous (all had normal hearing) group, decreasing limitations of
73
convenience sampling and serving as a good representative population. Data
collection followed an experimental design which made it possible to carefully
control for influential factors, except those whose possible effects are the
focus of investigation. Because of the fact that all TEOAE measurements
followed a strict protocol, environmental factors could be controlled. TEOAE
measurements are relatively objective (not requiring subject’s active
participation/ based on physiological rather than behavioural responses), so
other factors or possible explanations for the results were greatly eliminated.
The reliability of the research methodology was considered in order to assess
the consistency with which the recording apparatus and method yielded a
specific (similar) result when the characteristic being measured had not
changed (Leedy & Ormrod, 2005).
Two forms of reliability in the data
collection procedures were confirmed when raw data was viewed. The first,
formally known as “Interrater reliability”, was observed when TEOAE
amplitudes were compared in different subjects, using the same test
procedures (Leedy & Ormrod, 2005). Although there is a degree of variability
in TEOAE amplitude amongst the subjects, all obtained TEOAE amplitudes
within the normal range, and the same effect from prolonged contralateral
stimulation was observed in all subjects. The whole data collection procedure
was repeated on three different days and revealed similar results for each
subject across the three days. This ensured the second form of reliability,
known as “Test-retest reliability” (Leedy & Ormrod, 2005).
3.9
Summary
The need for investigating TEOAE suppression during continual CAS in order
to understand the extent of prolonged MOCS inhibition on OHC’s inspired this
research project. The aim of this study was to investigate TEOAE suppression
as a function of prolonged CAS. Ten healthy normal-hearing volunteers
between the ages of 20 and 26 were selected. Their hearing thresholds were
less than 15 dB HL at standard audiometric frequencies (250Hz – 8000Hz).
60 dB SPL (±3 dB) linear clicks were presented to record TEOAEs in the right
74
ear using the Otodynamics V6 Analyzer hardware and software in the
presence and then the absence of a 45 dB SL contralateral broadband noise,
using a GSI 61 clinical Audiometer and insert earphones. TEOAE recordings
were measured in the two conditions, namely the control and experimental
conditions. The control condition comprised of a series of TEOAE recordings
without the presentation to the non-test ear, four with a time interval of five
minutes between the on- and offset of recordings, followed by two recordings,
one and three minutes after the fourth recording. This was done to establish a
baseline. The experimental condition consisted of four TEOAE recordings
(with the same time intervals as in control condition) in the presence of
contralateral broadband noise, followed by two recordings, one and three
minutes after noise termination, to determine if the suppressive effect
persisted after stimulus offset. Final averages were accepted when the
reproducibility was 70% or more and the stimulus stability was maintained at
greater than 80%. The whole procedure was repeated thee time on each
subject. Noise levels, as well as averaged amplitudes of the TEOAE overall
response and half-octave frequency (1.0 to 4 kHz) for each recording, were
noted and included in spreadsheets. Data analysis determined descriptive
values and compared TEOAE over time and between conditions, in an
attempt to describe the significance in amplitude reduction at each recording
and the change/sustainability over prolonged CAS and in the two recordings
after noise offset. Suppression values were calculated by subtracting the
without-noise recordings from the with-noise recordings and
further
manipulated in Excel for Windows 2007 to create visual representations.
75
Chapter 4: Results
The results of the current study are discussed according to the research aims
that were described in Chapter t3. In order to achieve the primary aim of this
study, that is, to investigate the relationship between the duration of CAS and
the degree of TEOAE suppression, subjects had to adhere to strict selection
criteria, also discussed in Chapter 3. TEOAE suppression was used to explore
the time course of the efferent reflex in order to determine if it remains
consistent over a long duration of CAS, or if it adapts and weakens over time.
Ten subjects were repeatedly tested over three different days. Tests followed
a strict protocol with little inter-subject and test-parameter variance, to
minimize the possibility of bias in the recordings The results were presented
according to each sub-aim, as described in the methodology.
The aim of this chapter is to present all the collected and processed data (in
displays and summaries - graphs, charts and tables) and to describe these
results according to the sub-aims that were discussed in chapter three.
4.1 Sub-aim 1: TEOAE amplitudes over time for control and experimental
conditions
As described in chapter three, a control (without contralateral noise
stimulation) and an experimental (with contralateral noise) condition were
used to calculate TEOAE suppression over a predetermined time. These
conditions were subdivided into individual TEOAE amplitude recordings with
specific time intervals of five minutes between each recording. Thus TEOAE
amplitudes were recorded four times with a five minute period between the
onsets of each recording in both the conditions. Two additional TEOAE
recordings were taken after the period of contralateral noise. These
recordings took place at one and three minutes after the CAS ended (postnoise periods).
76
This results section presents the distribution of overall and half-octave
frequency TEOAE amplitudes elicited in both with and without contralateral
noise conditions and in the two additional post-noise periods of normal
hearing subjects. It also describes the noise levels during the period of
measurement in terms of consistency over time and between conditions.
4.1.1 Consistency of noise levels in control and experimental
conditions over time
In both conditions the noise levels (average SPL detected by the microphone
during the samples that were not rejected by the software algorithm) were
monitored for each of the recordings over the duration of measurement. Noise
levels were then noted and analyzed to examine stability over the duration of
the measurements. This was necessary to determine whether variability in
the average noise levels of TEOAE’s elicited across the course of
measurements existed within or between conditions. The descriptive analyses
(means, standard deviations, range) of the noise levels (in SPL) of the overall
and half-octave frequencies for all subjects are summarized and presented in
Table 4.1.
Table 4.1 Descriptive analysis of overall and half-octave frequency noise
levels as calculated for both conditions
Mean Noise level in
amplitude (dB SPL)
Standard
deviation (dB)
Range
(dB SPL)
Overall Response
-2.7
2.12
-6.2 - 5.4
1 kHz
-12.5
4.72
-22.1 - 3.6
1.5 kHz
-13.9
3.2
-20.6 - -2.8
2 kHz
-11.8
1.9
-16.5 - -4.3
3 kHz
-11.7
1.3
-14.9 - -6.8
4 kHz
-11.4
1.1
-14.7 - -7.6
In terms of stability between recordings over time, no significant differences
(p-value <0.05; Friedman test statistic) between recordings in the control
conditions were found. This was also true for all the recordings over time in
77
the experimental conditions. Thus the noise levels were stable during the
entire course of measurement within each condition.
A comparison of noise levels between recordings measured in the control
(without contralateral stimulation) and the corresponding time interval
recording in the experimental condition (with contralateral stimulation) was
also conducted to determine the variability between corresponding noise
levels across both conditions. From these results we can construe that the
mean values of the noise levels for TEOAE’s without contralateral stimulation
did not differ from the mean values of noise levels for TEOAE’s elicited in the
presence of CAS and that this noise variable did not influence results. The
results of a valid Wilcoxon test with the level of significant difference set to 5%
indicated no significant differences in the means of the TEOAE noise levels
for the overall response across the duration of measurement between both
conditions. All noise levels measured at half-octave intervals were also not
significantly different, except for two half-octave measurements at 1.5 kHz and
2 kHz recorded at the initial recording, where a significant difference was
present. Here the noise levels in the experimental condition showed
significantly higher noise levels than in the control. These two frequencies
showed no further significant differences (p-value < 0.05: Wilcoxen test of
Significance) between conditions for all recordings after the initial recording for
the entire duration of measurement. Taking into consideration the total noise
level in this initial recording also demonstrates that the significant differences
at the half-octave frequencies do not result in a significant difference in the
total noise levels. It is necessary to consider that this small difference found
only at two half-frequencies during the initial measurements may be the result
of the patient’s initial restlessness. With minimal differences between noise
levels between the recordings with and without noise, comparisons of TEOAE
amplitudes across conditions could, therefore, be considered valid and
reliable.
78
4.1.2 TEOAE amplitudes for control and experimental conditions over
time
TEOAE’s were present in 100% of subjects. The overall and half-octave
frequency (1 kHz, 1.5 kHz, 2 kHz, 3 kHz and 4 kHz) TEOAE amplitudes for
both the control and experimental conditions at each specified time period
over the duration of measurement (20 minutes) were recorded in all subjects.
This procedure was repeated over three different days of one week. The
mean TEOAE amplitudes of the three trials of control and experimental
conditions for each subject were calculated and processed for descriptive and
comparative statistical analyses.
The descriptive analyses (means, standard deviations, range) of the overall
and half octave frequency TEOAE amplitudes at specified recording periods
for all subjects are summarized and presented in Table 4.2.
79
Table 4.2 Descriptive analysis of TEOAE amplitudes in control and experimental condition
1 kHz
*C
*E
1.5 kHz
C
E
2 kHz
C
E
3 kHz
C
E
4 kHz
C
E
Overall
C
E
Initial recording
5 min
Mean
(SD)
-0.4
(4.1)
-1.9
(4.2)
0.5
(3,5)
-1
(4.3)
4.4
(4)
2.2
(3.8)
-0.4
(3.6)
-1.7
(3.6)
-9
(6)
-9.4
(5.3)
7.9
(3.1)
6.7
(3.2)
Mean
(SD)
0.1
(4)
-1.3
(3.7)
0.3
(4)
-0.8
(4.2)
3.1
(4.1)
2.3
(3.9)
-0.7
(3.3)
-1
(3.2)
-9
(6.2)
-9.6
(6.2)
7.7
(3.2)
6.6
(3.5)
Range
-5.1 - 7.3
-6.2 - 5.3
-7 - 4.4
-11.7 - 2.3
-2.6 - 10.5
-2.7 - 9.3
-5.2 - 5.8
-6.8 – 4.8
-15.8-1.9
-16.5 – 0.4
4.1-13.2
2.6 – 11.7
10 min
Range
-3.8 – 8.2
-5.7 – 8.2
-7.8 – 5
-11.3 – 2.8
-2.9 – 11.1
-2.1 – 9.8
-5.9 – 5.1
-5.7 – 5.3
-16.6 – 1.7
-18.3 – 0.6
3.7 – 13.8
3.4 – 11.8
Mean
(SD)
0.5
(3.7)
-1.3
(3.8)
0.6
(3.7)
-0.5
(3.7)
3.3
(3.9)
2.3
(3.8)
-0.4
(3.2)
-0.9
(3.4)
-8.9
(5.9)
-9.2
(5.9)
7.9
(3)
6.8
(3)
15 min
Range
-4 – 8.8
-6.7 – 5.5
-6.9 – 5.5
-9.3 -3.6
-2 – 11.3
-1.5 – 9.4
-5.2 – 5.2
-5.3 – 5.1
-14.6 – 1.7
-14.7 – 1.7
4.4 – 14.2
2.8 – 11.5
Mean
(SD)
0.4
(4.2)
-1.7
(4)
0.9
(3.7)
-0.9
(4)
3.6
(4.2)
2.3
(3.9)
-0.3
(3.4)
-0.8
(3.3)
-8.5
(5.9)
-9.2
(6)
8.3
(3.2)
6.8
(3)
Range
-5.6 – 9.2
-8.6 – 5.2
-6.7 – 6
-10.5 – 2.9
-2.4 – 11.9
-1.8 – 9.2
-6 – 5.4
-5.6 – 5.1
-15.9 – 1.6
-15.9 – 1.3
3.8 – 14.7
2.4 – 11
1 min post-noise
3 min post-noise
Mean
(SD)
0.5
(3.7)
0
(4.2)
0.9
(3.8)
0.9
(4.2)
3.3
(4)
3.7
(4)
-0.4
(3.4)
0.6
(2.9)
-8.9
(5.9)
-8.6
(6.1)
8.1
(3)
8.3
(3)
Mean
(SD)
0.2
(4.3)
0.1
(4)
0.6
(4.3)
0.8
(4.1)
3
(4.4)
3.7
(3.8)
-0.4
(3.3)
0.5
(2.8)
-8.9
(6.5)
-7.9
(5.8)
7.8
(3.5)
8.4
(2.9)
Range
-2 – 9
-4 – 8.7
-7.2 – 5.9
-9 – 6.1
-1.8 – 11.6
-1.2 – 12.2
-6.3 – 5.4
-3.9 – 5.4
-16.4 – 0.9
-15 – 2.5
4.2 – 14.5
4.6 – 14.7
Range
-4.7 – 9
-3.4 – 8
-8.3 – 5.9
-9.1 – 5.3
-3.2 – 11.5
-0.8 – 11.8
-6.2 – 5.2
-3.7 – 5.3
-18.4 – 1.4
-14.3 – 2.9
3.1 – 14.4
5.1 – 14.4
*E = Experimental condition
*C = Control
80
The results of the TEOAE amplitude differences between the control and
experimental condition in the overall and half-octave frequency measurements
were analyzed separately. Comparisons between conditions furthermore focused
on two different recording aspects.
The first comparisons between the two
conditions were between the first four recordings to ascertain the effect of
contralateral noise over time. The second set of comparisons was between OAE
amplitudes in the post-noise periods for the experimental and control conditions,
to determine if any residual suppression was evident in the post-noise timeframe.
**
TEOAE amplitude for overall response
Figure 4.1 illustrates the mean overall TEOAE amplitude elicited at each
specified time interval in both the control and experimental condition.
**
*
**
*
**
*
*
*
**
Figure 4.1 Mean overall OAE amplitudes over time across conditions.
*significant difference between conditions
As seen in figure 4.1 a decrease in mean overall TEOAE amplitude is evident in
the experimental condition with the onset of the contralateral stimulation. This
reduction in TEOAE amplitude with contralateral stimuli was observed in all
subjects. The difference in mean TEOAE amplitude between the control and
experimental condition was significant (p>0.05) for all recordings from the onset
of CAS up to 15 minutes of contralateral noise stimulation. In the initial recording,
81
the mean overall TEOAE amplitude significantly reduced from 7.9 to 6.7 dB SPL
when contralateral noise was introduced. At five, ten and 15 minutes of
contralateral noise, the experimental mean TEOAE amplitude reduced from 7.7
to 6.6, 7.9 to 6.8 and 8.3 to 6.8 dB SPL, respectively.
One minute after the contralateral stimulus was terminated (post noise period),
the mean TEOAE amplitude in the experimental condition rapidly increased to a
value that does not significantly differ (p<0.05) from the corresponding TEOAE
recording in the control condition (difference=0.18 dB SPL+-0.4). Thereafter the
mean amplitude in the experimental condition continued to increase to 8.4 dB
SPL three minutes after the offset of contralateral noise (three minutes post-noise
period) above its initial value (7.8 dB SPL) in the corresponding time recording of
the control condition (difference = 1.58 dB SPL ±0.82). This increase in OAE
amplitude in the experimental condition at three minutes post noise was
significantly (p>0.05) higher than the control condition. The amplitude in the
experimental condition exceeding the corresponding condition in the recording
three minutes after noise termination was observed in 90% of subjects (9/10).
TEOAE amplitude at half-octave frequency intervals
Figure 4.2 shows the mean TEOAE amplitude over time in both the control and
experimental conditions plotted against bandwidth for each of the five half-octave
frequencies.
82
83
**
**
**
**
**
**
**
**
**
**
**
**
**
Figure 4.2 Mean OAE amplitudes at half-octave frequency intervals for
control and experimental conditions over time
** significant differences between conditions
84
Like the overall TEOAE, the presentation of CAS decreased TEOAE amplitudes
over all half-octave frequency bands for the entire duration of stimulation. For
lower frequency bands such as 1 kHz, 1.5 kHz and 2 kHz this decrease in mean
TEOAE amplitude was significant (p>0.05) at all intervals measured over time.
The smallest differences between the amplitude of the experimental and control
conditions was for the higher frequencies (3 and 4 kHz) with no significant
(p<0.05) differences over time, except for the first recording in the 3 kHz
bandwidth, which was significantly reduced. The differences between mean OAE
amplitudes elicited in the control and experimental conditions decreased with an
increase in frequency, with the largest difference at 1 kHz and the smallest
difference at 3 kHz and 4 kHz measurements. These differences are illustrated
and discussed in more depth elsewhere in this chapter.
All TEOAE amplitudes at half-octave frequency intervals in the experimental
condition significantly (p<0.05) increased from the last recording with
contralateral noise to one minute after noise termination. The TEOAE amplitude
in the experimental condition was larger at three minutes post-noise compared to
one minute post-noise in four of the five half-octave frequency bands (3 kHz
demonstrated a decrease). In three out of four half-octave frequencies (higher
frequencies), the TEOAE amplitude in the experimental condition exceeded the
initial value in the corresponding control recording), one minute after noise
termination. There was an exceptionally difference between conditions at 3 kHz.
Four of the five half-octaves had amplitudes in the experimental condition that
exceeded those in the control, three minutes after the cessation of contralateral
noise.
Figure 4.3 illustrates the mean amplitude differences between the control and
experimental conditions in the post-noise recordings for each half-octave
frequency band.
85
1.00
0.50
(dB SPL)
Post-noise periods
Amplitude differences between conditions
1.50
0.00
-0.50
-1.00
1 minute post-noise
3 minutes post-noise
1kHz
-0.54
-0.06
1.5kHz
0.00
0.15
2kHz
0.31
0.67
3kHz
0.98
0.91
4kHz
0.26
0.95
Figure 4.3 Mean amplitude differences between control and experimental
conditions in the post-noise period.
Although not significant (p<0.05), amplitudes of the 1 kHz band demonstrated a
mean decrease in the experimental compared to the control condition, which is
evident in the negative values (inversion) in figure 4.3 For the recording one
minute after contralateral noise, 3 kHz showed the largest difference (which was
also the only significant difference between conditions). The recording at 1.5 kHz
one minute after contralateral noise, revealed almost no differences between
conditions. Differences between conditions at 3 kHz slightly decreased at the
three minute post-noise period, whereas the other frequencies increased, leaving
4 kHz with the biggest difference.
86
4.2 Sub-aim 2: Relationship between the duration of CAS and TEOAE
suppression
Suppression amplitudes (overall and half-octave frequencies) were calculated by
subtracting the average OAE amplitudes elicited in the three trials with CAS from
the average OAE amplitudes elicited in the corresponding three trials without
CAS. These calculations were done for the initial recording (recorded at the onset
of the CAS) and the recordings after five, ten and fifteen minutes of contralateral
stimulation. As significant reductions of TEOAE amplitudes in the experimental
conditions were not observed in the recordings after contralateral noise was
terminated, these are not further discussed in this section.
This section presents the result of TEOAE suppression over a 15 minute time
interval in the presence of sustained CAS to describe suppression characteristics
over prolonged periods.
A descriptive analysis is provided for suppression amplitude (overall and halfoctave frequencies) at each recording interval over the period of sustained
contralateral noise stimulation. Figure 4.4 illustrates these average suppression
values.
87
TEOAE suppression (dB SPL)
2.50
2.00
1.50
1.00
0.50
0.00
initial recording
5min
10min
15min
Overall
1.28
1.07
1.14
1.51
1kHz
1.50
1.39
1.74
2.18
1.5kHz
1.53
1.04
1.22
1.75
2kHz
1.23
0.82
0.99
1.25
3kHz
1.23
0.39
0.43
0.51
4kHz
0.49
0.49
0.28
0.68
Minutes of contralateral white noise stimulation
Figure 4.4 Mean TEOAE (overall & half-octave frequencies) suppression as
a function of CAS
Suppression values plotted in figure 4.4 correspond to the difference in
magnitude between TEOAE’s recorded in the presence and absence of CAS and
are consistent with those obtained by the Euclidean distance calculation method
(Chery-Croze et al., 1994). Generally the largest mean suppression at each
recording time interval was observed at lower half-octave frequency, 1 kHz,
except for the initial time interval of 1.5 kHz showing larger suppression values.
In the initial recording, the higher frequencies (3 kHz and 4 kHz) revealed the
least suppression over all time recordings, except in the initial time interval
recording of 3 kHz, which showed a significant difference between conditions.
Descriptive illustrations of overall and half-octave frequencies over time, including
the standard deviation, are presented in figures 4.6 and 4.7.
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4.2.1 Overall suppression
Figure 4.5 demonstrates the mean and standard deviation of overall TEOAE
suppression at the four recording intervals as a result of sustained CAS.
Figure 4.5 Overall TEOAE suppression as a function of contralateral noise
duration (Mean ± SD)
The first recording with CAS revealed suppression within the ranges of 0.66 to
2.8 dB SPL with a mean of 1.28 dB SPL (SD ± 0.63). In this time interval
recording only two of the 10 subjects had overall suppression below 1.0 dB SPL,
while the majority had normal suppression values of between 1.0 and 3 dB SPL.
At the five minute time interval recording suppression ranged from 0 dB to 2.1 dB
SPL with a mean of 1.0 dB SPL (SD ± 0.74) and three of the ten subjects showed
suppression of less than 1.0 dB SPL. Although not significant (p<0.05), a slight
reduction in mean suppression was observed from the initial time interval
recording to the recording of five minutes contralateral noise stimulation. At the
ten minute time interval recording the mean suppression was 1.14 dB SPL (SD ±
1.28) and ranged from -1 dB SPL (no suppression) to 3.33 dB SPL with two
89
subjects presenting with suppression below 1.0 dB SPL. From the 10 minute
recording, overall mean suppression at the 15 minute time interval recording
slightly increased (though not statistically significantly) to 1.51 dB SPL (SD =
1.31), ranging from 0.42 to 4.1 dB SPL. Only 2 subjects presented with
suppression of less than 1.0 dB in this time interval recording. In conclusion,
statistical analysis revealed no significant differences in suppression values
between recordings over time, as a result of small values and low subject
numbers, but a small increase in mean suppression was observed from the time
interval recordings of five minutes to 15 minutes of contralateral stimulation.
4.2.2 Suppression at half-octave frequency bands
Figure 4.7 illustrates the mean and standard deviation of TEOAE suppression in
frequency bands over the duration of CAS on a relative scale.
90
Figure 4.6 TEOAE suppression as a function of CAS duration at half-octave
frequencies (Mean ± SD)
91
4.2.3 Frequency suppression differences over time
Figure 4.7 illustrates the mean suppression values at half-octave frequencies
over a sustained period of contralateral noise stimulation.
Frequency Differences in Mean Suppression
over Time
2.5
1kHz
dBSPL
2.0
1.5kHz
1.5
2kHz
1.0
3kHz
0.5
4kHz
0.0
Initial
recording
5min
10min
15min
Time of recording
Figure 4.7 Mean suppression across frequency bands as a function of CAS
duration
Clear suppression could be observed in the frequency bands where the effect
was the greatest (1-2 kHz). As the frequency increased, mean suppression was
decreased, with the least suppression in 3 and 4 kHz half-octave frequencies.
However, because mean suppression values differ minimally and have widely
overlapping standard deviations, no statistically significant differences were found
between frequencies at each time interval recording. Among these half-octave
frequencies, it was also found that there was no significant change in
suppression over time as the CAS duration increased, indicating a stable
suppressive effect over prolonged stimulation. With the exception of 4 kHz, an
upward tendency was observed in the mean suppression values from five
minutes of CAS to 15 minutes of CAS and in the mean overall suppression, as
seen in figure 5.7.
92
4.3 Conclusion
From the results it could be concluded that a contralateral 45 dB SL white noise
significantly reduces ipsilateral TEOAE’s over a time period of 15 minutes, except
for the high frequencies of 3 to 4 kHz). Suppression was observed for the entire
duration of CAS. The amount of suppression stayed relatively stable for over 15
minutes and did not significantly reduce as a result of prolonged contralateral
stimulation. However, a slight increase in mean suppression was observed from
five to 15 minutes of stimulation in the overall and half-octave frequency
measurements. After the offset of CAS, TEOAE amplitudes rapidly increased to
values above those seen in the control condition, and no traces of suppression
were observed after one minute of noise termination, except at 1 kHz, although
suppression was minor and not significant (p<0.05). The amplitude in the
experimental condition at 3 min after the CAS termination indicated a significant
increase in overall TEOAE and some frequency band (2, 3 and 4 kHz)
amplitudes compared to the control condition. The average overshoot was
largest at 4 kHz (0.95 dB) followed by 3 kHz (0.91 dB) and 2 kHz (0.69 dB). The
average overshoot (0.58 dB) of the overall TEOAE amplitude in the experimental
condition was statistically significant (p > 0.05) at the 3 min post-noise recording
interval compared to the control condition. This overshoot, or amplitude
enhancement, above control values was observed in 7 out of the 10 subjects with
only one individual showing the same amplitudes and two individuals showing
control amplitudes slightly higher than experimental amplitudes in the recording 3
min post-CAS termination.
4.4 Summary
In the TEOAE (overall and filtered frequencies), the noise levels were low and
stable during the entire course of measurement within each condition. This
suggested that all TEOAE’s elicited during the control and experimental
conditions were valid and reliable and allowed the researcher to confidently
93
investigate the distribution of OAE amplitudes over the duration of measurement.
The mean OAE amplitudes of the control condition stayed relatively stable over
the course of measurement with no statistically significant differences between
the amplitude of the different time interval recordings, thus serving as a valid
control to compare with amplitudes in the experimental condition and the
calculation of suppression. TEOAE amplitudes (overall and half-octave
frequencies) were reduced under contralateral stimulation at 45 dB SL compared
to those found without contralateral stimulation. This reduction with CAS was
significant for all recordings within the overall TEOAE measurement and half
octave frequencies of 1 to 2 kHz. The loss of suppression (not significant) in
higher frequencies (3 and 4 kHz). Only the initial recording of 3 kHz showed a
significant reduction. The overall and all half-octave mean TEOAE amplitudes in
the experimental condition significantly increased from the last recording with
contralateral noise to one minute after noise termination.
From there mean
TEOAE amplitudes continued to increase up to three minutes post noise offset. A
reduction in amplitude after stimulus offset was present only in 1 kHz and the
overall TEOAE measurement and the majority of filtered frequencies showed
amplitudes in the experimental condition greater than in the corresponding
control condition. 1 kHz revealed the largest mean suppression for all recordings
during the measurement and the higher frequencies (3 kHz and 4 KHz) the least.
No significant change in suppression was found over time as the CAS duration
increased, indicating a stable suppressive effect over prolonged stimulation.
However, an upward trend from five minutes of CAS to 15 minutes of CAS was
observed in the mean suppression values in overall suppression and the majority
of half-octave frequencies.
94
Chapter 5: Discussion
5.1 Introduction
Suppression of otoacoustic emissions (OAE’s) is a relatively new and exciting
research area in the field of audiology. It has a great future with possible
applications in such areas as the diagnosis of eighth cranial nerve tumours and
Meniere’s disease. Clinical audiologists may be surprised to learn that the
literature describing investigations of OAE suppression by acoustic stimulation is
quite extensive, although the majority of the studies were conducted using animal
subjects. These studies all agree on the general conclusions, namely: a) efferent
suppression of OAE’s results in a reduction of emission amplitude, occurring
immediately after the onset of the suppression stimulus and being reversed
moments after its cessation; b) there is an inverse relationship between the
intensity of the contralateral stimulus and the amount of OAE amplitude
reduction, and c) broadband noise is the most effective suppressor. Although
extensive research has been done to reveal these properties in OAE
suppression, limited research exists describing the effect of prolonged
contralateral stimulation on OAE suppression.
The aim of this chapter is to discuss all findings and interpret the significance
thereof. These findings will be discussed according to sub-aims. Firstly, TEOAE
amplitudes will be discussed as a function of duration of CAS for both the control
and experimental conditions. This section will also discuss noise levels during the
period of measurement, the differences in TEOAE amplitude between conditions
at corresponding time interval recordings and after the termination of CAS (post
noise periods). Secondly, the correlation between the different durations of CAS
and TEOAE suppression will be interpreted and discussed.
95
5.2 Sub-aim 1: TEOAE amplitudes over time for control and experimental
conditions
TEOAE amplitudes were monitored for the duration of 15 minutes with and
without CAS in five minute interval recordings, with the first recording (in the
experimental condition) directly at the onset of CAS. Thereafter amplitudes were
observed in the post-noise periods of one and three minutes after CAS offset. To
investigate TEOAE amplitudes for both conditions over the duration of
measurement it was necessary to exclude any influences of noise levels related
to the subject or ambient noise.
5.2.1 Consistency of noise levels within and between conditions
Noise levels in both the control and experimental condition were noted and found
to be stable, with no significant differences (p<0.05) between recordings over
time in the both conditions and no significant differences (p<0.05) between
corresponding time recordings of the control and experimental condition.
All noise levels measured at half-octave intervals were also not significantly
different, except for two half-octave frequencies (1.5 kHz and 2 kHz) recorded at
the initial recording, where a significant difference was present. Noise levels in
the experimental condition were found to be significantly higher than in the
control at these frequencies. With the majority of half-octave frequency and the
overall measurement showing no significant variability between average noise
levels of TEOAE’s elicited across the duration of measurements within or
between conditions, the researcher could infer that all TEOAE’s elicited during
the control and experimental condition were valid and reliable. The knowledge
that there was no significant variability present in the noise levels in recordings
over time and between conditions allowed the researcher to investigate the
distribution of OAE amplitudes over the duration of measurement.
96
5.2.2 TEOAE amplitude reduction as a function of CAS
Comparisons were made between the first four recordings of the two conditions
in order to study the differences in TEOAE amplitudes over time in the with-noise
and without-noise traces.
Results showed that there was a significant reduction (p>0.05) in the mean
amplitude of the overall TEOAE when contralateral noise was introduced for the
duration of the CAS. As can be seen in figure 4.1, amplitude reduction in the
experimental condition was observed from the first recording (which lasted 60 s),
directly after CAS onset and at the recordings of five, ten and fifteen minutes of
CAS. Thus it can be assumed that amplitude reduction lasted for the entire
duration of contralateral stimulation. The time required for contralateral sound
suppression to reach maximal effect is in the order of 100 ms (Warren &
Liberman, 1989). This reduction may be the result of activation of medial efferent
neurons, as has been reported earlier (Norman & Thornton, 1993). Activation of
medial efferent neurons results in the release of acetylcholine at the synapse,
which in turn induces alteration in the shape and/ or compliance of outer hair
cells. These alterations can damp the micro-mechanical activity, reduce the
sensitivity of the basilar membrane (Neely & Kim, 1986), and thus reduce the
amplitude of TEOAE’s.
As was found for overall TEOAE, the presentation of a CAS decreased TEOAE
amplitudes over all half-octave frequency bands for each recording with CAS. As
can been seen in figure 4.2, it is evident that TEOAE amplitude reduction was
more pronounced in the lower half-octave frequency bands. Lower frequencies
(1, 1.5, & 2 kHz) revealed significant (p<0.05) amplitude reduction, which
persisted for each recording during the 15 minutes of CAS (excluding the last
recording at 2 kHz), whereas higher (3 and 4 kHz) half-octave frequencies
showed much less TEOAE amplitude reduction (not significantly reduced;
p<0.05). This observation is in agreement with other studies (Berlin et al., 1993;
Collet et al. 1992; Norman & Thornton, 1993). These investigators all reported
97
that the largest reduction of TEOAE amplitudes occurs in the range of 1 to 4 kHz
and that it tends to be greatest within the 1 to 2 kHz region, with a broadband
noise suppressor or when narrow band suppressors are compared (Veuillet et al
1991). The reason for more reduction in this area may be ascribed to the greater
density of MOC efferent innervations and more efferent control of OHC’s in the
area of the cochlea that responds to this frequency range (Guinan et al., 1983;
Liberman & Guinan, 1998; Warr et al., 1986).
5.2.3 TEOAE amplitude characteristics after noise termination
In the first recording, one minute after the CAS offset, no significant reduction
(p<0.05) in TEOAE amplitude was present in the overall TEOAE recordings
(figure 4.1). These results differ from the slow recovery of DPOAE amplitude
observed after longer CAS durations (Moulin & Carrier, 1998). In their study they
reported that DPOAE amplitude reduction continued for over one minute after the
CAS offset at a rate of 0.32 dB/min in human subjects. This was attributed to the
persistence of an efferent effect on the OHC’s after the CAS is terminated. The
present study in humans showed no continued reduction in overall TEOAE
amplitude after the first minute of CAS offset (figure 4.3). However, the results of
the current study suggest that lower half-octave frequency (1 kHz) revealed
different effects within one minute after CAS termination. Amplitudes in the lower,
1 kHz half-octave frequency continued to decrease one minute after stimulus
offset, although this decrease was small (0.54 dB) and not significant. Moulin and
Carrier (1998) measured 2f1-f2 DPOAE stimuli with F2 fixed at 1501 Hz,
measuring this specific area in the cochlea. It may be that different areas in the
cochlea respond differently after prolonged OCB stimulation and that efferent
neurons on OHC’s surrounding lower frequency areas are able to maintain their
discharge beyond CAS, leading to a slow recovery at the offset of CAS.
Nevertheless, the overall TEOAE amplitudes and the majority of half-octave
frequency amplitudes in the experimental condition indicated complete recovery
when measured at one minute after CAS offset. A quick amplitude recovery after
98
CAS termination correlates with a study conducted on animal models, concluding
that suppression disappears roughly 80 ms after the contralateral suppressor is
turned off (Warren & Liberman, 1989). Moreover, in the current study, amplitudes
in the higher half-octave frequencies (2, 3 and 4 kHz) further increased, to values
exceeding corresponding amplitudes in the control condition (figure 4.7).
However, this increase was small and only 3 kHz showed a statistically
significant difference (p>0.05) between the conditions. An interesting finding was
that the amplitudes after three minutes of CAS offset, in the overall TEOAE and
some half-octave frequencies (2 kHz & 4 kHz), continued to increase above
control values. The overall recording revealed a significant (p>0.05) increase in
TEOAE amplitude above the control, three minutes after CAS termination.
Elevated TEOAE amplitudes in the experimental condition of the post-noise
periods lead to the impression that emission amplitude, after a prolonged CAS,
increases even more than it would in total absence of noise over the same
duration, up to at least three minutes post-noise. This raises the question of
improved OHC sensitivity after prolonged contralateral stimulation. No studies
have been done to investigate these findings, and thus further investigation into
OAE amplitude changes after stimulus offset may be needed to confirm these
results.
5.3 Sub-aim 2: Relationship between the duration of CAS and TEOAE
suppression
The present study aimed to monitor TEOAE suppression over specific time
interval recordings during the presentation of 15 minutes of CAS. Of particular
interest was the change (increase or decrease) in suppression in response to
continuous presentation of noise, observed by calculating suppression at each
recording time interval.
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5.3.1 Suppression as a function of CAS duration
Results of overall TEOAE’s showed that there was no statistically significant
(p<0.05) change in suppression between time interval recordings as the CAS
duration increased, indicating a stable suppressive effect over prolonged
stimulation. Therefore, it can be assumed that the efficiency of the MOC reflex in
exerting its efferent effect on OHC’s was the same throughout the 15 minutes of
CAS. It can reasonably be supposed that acoustic stimulation has the same longlasting effectiveness in OCB activation as sustained electrical stimulation of six
minutes has been shown to have (Wiederhold & Kiang, 1970). The results of the
present study also agree with the continued decrease in CAP suppression
observed in guinea pigs during efferent electrical stimulation, which was ascribed
to the continual fast effect on efferent neurons (Shridar, Liberman & Brown,
1995). Furthermore, it was found that the suppressive effect on the EBA brought
about by two hours of CAS revealed no decrease for the entire stimulation
duration (Da Costa, et al., 1997). This sustained suppression over time was also
seen in other studies using evoked OAE’s on human subjects for a duration of up
to 20 minutes of CAS (Moulin & Carrier, 1998; Giraud et al., 1997). In the study
by Moulin and Carrier (1988), the sustained suppressive response on DPOAE
amplitude was attributed to the MOCS being capable of a sustained efferent
effect on OAE’s (Moulin & Carrier, 1998).
Though no significant change (p<0.05) in overall suppression was observed over
time, a definite trend was noted amongst overall and half-octave frequencies.
From the initial recording to the second recording at five minutes CAS, the
majority of half-octave frequencies (1, 1.5, 2 and 3 kHz) and the overall TEOAE
measurement showed a small decrease in suppression. From then on, a slow
increase in suppression was observed from five minutes up to the last recording,
made at 15 minutes of CAS. This continued increase in suppression over time
was present in the overall TEAOE recordings and all half-octave frequency
recordings (figures 4.6 and 4.8). The decrease in amplitude may be attributed to
the slow effect reported in guinea pigs (Shridar, Liberman & Brown, 1995), which
100
consisted of an increase of the CAP efferent suppression during prolonged
electrical MOC stimulation. A similar slow effect was observed in EBA of the VIII
nerve of guinea pigs when stimulated with a low-level contralateral broadband
noise (Da Costa et al., 1997). It is believed that this slow effect is caused by OC
fibres slowly increasing their rate of discharge during repeated electrical
stimulation of the brainstem due to direct or indirect effects at their cell bodies in
the brainstem (Shridar, Liberman & Brown, 1995).
However, this slow effect of the MOC was only measured during one minute after
efferent electrical stimulation onset in guinea pigs (Shridar, Liberman & Brown,
1995) and not for longer durations. Differences in methods of MOCS stimulation
could account for the difference in the duration of this slow efferent effect.
Electrical stimulation of the MOC could shorten the duration of this slow effect by
influencing natural acetylcholine discharge patterns, whereas acoustic stimulation
could result in a more physiological activation of the MOC. A slow amplitude
decrease after stimulation onset with a longer duration (3 to 10 minutes) has also
been demonstrated in DPOAE’s on animal subjects (Brown, 1988).
5.3.2 Suppression across half-octave frequencies
Statistically, no significant differences in suppression were found between
frequencies at each time interval recording. It was also found that there was no
significant change (p<0.05) in suppression as a function of CAS duration
between these frequencies. Nevertheless, a clear trend was observed amongst
these half-octave frequencies. Results showed evident suppression in the lower
half-octave frequencies bands (1 to 2 kHz), where the effect is the greatest. As
the frequency increased, a clear decrease in suppression was observed, with the
least suppression in higher half-octave frequencies (3 and 4 kHz). This difference
can be seen in figure 4.8. This was also discussed in section 5.2.2 in the TEOAE
amplitude differences between conditions, where this observation was reported in
previous studies (Berlin et al., 1993; Collet et al., 1992; Norman & Thornton,
1993; Veuillet et al., 1991) and possibly ascribed to a greater density of MOC
101
efferent innervation in this frequency area (Guinan et al., 1983, Liberman &
Guinan, 1998; Warr et al., 1986). In such a case, the efferent effect on OHC’s in
the lower frequency area of the cochlea is expected to be less than in higher
frequency areas. This may also imply that the efferent protective function against
acoustic trauma is less in lower frequency areas (Rajan, 1995). It is generally
known that noise-induced hearing loss is a result of hair cell trauma in the region
between 4 and 6 kHz. The greater impact on this area of the cochlea is ascribed
to the fact that broadband noise (with equal energy across low to mid frequencies
of 2 to 3 kHz) damage is found to occur in the octave-band above the band of the
noise, as a result of natural resonance of the external ear (Ward, 1973). The
reduced ability of the MOC efferent system to protect OHC’s against acoustic
trauma in higher frequencies may contribute to this area being more susceptible
to noise damage.
5.4 Sustained suppression over prolonged stimulation durations
The results have important implications for understanding the physiology of
efferent auditory system and more particularly, efferent inhibition. The current
study shows that the MOCS is capable of a sustained suppressive response on
TEOAE’s over the duration of 15 min and that MOC neuron adaptation during
prolonged stimulation is unlikely. Minimal adaptation of the MOC to neurons has
been reported in anesthetized guinea pigs by measuring the adaptation of a
single olivocochlear neuron’s response to noise stimulation of 10 second
durations (Brown, 2001). It has also been found that there are minimal declines in
the response rate to continuous tones for MOC neurons (Liberman and Brown,
1986). Compared with auditory nerve response to long-term stimulation, the
MOC response was found to be much more sustained over a period of several
minutes. It was hypothesized that the decline in input provided by the auditory
nerve fibres, as they adapt over time, may be compensated for by elements
within the MOCS at a more central location (Brown, 2001). The lack of long-term
102
adaptation suggests relatively constant effects on the MOC targets when the
efferent system is activated by a contralateral noise.
Sustained efferent suppression for prolonged contralateral stimulation supports
the possibility that the role of the efferent system is predominantly for ongoing
stimuli, which corresponds with a role as a permanent gain control of the
cochlear amplifier, adjusting the gain to the level of background noise and with a
role in hearing protection against overstimulation. Indeed, the suppression effect
of MOC efferent neurons has been found to exert its protective function from
acoustic overstimulation for sound exposures of hours in duration (Kujawa &
Liberman, 1997; Zeng et al., 1997 a, b). A study measuring OAE suppression in
individuals with occupational exposure to noise showed that suppression was
decreased in the experimental, exposed group when compared with suppression
in the control, non-exposed individuals (Sliwinska & Kotylo, 2002). It was
reported that efferent auditory neurons were damaged in the individuals exposed
to noise, which may suggest a weakening of the protective function of the MOC
efferent effect over time. It should be taken into consideration that these noiseexposed individuals had abnormal hearing thresholds in higher frequencies (4
kHz, 6 kHz and 10 kHz) and poorer OAE’s (Sliwinska & Kotylo, 2002). The
decrease in OAE suppression in the exposed group as compared with the control
group may have been a direct result of noise exposure damage to the cochlea,
and not as a result of a declining efferent protective function due to prolonged
noise exposure.
Indeed, the definite trend of increased suppression over time, as was found in the
current study (figure 4.6 & 4.8), may suggest the exact opposite of Sliwinska &
Kotylo (2002) findings. An increase in efferent suppression may be caused by a
slow increase in the discharge rate of MOC fibres during prolonged CAS
(Shridar, Liberman & Brown, 1995). In such a case, the protective function of the
efferent auditory neurons is expected to not only remain stable, but to also
strengthen when the cochlea is exposed to noise for long durations.
103
5.5 Conclusion
The main effect of efferent stimulation is the physiological alteration of outer hair
cells. It is reported that efferent innervation of OHC’s probably controls the
dynamic range of the cochlea, reduces the masking effects of noise, and protects
the cochlea from the negative effects of acoustic overstimulation (Geisler, 1974;
Rajan, 1995). This study shows a stable TEOAE suppressive effect over 15
minutes of contralateral stimulation, indicating that the MOCS is capable of a
sustained efferent effect on OHC’s for prolonged durations. This supports a
sustained MOC role of protection against acoustic overstimulation, and adjusting
the dynamic range of the cochlea with no adaptation of MOC neurons over longer
periods. Moreover, a slow increase in suppression over time is observed, which
may be a result of the slow effect after stimulus onset described by Shrider et al.
(1995) in guinea pigs. However, one minute after CAS offset, TEOAE
suppression is found to completely vanish, suggesting the absence of the slow
recovery after stimulus offset that has been observed with DPOAE’s (Moulin &
Carrier, 1998). In addition, TEOAE amplitudes in the experimental condition were
found to exceed corresponding control amplitudes in the recordings three
minutes after CAS offset, suggesting a possibility of increased cochlear
sensitivity minutes after stimulus offset.
5.6 Summary
This chapter discussed and interpreted the significance of findings in line with
previous reports that studied similar characteristics of efferent suppression by
either electrical or acoustic stimulation of the MOCS. It shed light on TEOAE
amplitudes as a function of the duration of CAS for both conditions, by discussing
the differences in TEOAE amplitude between conditions at corresponding time
interval recordings with and without CAS and in the post-noise periods after CAS
offset. It also discussed TEAOE suppression as a function of different CAS
durations and provided possible physiological reasons for significant suppression
104
over different time recordings. This chapter included previous literature
describing the importance and different roles of the MOCS stimulation and
attempted to discuss the current results according to these roles.
105
Chapter 6: Conclusion and Recommendations
The main aim of this study was to investigate TEOAE suppression as a function
of prolonged contralateral acoustic stimulation (CAS). This was done by
comparing replicated recordings of TEOAE amplitude obtained over identical
recording periods in conditions with and without CAS and by defining the
relationship between the duration of CAS and TEOAE suppression amplitude.
The aim of this chapter is to draw conclusions from the reported results and to
critically review the research process. Implications of the research are presented
along with recommendations for further research
6.1 Conclusion
The research process described in this report was successful in attaining the
main aim set for this study, namely describing TEOAE amplitude suppression
over prolonged periods of CAS. The purpose of the present study was to
evaluate the relationship between the duration of contralateral acoustic
stimulation and the suppression of transient evoked otoacoustic emissions
(TEOAE’s) in normal-hearing subjects. TEOAE recordings with specific time
intervals were measured in two conditions, namely 1) in the presence of 15
minutes of continuous 45 dB SL contralateral white noise, followed by two
recordings at different time-intervals after the noise was terminated, and 2)
identical interval recordings without any noise to serve as a control condition.
Results revealed visible reduction in TEOAE amplitude for the entire duration of
contralateral
acoustic
stimulation.
Although
not
statistically
significant,
suppression tended to increase as contralateral noise duration increased. After
noise termination, TEOAE amplitudes increased to values above control
recordings.
106
It has been reported that the main effect of efferent stimulation is the
physiological alteration of outer hair cells (OHC’s) and that efferent innervation of
OHC’s may control the dynamic range of the cochlea, reducing the masking
effects of noise, and protecting the cochlea from acoustic overstimulation (Rajan,
1995).
The current study revealed the following effects on TEAOE amplitude
suppression as a result of prolonged CAS:
•
A stable TEOAE suppressive effect over 15 minutes of contralateral
acoustic stimulation was observed, indicating that the MOCS is capable of
a sustained efferent effect on OHC’s for prolonged durations (up to at least
15 minutes). This result suggests a sustained MOC role of protection
against acoustic overstimulation, and adjusting the dynamic range of the
cochlea with no adaptation of MOC neurons over longer intervals.
•
A gradual increase (though not statistically significant) in suppression over
time was recorded, which may be a result of the slow effect after stimulus
onset described by Shrider et al. (1995) in guinea pigs. This may
imply/suggest that the protective function of the MOC can also be expected
to strengthen when the cochlea is exposed to noise for long durations.
•
In the recordings one minute after CAS offset, TEOAE suppression is
found to completely abolished, which may suggest the absence of a slow
recovery, or much faster recovery (within one minute) after stimulus offset,
as has been documented with DPOAE’s (Moulin & Carrier, 1998).
•
In the recordings three minutes after CAS, TEOAE amplitudes in the
experimental condition were observed to exceed corresponding control
107
amplitudes,
suggesting
the
possibility
of
increased
cochlear
responsiveness minutes after stimulus offset.
Sustained suppression over a prolonged duration of contralateral stimulation may
imply that the MOCS has a reasonably consistent and sustained effect over
prolonged periods of time, supporting the role for this efferent system during
ongoing stimulation.
6.2 Implications of study
Apart from the clinical applications of OAE’s, advances in the field are a continual
area of research interest. Recently numerous studies have investigated the
suppression of OAE’s by CAS, because of the ability of OAE suppression to
measure alterations of cochlear micromechanics by the medial olivocochlear
bundle (MOCB), activated by acoustic stimulation of the contralateral ear
(Maison, Micheyl & Collet, 1995). One of the purposes of the current study was to
shed light on the relationship between OHC integrity, as measured with
TEOAE’s, and the duration of efferent inhibition in an attempt to understand the
MOCS more thoroughly. The auditory efferent nerve fibres are known to have an
inhibitory influence on the auditory periphery, which in turn serves as a protective
reflex against loud, damaging sounds. The proposed study investigated the
inhibitory effect of the efferent system to determine if it remains unchanged over
a prolonged period of acoustic stimulation or if the medial efferent neurons adapt
or show fatigue. The results of the present study indicated a sustained MOC role
of protection against prolonged acoustic stimulation, and in adjusting the dynamic
range of the cochlea with no adaptation of MOC neurons over longer intervals.
The current study thereby provided information of the duration characteristics of
CAS. This knowledge may be useful in our understanding of the exposure
durations for which MOC protection is effective and the duration for which the
MOC system plays a role in reducing the effects of noise masking. The definite
trend of increased TEOAE suppression over time may provide evidence for an
108
increase in the discharge rate of MOC fibres during prolonged CAS. This is an
interesting finding and may be an indication of an increased or strengthened
protective function of the MOC during prolonged exposure durations. The
significant increase in TEOAE suppression amplitude after stimulus offset may
also be important knowledge for clinical use. With total TEOAE suppression
recovery within one minute after noise termination, it may complicate suppression
measurements using a forward masking paradigm, where the suppression
amplitude is usually measured directly after stimulation. It should be
acknowledged that this paradigm is used mostly in the case of ipsilateral or
bilateral acoustic stimulation.
109
Table 6.1 Critical evaluation of test method variables as applied in current study
Option chosen for
current study
TEOAE broadband click
Time interval
between TEOAE
recordings
4 minutes in between
recordings in the with and
without noise
measurements and 1
minute between recordings
in the post-noise periods
Stimulus
polarity
Linear
Strengths
A single TEOAE measurement takes ± 1
minute to collect whereas DPOAE’s require
more time and are often inconsistent. Using
DPOAE’s would have complicated the test
procedure (Figure 3.1). Click TEOA’Es could
be measured every few minutes during
continuous contralateral stimulation, because
the responses could be collected rapidly.
As mentioned in the above section, TEAOE
responses took ± 1 minute to collect. Thus
time intervals between recordings needed to
be more than 1 minute. A 4-minute interval
was chosen to investigate the suppression
over a time.
A constant linear stimulus polarity can be
used, because lower click intensity levels are
used in the evaluation of contralateral
suppression effects.
60 dB SPL
Stimuli
intensity
STIMULUS
OAE Type
Test
variable
A contralateral BBN stimulus is most effective
in suppressing TEOAE’s when the intensity of
the click stimulus used to evoke an emission is
between 55 and 65 dB SPL (Hood et al.,
1996)
Limitations
DPOAE measurements can provide more frequency-specific
information. DPOAE’s have been used in a similar study
(Moulin and Carrier, 1998), but F2 was fixed at frequency
1501 Hz in order to shorten the time to collect responses. The
use of an broadband clicks ensured that the components of
the TEOAE across a broad frequency range could be elicited
If TEOAE’s could be recorded more rapidly, it would provide
more specific information on the changes over time, for
example, a TEOAE recording every minute. It also would
have been very useful to record directly after the noise was
terminated.
Nonlinear stimuli have the advantage of largely eliminating
the stimulus artefact
Although nonlinear stimuli are most often used for TEOAE
recordings, they may result in some growth in the emission
and they also have a higher level of residual noise.
In some individuals, the use of such low stimulus intensities
reduced the response reproducibility and stability. When the
intensity was increased during the pilot study this problem
was eliminated. Nevertheless responses were only accepted
if the reproducibility was above 70% and the stability above
80%.
110
Contralateral vs. Ipsilateral and Bilateral
Suppressor intensity Suppressor
presentation
SUPPRESSOR
Test
variable
Option chosen for
current study
Contralateral suppressor
Sensation level intensity of
45 dB
Noise generated by a
clinical audiometer
Strengths
Limitations
A contralateral suppressor made it possible to
present the continuous noise suppressor
simultaneously with the TEOAE-evoking
recordings in the opposite ear.
With ipsi- and bilateral suppressors two
approaches of assessing are used. Firstly, a
forward masking paradigm, where a
suppressor signal is presented to an ear prior
to the presentation of an OAE stimulus. In this
case, the suppressor must be terminated
before the OAE recording, which thus makes it
impossible to measure the effect of continual
CAS on OAE’s. The second approach is to use
a simultaneous masking paradigm. Here the
suppressor is presented simultaneously with
an OAE, but a special custom-made dual
probe is needed.
Sensation level intensities of 30-50 dB SL
have been found to be low enough to prevent
any crossover and contraction of the
contralateral stapedius muscle (Berlin et al.,
1994; Ryan et al., 1991).
Sensation level intensities of each subject
could be estimated by determining the
threshold for white noise and presenting the
suppressor 45 dB above the threshold.
In relation to contralateral stimulation, ipsilateral and
contralateral suppressors can result in more pronounced
suppression of evoked OAE’s (Kemp & Chum, 1980;
Travartkiladse, Frolenkov, Kruglov, Artamasov, 1994; Wilson,
1980)
In order to measure the effect of continual CAS on OAE’s by
using ipsilateral and contralateral suppression
measurements, special equipment (custom-designed
acoustic probes, consisting of a microphone and two
electroacoustic transducers which provide the same ear with
the suppressor signal and the recording-evoked OAE) is
required.
Click stimulus intensities of below 65 dB SPL have been
found to be the most effective in suppressing TEOAE
amplitudes when using contralateral white noise stimuli (Hood
et al., 1996a:117). Hood et al. (1996) suggested using 55 or
60 dB peak SPL with the overall intensity level of the noise
set at or 5 dB higher than the click intensity (Hood et al.,
1996a:117),. The click intensity used in the current study is
65 dB SL. It is imperative to avoid using high click intensities
(e.g. 70 dB SPL) in order to minimize the risk of major
participation of the middle ear muscle reflexes
White noise generated by a GSI 61 clinical audiometer is
presented in dB HL and not in SPL. SPL’s in the ear canal
during noise presentation could not be measured, because of
lack of appropriate equipment.
111
Subject characteristics
Suppression
measurements
TEST PROCEDURE
SUBJECTS
Test
variable
Option chosen for
current study
Auditory characteristics:
Thresholds ≤ 15 dB HL
from 250 to 8000 Hz,
normal otoscopic
examination,
tympanograms and
otologic history, no history
of noise or ototoxic
exposure or complaints of
middle ear problems
Age range: 20-30 years
The entire procedure was
repeated three times, each
on separate days, in all the
subjects, to increase the
reliability of the results.
Strengths
Limitations
Selection criteria ensured homogeneity of
research sample and controlled factors that
could possibly affect TEOAE recordings
Required a series of selection procedures.
One session of data collection took
approximately an hour to conduct. The
experiment is a lengthy procedure and thus it
was decided to do the recordings over three
different days to decrease subject discomfort.
Subject exposure to noise during this period of days could not
be closely controlled. If the subject were to be exposed to
excessive noise during these three days OAE results could
be negatively influenced. It is well known that OAE’s are
highly sensitive to noise damage, even before threshold
changes are observed with audiometric measurements.
112
6.3 Recommendations for further research
The experimentation conducted in this project provided insight into the effect of
prolonged CAS on TEOAE’s and the effects after CAS offset. Investigating the
TEOAE amplitudes, in the presence and absence of CAS, provided a noninvasive, objective, approach for investigating MOCS efferent feedback in
subjects (Giraud, Collet, Chery-Croze, Magnan, & Chays, 1995). However,
additional research could prove valuable in further refining the test methodology.
The current research findings could also stimulate further research. The
recommendations for further improvement on the current study and new findings
that may be worth investigating are summarized in Table 6.2.
Table 6.2 Recommendations on improving test method and new research
level
Suppressor transducer
Suppressor intensity
METHODOLOGY
areas to be investigated
Current study
Recommendations for futures studies
In the current study prolonged continual
contralateral acoustic stimulation was
used to investigate suppression on
TEOAE’s. It has been found that ipsilateral
and contralateral suppressors can result in
more pronounced suppression of evoked
OAE’s than contralateral suppressors
(Kemp & Chum, 1980; Travartkiladse,
Frolenkov, Kruglov & Artamasov, 1994;
Wilson, 1980). The reason why this
approach was not followed in the current
study is mainly due to the lack of a dual
probe consisting of microphone and two
electroacoustic transducers, which provide
the same ear with the suppressor signal
and the recording OAE.
White noise was generated by a clinical
audiometer which presents the noise in dB
HL and not in dB SPL. SPL’s in the ear
canal during noise presentation could not
be measured, because of the lack of
appropriate equipment.
With the use of a custom-made dual probe,
prolonged continual ipsilateral and bilateral
suppression on TEOAEs can be investigated
using the simultaneous masking paradigm.
The use of ipsilateral and bilateral suppressors
may result in more robust suppression, which
may show clearer effects of acoustic
stimulation on TEOAE’s. It is important,
however, to take into consideration that
bilateral and ipsilateral suppression of
TEOAE’s may not be attributed only to the
MOC bundle but also to intracochlear
processes.
Some OAE apparatus are equipped with noise
generation software, which presents noise in
SPL. Often in the literature, research on OAE
suppression was done by using noise
presented in SPL. It would be useful to
conduct similar studies using suppressor
intensities measured in SPL in order to
compare the results with these studies and to
ensure that the level stays under 70dB SPL in
order to minimize the risk of major participation
of the middle ear muscle reflexes.
113
Slow effect of MOC stimulation
TEOAEs after CAS termination
NEW RESEARCH
Current study
Recommendations for futures studies
A slow increase (though not significant) in
suppression over time is recorded, which
may be a result of the slow effect after
stimulus onset described by Shrider et al.
(1995) in guinea pigs. This slow effect
may be caused by a slow increase in the
discharge rate of MOC fibers during
prolonged contralateral stimulation
(Shridar, Liberman & Brown, 1995).
However, this slow effect of the MOC was
only perceived during one minute after
efferent electrical stimulation onset and
not for longer durations, as in the current
study.
In the post noise periods two interesting
results were found:
• Firstly, one minute after CAS offset,
TEOAE suppression was found to have
been completely eliminated, suggesting
the absence of a slow recovery after
stimulus offset as been documented in
previous reports (Moulin & Carrier,
1998).
• Secondly, in the recordings three
minutes after CAS, TEOAE amplitudes
in the experimental condition were
found to exceed corresponding control
amplitudes, which may suggest the
possibility of increased cochlear
sensitivity minutes after stimulus offset.
Differences in methods of MOCS stimulation
could account for the difference in the duration
of this slow efferent effect. Electrical
stimulation of the MOC could shorten the
duration of this slow effect by influencing
natural acetylcholine discharge patterns,
whereas acoustic stimulation leads to a more
physiological activation of the MOC. It would
be interesting to investigate the slow effects by
measuring the discharge rate of MOC fibers
by acoustically stimulating the MOC
It would be interesting to conduct a experiment
recording OAE responses before and after
prolonged noise stimulation, to investigate if
there are changes in the OAE amplitude. I,
this were to happen, and a slowly increasing
OAE amplitude, as has been found in current
study, were observed, these results would be
confirmed. If a slow increase were observed,
as compared with recordings before noise
stimulation, it might suggest that there is
increased cochlear sensitivity after prolonged
noise stimulation.
6.4 Final conclusion
The research project described in this study has been successful in answering
the research questions posed in the first two chapters. The different methods that
were used by previous researchers in the investigation of OAE suppression,
especially TEOAE’s, were extensively explored and critically discussed to
provide a framework that served to guide the methodology of the present study.
The experiment provided new information on the effect of prolonged CAS on
TEOAE’s in normal-hearing subjects, which indirectly provides insight in the
efferent response of the MOCS to constant acoustic stimulation. These findings
provide further support for the protective function of the efferent auditory system,
even when the auditory system is subjected to acoustic overstimulation for long
periods.
114
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Appendixes
138
Appendix A: The interview
Personal information
Name:
Subject file:
Date of birth:
Gender:
Information regarding hearing status
Do you have any problems with your hearing?
Did you suffer from any pain in your ear/s within the last 6 months?
Did you have any discharge coming from your ear/s recently?
Did you suffer from or are you suffering from any other ear, nose or throat
problem?
Does anyone in your family have a childhood history of hearing loss?
Do you hear any unusual sounds or noises in your ear/s?
Do you practice any hobbies that involve exposure to very loud noise?
Have you been exposed to very loud noises?
Did you injure your head and/or ear/s that affected your hearing?
What types of medication are you currently using?
________________________________________
139
Appendix B: Informed Consent
Department of Communication Pathology
Speech, Voice and Hearing Clinic
Tel
: +27 124202355
Fax
: +27 124203517
Email
: [email protected]
Researcher: Altelani van Zyl
Tel: 082 925 0830
E-mail: [email protected]
Date: ____________
To Whom It May Concern:
Thank you for showing interest in this research project being conducted at the
Department of Communication Pathology, University of Pretoria.
We are currently investigating the effects of prolonged contralateral acoustic
stimulation on Otoacoustic emissions (OAEs). This will give us a better
understanding of the way the outer ear hair cells of the inner ear react to
prolonged stimulation of broad band noise. This procedure is completely
harmless and non-invasive. Participation in the study is voluntary and you may
withdraw at any time if you wish to. If you do participate the following procedures
will apply to you:
• An otoscopic examination, followed by immittance measurements, will be
carried out. You will be asked to sit quietly, while the researcher examines
your outer ear canal, eardrum and your middel ear functioning. These
procedures do not require any response from you and will take
approximately 5 minutes.
•
You will then undergo a standard hearing evaluation (pure tone
behavioural audiometry), where you are required to respond to the
presence of a sound. This procedure takes approximately 10 minutes.
140
•
An otoacoustic emission (OAE) test will then be conducted. This
procedure is also objective and does not require a response from you.
During the TEOAE measurement a small probe will be placed in one ear
while an earphone will be placed on your other ear. The test will be
repeated two times without and with noise presented to the non-test ear. It
is important to know that the noise level that will be used will be at a
comfortable level. The entire procedure will last for approximately 1 hour.
All the procedures (tests) are non-invasive and only the behavioural (pure tone)
procedures require responses from you. All acquired information will be treated
as confidential and no names will be used. The results will be used for research
purposes as part of a dissertation and possibly future articles and presentations.
The data will be stored for archiving and research purposes for 15 years. By
agreeing to participate in this study you acknowledge that future research using
the acquired data may be conducted at a later stage. A copy of your results will
be made available to you, should you request it. You are free to withdraw from
the study at anytime without any negative consequences.
Should you require any further information, you are welcome to contact us.
Sincerely,
Altelani van Zyl
Researcher
Dr De Wet Swanepoel
Supervisor
Professor Brenda Louw
HEAD: Dept of Communication Pathology
141
University of Pretoria
Department Communication Pathology: Audiology
The effect of prolonged contralateral noise on the amplitude of TEOAE
suppression
Surname____________________
Name______________________
Age________________________
Please complete the following:
I
hereby agree to participate in this project and
acknowledge that the data may be used for research purposes. I am aware that I
can withdraw from this project, at any time, should I want to.
Signature
Date
142
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