Benefits of Localization and Speech Perception with Multiple Noise

Benefits of Localization and Speech Perception with Multiple Noise
J Am Acad Audiol 21:44–51 (2010)
Benefits of Localization and Speech Perception
with Multiple Noise Sources in Listeners with
a Short-Electrode Cochlear Implant
DOI: 10.3766/jaaa.21.1.6
Camille C. Dunn*
Ann Perreau*
Bruce Gantz*
Richard S. Tyler*
Abstract
Background: Research suggests that for individuals with significant low-frequency hearing, implantation
of a short-electrode cochlear implant may provide benefits of improved speech perception abilities.
Because this strategy combines acoustic and electrical hearing within the same ear while at the same
time preserving low-frequency residual acoustic hearing in both ears, localization abilities may also be
improved. However, very little research has focused on the localization and spatial hearing abilities of
users with a short-electrode cochlear implant.
Purpose: The purpose of this study was to evaluate localization abilities for listeners with a short-electrode
cochlear implant who continue to wear hearing aids in both ears. A secondary purpose was to document
speech perception abilities using a speech-in-noise test with spatially separate noise sources.
Research Design: Eleven subjects that utilized a short-electrode cochlear implant and bilateral hearing aids were tested on localization and speech perception with multiple noise locations using an
eight-loudspeaker array. Performance was assessed across four listening conditions using various combinations of cochlear implant and/or hearing aid use.
Results: Results for localization showed no significant difference between using bilateral hearing aids
and bilateral hearing aids plus the cochlear implant. However, there was a significant difference between
the bilateral hearing aid condition and the implant plus use of a contralateral hearing aid for all 11 subjects.
Results for speech perception showed a significant benefit when using bilateral hearing aids plus the
cochlear implant over use of the implant plus only one hearing aid.
Conclusion: Combined use of both hearing aids and the cochlear implant show significant benefits for
both localization and speech perception in noise for users with a short-electrode cochlear implant. These
results emphasize the importance of low-frequency information in two ears for the purpose of localization
and speech perception in noise.
Key Words: Cochlear implant and hearing aid, hybrid cochlear implant, localization, short-electrode,
speech perception
Abbreviations: ILD 5 interaural level difference; ITD 5 interaural timing difference; RMS 5 root mean
square; S/N = signal to noise
T
he use of cochlear implants as an intervention
to remediate hearing loss in listeners with
bilateral, profound hearing loss has become
widely accepted. Due to the success of cochlear
implantation, many people are able to regain hearing
function and communicate more effectively in their
*Department of Otolaryngology—Head and Neck Surgery, University of Iowa
Camille C. Dunn, Ph.D., Assistant Research Scientist, Department of Otolaryngology—Head and Neck Surgery, University of Iowa, 200 Hawkins
Drive, PFP 21038, Iowa City, IA 52242-1078; E-mail: camille-dunn@uiowa.edu
This research was supported in part by research grant 5 P50 DC00242 from the National Institute on Deafness and Other Communication
Disorders, National Institutes of Health; grant M01-RR-59 from the General Clinical Research Centers Program, Division of Research Resources,
National Institutes of Health; the Lions Clubs International Foundation; and the Iowa Lions Foundation.
44
Localization Short-Electrode Cochlear Implant/Dunn et al
everyday lives. Furthermore, some cochlear implant
users maintain a degree of residual hearing in the
opposite ear and choose to use a hearing aid in the nonimplanted ear.
Research suggests that use of a cochlear implant plus
hearing aid in opposite ears provides binaural advantages to these users and, as a result, improved speech
perception abilities (Shallop et al, 1992; Chmiel et al,
1995; Simon-McCandless and Shelton, 2000; Tyler
et al, 2002; Dunn et al, 2005; Kong et al, 2005; Luntz
et al, 2005; Mok et al, 2006). Another important aspect
of hearing, the ability to localize a sound source, also
has implications for those using cochlear implants plus
hearing aids in opposite ears. Some studies have shown
improved localization for subjects using a contralateral
hearing aid plus a cochlear implant versus use of either
device alone (Tyler et al, 2002; Ching et al, 2004). However, other results found that only a few listeners (2 out
of 12) showed an improvement in localization despite
wearing a cochlear implant and hearing aid in opposite
ears (Dunn et al, 2005). Additionally, unpublished data
on four subjects at the University of Iowa showed a significant decrement in localization abilities when comparing preoperative abilities using two hearing aids
to abilities after 12 mo of cochlear implant plus hearing
aid use.
One reason for these findings may be the differences
in signal processing between the acoustic and electric
signals, which can interfere or even distort the interaural timing and level differences of the incoming signal.
Research suggests that cochlear implant and hearing
aid devices might not accurately convey interaural timing and level differences (Tyler et al, 2002; Tyler et al,
2006; Ching et al, 2007; Francart et al, 2008). This is
likely due to the following: (1) differences in the place
of stimulation in the cochlea across ears, (2) the inability of cochlear implant strategies to accurately encode
fine structure information in the cycle-by-cycle structure of the stimulus (Kong et al, 2005; Grantham
et al, 2007); (3) differences in processing of time delays
across the two ears, (Tyler et al, 2002, 2006; Ching et al,
2007) and (4) loudness differences across ears (Ching
et al, 2001; Tyler et al, 2002). However, more similar
signal processing across ears (i.e., two hearing aids or
two cochlear implants) likely results in better coding
for spectral, level, and timing cues. Despite this, many
people have high-frequency severe-to-profound hearing
loss, and continuing with two hearing aids does not
effectively help them overcome their hearing difficulties. The amount of residual low-frequency hearing
may be adequate enough in this group of listeners that
they would not be considered candidates for the traditional cochlear implant, yet communication difficulties
remain. An alternative method is to implant a shorterlength cochlear implant into the basal end of the cochlea
that combines acoustic and electrical hearing within the
same ear, while simultaneously preserving low-frequency residual acoustic hearing bilaterally.
Research studying combined unilateral acoustic and
electrical hearing shows that listeners are able to successfully integrate low-frequency acoustic hearing with
high-frequency electrical stimulation for improved
speech perception abilities (Gantz and Turner, 2003,
2004; Gantz, et al, 2009). Research has also suggested
that a short-electrode cochlear implant may provide
more benefits than a standard length cochlear implant
for individuals with significant low-frequency hearing
(Gantz et al, 2005; Novak et al, 2007). It is thought
that listeners with preserved low-frequency hearing
will obtain better speech understanding due to better
frequency selectivity from available low-frequency
cues (Turner et al, 2007). When compared to preoperative hearing with bilateral hearing aids, these listeners
with combined acoustic and electrical signal processing
in the same ear showed improvements in word understanding, speech in noise, and melody recognition (Gantz
et al, 2005). A study by Turner et al (2004) compared performance on a speech-perception-in-noise task for two
groups of subjects who were matched according to
their speech recognition ability in quiet: those
with combined acoustic and electric hearing in a
unilateral ear to those with a single long cochlear
implant. Results showed a 9 dB advantage in signalto-noise ratio for the users with combined acoustic plus
electric hearing in the same ear. In addition, studies
have shown that preserved low-frequency hearing along
with use of a short-electrode cochlear implant provides
listeners with improved musical performance, including
better melody recognition and greater music appreciation (Gantz and Turner, 2003; Gantz and Turner,
2004; Gantz et al, 2005; Gantz et al, 2006; Gfeller
et al, 2006; Turner et al, 2007).
To date, very little research has focused on localization and binaural hearing abilities for users of a shortelectrode cochlear implant. Localization has important
applications for all listeners because it allows for accurate identification of environmental sounds and alarms
and allows listeners to attend to a target or speaker
when several talkers in a background of noise are
presented to the listener. A potential advantage of
the short-electrode cochlear implant is that most of
these listeners often continue wearing bilateral hearing aids after receiving the cochlear implant, which
provides them with similar processing across ears.
Compared to the traditional standard length cochlear
implant worn in one ear and a hearing aid worn on
the opposite ear, this could provide a clear advantage
for these listeners because important timing and
level cues would no longer be dissimilar across ears.
The purpose of this study was to evaluate localization
abilities for listeners with a short-electrode cochlear
implant who continue to wear bilateral hearing aids.
45
Journal of the American Academy of Audiology/Volume 21, Number 1, 2010
A secondary purpose was to document speech perception abilities using a speech-in-noise test with spatially separate noise sources.
METHOD
All subjects reportedly wore their hearing aids consistently following implantation except for subject SE9,
who reported that he occasionally did not wear the
hearing aid contralateral to the implant. All subjects
had at least 6 mo of cochlear implant experience at
the time of testing.
Subjects
Pure-Tone Acoustic Thresholds
Eleven adults (M 5 61.3 yr; range 5 51–81 yr)
implanted with a Nucleus 24 Hybrid 10 mm shortelectrode cochlear implant served as subjects for this
study. Table 1 shows demographic data for all 11 subjects, including age, ear implanted, duration of cochlear implant use, frequency range of the cochlear
implant, hearing aid use prior to cochlear implantation, and hearing aid type. Seven subjects, denoted
with an “A” in their subject name (A2, A4, A7, A8,
A9, A10, A12), were implanted with a CI24RE shortelectrode cochlear implant, and four subjects, denoted
with an “SE” in their subject name (SE5, SE8, SE9,
SE11), were implanted with a CI24M short-electrode
cochlear implant. Both cochlear implants had six
intracochlear electrodes; however, the earlier CI24M
device allowed for stimulation rates up to 2400 Hz
whereas the CI24RE internal device allowed for stimulation rates up to 3500 Hz. All other parameters and
specifications were essentially the same across these
two implant types. All subjects were provided with a
hearing aid in the ipsilateral ear to use in conjunction
with the short-electrode cochlear implant following
surgery. This hearing aid was a 15-channel digital,
Phonak Aero 33 in-the-ear (ITE) hearing aid. No noise
reduction or directional microphones were active during testing. On the contralateral ear, subjects used
their own hearing aid that was not controlled for in
this study. As seen in Table 1, 10 of the subjects used
bilateral hearing aids for 5 yr or longer prior to cochlear implantation. Subject A9 had approximately
3 mo of hearing aid use prior to cochlear implantation.
Pure-tone acoustic thresholds were measured at the
time of testing using insert earphones and a clinical
audiometer in both ears for all 11 subjects. Figure 1 displays pure-tone acoustic thresholds in the ipsilateral (or
same side as the implanted ear) (A) and contralateral
(B) ears for each subject. Mean thresholds were also calculated and are plotted in bold overlying the individual
data. Thresholds in the implanted ear varied from mildto-severe hearing loss levels for frequencies 125, 250,
and 500 Hz, and reached profound hearing loss levels
at 2000 Hz and above. Few subjects had responses in
the implanted ear from 4000 to 8000 Hz. Results for
the contralateral ear showed that all subjects had moderate hearing loss or better at 125, 250, and 500 Hz,
sloping to a profound hearing loss at 2000 Hz and above.
Per the FDA protocol for the Nucleus Hybrid cochlear
implant, the poorer hearing ear always received the
cochlear implant.
Hearing Aid and Cochlear Implant Fitting
Hearing aid verification using real ear probe measurements was completed bilaterally for all subjects.
The hearing aid settings were adjusted to approximate
NAL-RP targets at all low frequencies. Cochlear
implant programming was completed for each subject
as part of their routine clinical follow-up. The cochlear
implant frequency response was set to supplement the
subject’s acoustic hearing as determined from their
audiogram (see Figure 1). Specifically, the cochlear
Table 1. Individual Subject Demographics
Subject
A8
A2
SE11
A12
A10
SE9
A4
SE8
A7
A9
SE5
46
Age
(years)
Implant
ear
56
61
69
54
81
51
51
69
74
51
57
Right
Left
Right
Left
Right
Right
Right
Right
Right
Left
Left
CI use
(yr, mo)
0,
1,
3,
0,
0,
3,
1,
3,
0,
0,
5,
7
6
1
6
6
6
3
10
6
6
11
Frequency range for
standard map
Pre-implant hearing
aid use (years used,
bilateral or unilateral)
Hearing aid type
(R 5 right, L 5 left)
563–7938 Hz
688–7938 Hz
688–7938 Hz
563–7938 Hz
688–7938 Hz
750–8000 Hz
688–7938 Hz
1063–7938 Hz
1063–7938 Hz
688–7938 Hz
688–7938 Hz
13, bilateral
15, bilateral
5, bilateral
25, L; 28, R
15, bilateral
10, bilateral
9, bilateral
12, bilateral
13, bilateral
.25, bilateral
25, bilateral
Phonak ITE, R; Phonak BTE, L
Phonak ITE, L; Phonak BTE, R
Phonak ITE, R; Phonak BTE, L
Phonak ITE, L; AVR Transpositional BTE, R
Phonak ITE, R; Beltone BTE, L
Phonak ITE, R; Phonak BTE, L
Phonak ITE, R; Phonak BTE, L
Phonak ITE, R; Phonak BTE, L
Phonak ITE, R; Phonak BTE, L
Phonak ITE, L; Phonak BTE, R
Phonak ITE, L; Phonak BTE, R
Localization Short-Electrode Cochlear Implant/Dunn et al
contralateral ears. The conditions consisted of the following: (1) combined, (2) hybrid, (3) bimodal, and (4)
bilateral hearing aids. In the combined condition, subjects used bilateral hearing aids in addition to the cochlear implant. The hybrid listening condition referred to
the cochlear implant and the hearing aid on the same
ear, but no hearing aid on the contralateral ear. In comparison, the bimodal condition implied use of the cochlear implant plus the contralateral hearing aid, but no
hearing aid on the ear with the cochlear implant.
Finally, the bilateral hearing aid mode consisted of
hearing aids on both ears, but the cochlear implant processor is turned off. A description of each condition is
further summarized in Table 2. The testing order for
the above conditions was randomized for each subject.
Ear plugs were placed in the subjects’ ears by the
experimenter when a hearing aid was removed from
a test condition. For all test conditions, no modifications
were made to the cochlear implant or hearing aid programming, and parameters were set identical across
test conditions. Subjects were tested acutely in the laboratory and did not have a listening trial with each of
the different conditions before testing began.
Test Measures
Localization
Figure 1. A: Individual and average implanted ear pure-tone
thresholds for all subjects. B: Individual and average contralateral
ear implanted pure-tone thresholds for all subjects.
implant was set to stimulate only the high frequencies
where acoustic hearing was greater than approximately
90 dB HL and to provide minimal frequency overlap
with the hearing aid.
Test Conditions
All subjects were tested in numerous conditions to
evaluate localization abilities using the short-electrode
cochlear implant and hearing aids in the ipsilateral and
Localization ability was assessed on all 11 subjects
using an Everyday Sounds Localization test (Dunn
et al, 2005). An array of eight loudspeakers spanning
a horizontal arc of 108° was used. Loudspeakers one
and eight were placed 54° to the left and to the right
of the straight-ahead (0°) position. Sixteen different
everyday sounds (i.e., child laughing, baby crying, glass
breaking, and telephone ringing) were each presented
six times randomly from one of the eight loudspeakers
at 70 dB(C) SPL. Subjects were asked to identify the
loudspeaker from which the sound originated using a
touch screen monitor placed in front of them. No feedback was provided throughout the test. Head movement
was restricted throughout the test, and subjects were
asked to fixate on an object placed directly in front of
Table 2. Test Conditions
Condition
number
Mode of testing
1
Combined
2
Hybrid
3
Bimodal
4
Bilateral
hearing aids
Description
Configuration
A cochlear implant and a hearing aid in one ear and a hearing
aid in the opposite ear.
A cochlear implant and a hearing aid in one ear. No hearing
aid on the opposite ear.
A cochlear implant in one ear and a hearing aid in the opposite
ear. No hearing aid on the ear with the cochlear implant.
Hearing aids on both ears and no cochlear implant.
CI 1 Ipsi HA 1 Contra HA
CI 1 Ipsi HA
CI 1 Contra HA
Ipsi HA 1 Contra HA
Note: CI 5 cochlear implant; Ipsi 5 same side as the cochlear implant; HA 5 hearing aid; Contra 5 opposite side of the cochlear implant.
47
Journal of the American Academy of Audiology/Volume 21, Number 1, 2010
Figure 2. Individual and average localization scores (RMS error in degrees) for the combined, hybrid, bimodal, and bilateral hearing aid
listening conditions. Better performance is reflected by a lower RMS error score.
them. Localization performance was determined by calculating the average root mean square (RMS) error in
degrees. Chance performance on the Everyday Sounds
Localization test was approximately 40 degrees RMS
error and a lower RMS error score indicated better
localization performance. All four test conditions (combined, hybrid, bimodal, and bilateral hearing aids) were
completed for the localization test.
Speech Perception
The Recognition with Multiple Jammers speechperception-in-noise test (see Tyler et al, 2006) was
administered to nine subjects. This test was used to
evaluate binaural hearing abilities as it simulates a situation where listeners have to separate a target signal
from similar competing sounds that are introduced
from multiple locations (Hawley et al, 1999; Culling
et al, 2004). In this test, the listener was to select the
target spondee word, heard in a background noise, from
12 possible words (Turner et al, 2004). An array of eight
loudspeakers spanning an arc of 108° in front of the subject was used to present both the target and background
noise. The target spondee word was presented from a
front-facing loudspeaker (either 68° from 0° azimuth),
and background noise was presented from two loudspeakers to the right and left of the subject (located either
at 154° and 238° azimuth, or at 138° and 254° azimuth.
The background noise consisted of sentences from randomly selected male and female talkers presented
simultaneously. The sentences were different from
trial to trial. Additionally, the level of the background
noise varied adaptively while the level of the spondee
word remained constant throughout the testing. The
target spondee word was played 0.8 sec following
48
the start of the background noise. Subjects manually
entered their responses using a touch screen monitor
placed in front of them. The signal-to-noise (S/N) ratio
yielding 50% correct was obtained with a two-up and
two-down adaptive rule with a total of 14 reversals.
Each test consisted of five runs, and the signal-tonoise (S/N) ratio was calculated based on the average
threshold of the last three runs. Because of time constraints while testing, only the combined, hybrid, and
bimodal test conditions were completed for this test.
RESULTS
Localization
Figure 2 displays individual data for each subject as
well as average results for the Everyday Sounds Localization test comparing the following conditions: combined, hybrid, bimodal, and bilateral hearing aids. A
repeated-measures analysis of variance revealed that
there was a significant difference in localization scores,
F(1.63, 14.67) 5 28.75, p , .001. Post hoc comparisons
using a Bonferroni adjustment revealed that the combined and bilateral hearing aid conditions were significantly better than the hybrid and bimodal conditions.
No significant difference was found between combined
and bilateral hearing aid conditions or between the
hybrid and bimodal conditions. Individual results
showed that all subjects were able to localize better than
chance performance in the combined and bilateral hearing aid test conditions and performed best in these two
conditions. In contrast, eight subjects (A12, A8, A2, SE9,
SE8, A7, A9, A10) scored above chance performance
when using the bimodal condition. Additionally, eight
subjects (SE5, A12, A8, A2, A4, A7, A9, SE11) also
Localization Short-Electrode Cochlear Implant/Dunn et al
Figure 3. Recognition with Multiple Jammers test individual and average signal-to-noise (S/N) ratio scores for combined, hybrid, and
bimodal testing conditions. Lower scores (more negative) indicate better speech perception performance.
scored above chance performance when using the hybrid
condition.
Speech Perception
Figure 3 displays individual and average results on the
Recognition with Multiple Jammers test comparing the
following conditions: combined, hybrid, and bimodal. A
repeated-measures analysis of variance revealed a significant difference, F(2, 16) 5 9.65, p , .01. Post hoc comparisons using Bonferroni adjustment showed that the
combined listening condition was significantly different
from the hybrid and bimodal conditions. Individual
resultsshowedthat four(SE9, A4, A9, A10)oftheninesubjects performed better when using the combined condition
over the hybrid and bimodal conditions. No subjects performed best with the hybrid or bimodal conditions, but
three subjects (SE5, A8, SE9) performed better in the
bimodal condition over the hybrid condition. Only one
(A10) subject performed better with the hybrid condition
over the bimodal condition. Finally, two subjects (A12,
SE8) showed no difference between any of the conditions.
DISCUSSION
O
ne approach to improving speech understanding
and hearing function for individuals with significant low-frequency residual hearing is to obtain a
shorter-length cochlear implant in addition to conventional hearing aid use. This type of implant stimulates
the high-frequencies in the basal region of the cochlea
while residual hearing is maintained by not disrupting
low-frequencies stimulated via hearing aids in the apex
of the cochlea. A potential benefit of providing electro-
acoustic stimulation in the same ear is that the listener
can utilize bilateral hearing aids and thus rely on similar signal processing across ears.
In this study, performance on sound source localization and speech perception in noise using spatially separate noise sources was evaluated using different
listening configurations: combined, bimodal, hybrid,
and bilateral hearing aids. Three of the four listening
conditions were conducted in the binaural mode, including combined, bimodal, and bilateral hearing aids, and
provided the listener with binaural hearing cues. The
binaural auditory system computes differences in interaural timing (ITD) and level (ILD) between ears to
determine the azimuthal location of sound sources as
well as provide benefits to speech perception. Below
about 800 Hz, ITDs are the primary cues used for localization (Carhart, 1965; Dirks and Wilson, 1969;
Durlach and Colburn, 1978; Yost and Dye, 1997) and
binaural squelch effects (the ability to combine the noise
at the ear with the poorer signal-to-noise (S/N) ratio
with the noise from the ear with the more favorable
S/N ratio [Carhart, 1965; Middlebrooks and Green,
1991; Zurek, 1993]). For higher frequency information
(above 1500 Hz), ILD information is the primary cue
used for localization and benefits from the head shadow
effect (the head acts as an acoustic barrier, which creates a spectral difference between the two ears resulting in a greater S/N ratio at one ear [Shaw, 1974]).
However, it should be noted that the ability to use head
diffraction is not a direct function of binaural processing
but is the physical consequence of sound diffraction around an object. From about 700 Hz to around
1200 Hz, both ITD and ILD information can be useful
for benefits of localization and speech perception (Dunn
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Journal of the American Academy of Audiology/Volume 21, Number 1, 2010
et al, 2006). In the current study, listeners are provided
with mostly low-frequency information through their
hearing aids (up to around 1000 Hz) and high-frequency
information through the cochlear implant (ranging
from 500 to 8000 Hz or 1000 to 8000 Hz).
The results from our study indicated that localization
abilities were significantly better when using bilateral
hearing aids and a cochlear implant worn together compared to using a single hearing aid and a cochlear
implant on opposite ears. This might not be surprising
given there is little overlap in the bilateral signals since
the bimodal condition provides mostly high-frequency
information via the cochlear implant and only low frequency information via the hearing aid. Additionally,
the differences in signal processing might interfere or
even distort the available ILD and ITD cues. It appears
that when listeners have similar processing bilaterally
through the use of bilateral hearing aids, listeners are
able to take advantage of the ITD cues, and the addition
of the cochlear implant did not disrupt their overall performance. In fact, when comparing results between the
bilateral hearing aid condition to the use of the cochlear
implant condition plus hearing aids, there was no significant difference in performance. In addition, even
though the bimodal condition, with a hearing aid in
one ear and a cochlear implant in the other, provided
binaural stimulation, performance was not significantly different from the hybrid condition where the
subjects wore a single cochlear implant and hearing
aid on the same ear.
A secondary purpose of this study is to document
speech-perception-in-noise abilities using a test with
multiple noise sources where listeners have to separate
a target signal from similar competing sounds. Results
showed that combined use of a cochlear implant plus
bilateral hearing aids facilitates the best speech perception performance compared to use of a cochlear implant
and a single hearing aid worn on opposite ears or worn on
the same ear (hybrid). As demonstrated in this study
with the localization, having two ears with similar signal
processing enabled the listeners to utilize ITD and ILD
cues to benefit them with speech perception in noise. It is
likely that the fine-structure information provided by the
use of bilateral hearing aids assisted in the detection of
the target signal by enabling the listeners to “squelch”
information provided by the spatially separated competing sound source (Turner et al, 2004; Dunn et al, 2006).
CONCLUSION
P
reservation of acoustic hearing is very important to
maintain localization abilities and speech perception in noise with spatially separate noise sources. While
speech perception abilities are often the goal of rehabilitation for individuals with hearing impairment, all
aspects of hearing such as sound source localization
50
should be considered. The results of this study indicate
that bilateral hearing aid use combined with a short-electrode cochlear implant provides listeners with additional
benefits for speech perception and localization. Future
studies should continue to investigate the salient features of combined cochlear implant plus hearing aid
use and provide more systematic fitting protocols for
these devices.
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