Key Words
contralateral routing of signal - listening - monaural - unilateral cochlear implant
INTRODUCTION
More than 5% of the world’s population, approximately 360 million people, has disabling
hearing loss, yet rehabilitative devices such as hearing aids and cochlear implants
(CIs) are only able to reach about 10% of the global need ([Blamey et al, 2001]). The deleterious effects of hearing loss are pervasive, resulting in increased
disability and handicap as well as reduction in quality of life ([Gatehouse and Noble, 2004]; [Silverman et al, 2006]; [Araujo et al, 2010]; [Sano et al, 2013]). The impact of hearing loss reaches far beyond just the individual affected. According
to the World Health Organization, unaddressed hearing loss costs upward of 750 billion
international dollars annually ([Blamey et al, 2001]). CIs have the widest utility in managing bilateral profound sensorineural hearing
loss (PSNHL). CIs have been demonstrated not only to be cost-effective, but also to
significantly reduce the associated societal burden. Still, the initial investment
required to obtain a CI is costly ([Blamey et al, 2001]). Despite long-term economic benefits, the high cost of these systems remains prohibitive,
particularly in low-income societies ([Bond et al, 2009]; [Lin and Albert, 2014]). Although a critical mass of evidence exists in support of bilateral implantation
for bilateral PSNHL ([Litovsky et al, 2004]; [Schleich et al, 2004]; [van Hoesel, 2004]; [Litovsky et al, 2006]; [Grantham et al, 2007]; [Godar and Litovsky, 2010]; [Balkany, 2012]), cost-benefit analysis suggests that the greatest benefit is received with unilateral
implants and that effectiveness is only incrementally enhanced with bilateral implants
([Summerfield et al, 2002]; [Lammers et al, 2011]; [Chen et al, 2014]). Such studies are highly influential in driving health-care policy. As such, access
to hearing health care around the globe limits most individuals with hearing loss
to unilateral treatment.
For unilateral CI users, however, the deficits inherently associated with monaural
listening remain despite the considerable benefits achieved through implantation ([Litovsky et al, 2004]; [Schleich et al, 2004]; [van Hoesel, 2004]; [Litovsky et al, 2006]; [Grantham et al, 2007]; [Godar and Litovsky, 2010]; [Balkany, 2012]). The association of adverse hearing outcomes with unilateral hearing impairment
has, in recent years, garnered considerable attention by the scientific community.
Studies of unilateral hearing loss have offered much insight into the deficits associated
with monaural listening, most notably inability to perceive speech in background noise
and localize sounds ([Welsh et al, 2004]; [Vermeire and Van de Heyning, 2009]; [Wie et al, 2010]; [Arndt et al, 2011]; [Snapp, Holt, et al, 2017]). This is due to the loss of auditory cues provided through binaural hearing. It
has been long known that the ability to process complex auditory signals is enhanced
by the ability to detect sound at the two ears. Binaural hearing provides listeners
with specific interaural timing and levels cues, which are processed in the auditory
cortex for interpretation of complex acoustic signals. In bilateral PSNHL, bilateral
cochlear implantation has been shown to provide binaural benefit as demonstrated by
improved speech perception in noise ([Fetterman and Domico, 2002]; [Litovsky et al, 2006]; [Litovsky et al, 2009]) and localization ability ([Litovsky et al, 2006]; [Grantham et al, 2007]; [Litovsky et al, 2009]). Bilaterally deafened adults who are unilaterally implanted are not able to take
advantage of this potential binaural benefit.
The primary deficit for monaural listeners (MLs) is the reduced ability to hear in
background noise ([Giolas and Wark, 1967]; [Sargent et al, 2001]). This effect is most significant when the signal is directed at the impaired ear
and noise is masking the good ear. Although binaural processing contributes to improved
listening in noise ability, the deficit incurred by MLs is largely a physiological
phenomenon known as the head shadow effect. In monaural listening, the head acts as
an acoustic barrier attenuating signals directed at the impaired ear by as much as
20 dB in the high frequencies ([Middlebrooks and Green, 1991]; [Gilkey and Good, 1995]). In MLs, loss of high-frequency cues as a result of the head shadow directly contributes
to poor perception of speech, particularly in noise.
Providing bilateral input to MLs through rerouting of signal overcomes some of these
disadvantages. Traditional MLs who have unilateral PSNHL with normal hearing (NH)
in the opposite ear have long benefited from devices that allow for lifting of the
head shadow ([Harford and Barry, 1965]; [Harford and Dodds, 1966]; [Gelfand, 1979]; [Niparko et al, 2003]; [Wazen et al, 2003]; [Lin et al, 2006]). Studies show that contralateral routing of signal (CROS) technology, where the
signal of interest is routed from the impaired (deaf) ear to the normal cochlea for
processing, is successful in improving speech perception in noise for individuals
with unilateral PSNHL ([Harford and Barry, 1965]; [Harford and Dodds, 1966]; [Gelfand, 1979]; [Snapp, Holt, et al, 2017]). Rerouting input from the impaired side to the NH side overcomes some of the auditory
deficits experienced by MLs, suggesting that unilateral CI recipients, who in effect
become MLs, may also benefit from rerouting input in the absence of binaural input.
Although bilateral CI does not fully restore binaural function, improved binaural
integration abilities are observed in bilateral CI recipients. Lifting of the head
shadow, however, can be seen in individuals who do not necessarily demonstrate other
tasks of binaural function, such as summation or localization. The same has been demonstrated
in traditional MLs who use CROS hearing aids as a treatment solution for unilateral
PSNHL (NH + CROS). Without any access to binaural cues, these individuals consistently
demonstrate improved speech-in-noise (SIN) performance through elimination of the
head shadow effect ([Niparko et al, 2003]; [Wazen et al, 2003]; [Hol et al, 2004]; [Lin et al, 2006]; [Hol et al, 2010]). Despite considerable gains in audibility, unilateral CI recipients continue to
demonstrate poor perception of speech in background noise ([Fetterman and Domico, 2002]; [Litovsky et al, 2006]; [Litovsky et al, 2009]) and localization ([Litovsky et al, 2006]; [Grantham et al, 2007]; [Litovsky et al, 2009]), consistent with traditional MLs.
Rerouting of bilateral input for MLs is a promising alternative for patients who cannot
benefit from binaural input ([Arora et al, 2013]; [Weder et al, 2015]; [Taal et al, 2016]). Early studies of CROS application in unilateral CI have demonstrated variable
results ([Arora et al, 2013]; [van Loon et al, 2014]; [Grewal et al, 2015]; [Guevara et al, 2015]; [Weder et al, 2015]). Outcomes were reported in terms of hearing performance in noise. SIN measures
can be either fixed where a percent correct score is obtained at a fixed signal-to-noise
ratio (SNR) level, or adaptive where either the speech or the noise is adjusted to
determine an individual’s threshold (SNR, where 50% of the presented speech can be
identified) for hearing in noise ([Taylor, 2003]). Studies measuring differences in overall percent correct scores suggested limited
benefit ([Arora et al, 2013]; [Grewal et al, 2015]; [Guevara et al, 2015]), whereas studies of speech reception thresholds suggested a positive effect of
CROS input to the CI ([van Loon et al, 2014]; [Weder et al, 2015]; [Taal et al, 2016]). Given the variability in speech perception among CI recipients, threshold measures
may be more resistant to these inherent variances in the CI population. As expected
([Niparko et al, 2003]; [Lin et al, 2006]), the assessment of CROS benefit when speech is in front of the unilateral CI listener
([Arora et al, 2013]; [Grewal et al, 2015]) results in marginal benefit compared with those designed to measure head shadow
effect ([van Loon et al, 2014]; [Taal et al, 2016]). Furthermore, these studies all used hearing aids ([Arora et al, 2013]; [Grewal et al, 2015]; [Guevara et al, 2015]; [Weder et al, 2015]; [Taal et al, 2016]) or lapel microphones ([van Loon et al, 2014]) hard-wired to a CI processor to deliver the contralateral microphone input. Although
promising, such systems are not optimized for application in CIs, and variables such
as directional microphones were not considered. Recent technological advances applied
in CROS technology may improve outcomes in traditional MLs over these early hard-wired
systems. However, novel CROS technology, which allows for wireless transmission of
the signal, has yet to be applied to unilateral CI users.
The present study aimed to investigate the benefit of CROS input in unilateral CI
recipients using a novel customized hearing device providing wireless CROS input to
the implanted ear (Advanced Bionics, LLC, Valencia, CA) using measures of SIN performance
and localization ability.
MATERIALS AND METHODS
Participants
University of Miami Institutional Review Board approval was obtained for this study.
Individuals aged 18 years and older, who are primary English speakers, were included
for the study. Enrollment was limited to postlingually deafened and experienced processor
users defined as consistent users of their hearing device for at least 6 months. Twelve
bilateral PSNHL patients ranging in age from 18 to 76 years (mean, 51 years ± 18 years)
using an Advanced Bionics CI in at least one ear were included for the study. Evaluation
and testing was conducted using the better performing ear defined as greater than
50% word recognition ability on Consonant-Nucleus-Consonant (CNC) word testing in
quiet in the aided condition. To determine if degree of CROS benefit in unilateral
CI recipients was comparable with that of traditional MLs, outcomes were compared
with a previously described ([Snapp, Hoffer, et al, 2017]) group of 12 unilateral PSNHL participants with no prior CROS hearing aid use and
NH in the contralateral ear using a standard CROS hearing aid.
Study Procedure
This study was designed as a prospective, within-subject repeated-measures experiment
in which each participant served as his/her own control. The participants were evaluated
in two phases randomized to either the monaural (CI Only) or CROS-aided (CI + CROS)
conditions. Each phase consisted of two intervals with random assignment to either
speech perception in noise or localization testing. All participants were allowed
a minimum 15-minute break between intervals. For the second phase, the participants
underwent testing in either the monaural (CI) or CROS (CI + CROS) conditions, alternate
to phase 1. All participants were evaluated acutely with no adaptation to the CROS
condition; however, when the CROS phase was initiated, a minimum of a 15-minute acclimatization
period was provided before laboratory testing.
Hearing Devices
Unilateral CI users were fitted with a customized Advanced Bionics Naida CI processor
designed to wirelessly route the acoustic signal from the nonimplanted ear to the
CI for the CROS condition ([Figure 1]). As with the traditional MLs using CROS, the CI + CROS used Hearing instrument
Body Area Network for the wireless streaming of acoustic signals by transmitting digital
data at a carrier frequency of 10.6 MHz via a digital magnetic induction link from
the CROS transmitter to the receiving antennae loaded within the sound processor ([Wolfe and Schafer, 2015]). The participants’ own programs were loaded to the research processor and no changes
to programming of the CI processor made before connecting the CROS microphone. The
input to the CI processor was set to a 50/50 split between the CI and the wireless
CROS microphone. As this is a single-participant study design, the change in performance
is based on the independent variable of the CROS condition. However, the following
requirements were maintained for all participants: (a) omnidirectional microphone
mode and (b) CI processors were maintained in front-mic mode.
Figure 1 Customized initial CROS prototype from Advanced Bionics designed to wirelessly route
the acoustic signal from the nonimplanted ear to a Naida CI processor.
Stimuli
Localization
Stimuli were presented in a custom 4-m × 4-m × 2-m sound booth with 19 loudspeakers
with a radius of 1.3 m spatially separated by 10 degrees, spanning ±90 degrees. Localization
stimuli were generated by a custom MATLAB front-end for TDT RX8 real-time multichannel
processor and series of Crown Audio CT-8150 eight-channel amplifiers (Tucker-Davis
Technologies, Gainesville, FL). Localization stimuli included a narrowband 350-msec
1/3 octave noise centered at 500 Hz, a narrowband 350-msec 1/3 octave noise centered
at 4 kHz, and a broadband 189-msec male-voiced “hey,” band-passed from 100 to 8000
Hz. Three trials of each stimulus were presented at 65-dB SPL roved by ±4 dB for a
total of 171 stimuli per participant per condition. The perceived location of the
sound sources was recorded using an Arduino-based 24-pushbutton feedback panel. Localization
was assessed in the CI only (monaural) and CI + CROS conditions. Localization results
were compared with outcomes in NH controls and traditional MLs with unilateral PSNHL
using a CROS hearing aid ([Snapp, Hoffer, et al, 2017]).
SIN
Recorded commercially available BKB-SINTM sentences were used for assessment of SIN
performance. The BKBSINTM uses Bamford-Kowal-Bench sentences ([Bench et al, 1979]) recorded in four-talker babble requiring the listener to identify key words. A
minimum of two lists of 10 sentences were completed for each condition and averaged
to determine the SNR level in dB required to obtain 50% accuracy, expressed as the
SNR-50, in dB ([Etymotic Research, 2005]). Speech stimuli were presented at 62-dB SPL. The test was conducted in the sound
field, and the participants were evaluated in the speech front/noise front (0 degrees/0
degrees azimuth), speech front/noise back (0 degrees/180 degrees azimuth), speech
deaf ear/noise monaural ear (90 degrees/270 degrees azimuth), and speech monaural
ear/noise deaf ear (90 degrees/270 degrees azimuth) configurations ([Figure 2]). All test protocols were repeated in the CROS (aided) condition to characterize
performance changes in this condition.
Figure 2 Diagram of the experimental setup for assessment of SIN and localization ability.
Gray speakers indicate the locations for the four SIN conditions. Localization was
presented in the front hemifield 90 degrees to 270 degrees.
Statistical analyses were performed using Wilcoxon signed rank tests for nonparametric
data and paired sample, as the outcome both on a histogram and a quantile–quantile
plot does not support an assumption of normality for this sample. Statistical significance
was set to p < 0.00625 assuming a type I error rate per test of 0.05 that was adjusted via a Bonferroni
correction to account for eight tests. All analyses were performed using SAS JMP™
software (SAS Institute version 12.1; Cary, NC).
RESULTS
Localization Performance
When compared with NH listeners ([Snapp, Hoffer, et al, 2017]), MLs displayed high degrees of localization error ([Figure 3]). Unilateral CI users had comparable performance on tasks of localization to traditional
MLs ([Snapp, Hoffer, et al, 2017]) ([Table 1]) with no significant difference observed between groups for any of the presented
stimuli (p > 0.00625). The root mean square (RMS) error ± 1 SD for each of the three localization
stimuli in the CROS-aided condition is reported in [Figure 3] for traditional MLs versus unilateral CI users. In the CROS-aided condition, neither
the traditional MLs nor the unilateral CI users gained any localization benefit ([Figure 3]), and there was no difference in aided performance between groups (p > 0.00625).
Figure 3 Aided localization performance represented in RMS error for the CI + CROS condition
(blue) for the voiced “hey” (A), 500 Hz 1/3 oct (B), and 4000 Hz 1/3 oct noise (C),
respectively. Aided responses are plotted against NH + CROS (red) and normal hearers
(black) ([Snapp, Hoffer, et al, 2017]).
Table 1
RMS Error for Three Localization Stimuli Demonstrating Unilateral CI Users Have Similar
Localization Performance to Traditional MLs ([Snapp, Hoffer, et al, 2017])
|
Normal Hearers
|
MLs
|
Unilateral CI
|
|
Broadband voiced “hey”
|
6.8°
|
41.8°
|
46.6°
|
|
1/3 Octave noise 500 Hz
|
10.8°
|
45.3°
|
48.7°
|
|
1/3 Octave noise 4000 Hz
|
10.0°
|
45.0°
|
43.3°
|
SIN Performance
The median-aided CROS benefit for each of the four experimental configurations for
CI + CROS users is presented against traditional MLs (NH + CROS) in [Figure 4]. Here, gain (improved performance) is presented as a positive change in dB value,
whereas loss (decrease in performance) is presented as a negative change in dB value.
Between-group analysis of overall aided benefit resulted in no statistically significant
difference between traditional MLs ([Snapp, Hoffer, et al, 2017]) and unilateral CI users in the CROS-aided condition for any of the listening configurations
(p > 0.00625, Wilcoxon signed rank). Differences in unaided and aided performance for
the CI + CROS group can be found in [Figures 5A and B] where the median absolute performance is presented as the SNR-50 or dB level of
the sentences relative to the noise required by the participant to correctly repeat
the BKB-SINTM sentences 50% of the time. For [Figure 5], better performance corresponds with lower dB values. For the unilateral CI user,
application of the CROS microphone resulted in significant improvement for the speech
front/noise front (p < 0.0005, median change = +2.5-dB benefit, Wilcoxon signed rank), and speech poorer
ear/noise better ear (p < 0.0005, median change = +9.75-dB benefit, Wilcoxon signed rank) configurations
([Figure 5A]). No significant change in performance was observed for speech front/noise back
(p > 0.00625, median change = +1.75 dB, Wilcoxon signed rank). A significant decrement
in performance was observed when speech was directed at the CI (better ear) and noise
was directed at the CROS microphone (p = 0.002, median change = −3.0 dB, Wilcoxon signed rank test). Variability for unaided
versus aided conditions for the CI + CROS group are presented in [Figure 5B].
Figure 4 Aided CROS benefit for each of the four experimental configurations in traditional
MLs (NH + CROS) ([Snapp, Hoffer, et al, 2017]) and unilateral CI (CI + CROS) groups is presented as a change in dB from the monaural
(no CROS) condition. Here, improved performance is presented as a positive dB value,
whereas decrease in performance is presented as a negative dB value. No significant
difference between systems is observed in aided benefit or absolute performance across
the listening configurations (p > 0.00625, Wilcoxon signed-rank).
Figure 5 SNR-50 for the unaided (unilateral CI) and aided (CI + CROS) conditions for the CI
group is presented in A where better performance corresponds with lower SNRs. (B)
This shows the 95% confidence intervals demonstrating variability across listening
configurations for unaided (CI only) vs. CI + CROS aided.
DISCUSSION
Despite considerable advancements in implantable technology and irrefutable evidence
that binaural hearing is essential for advanced tasks of auditory function ([Balkany et al, 1988]; [Litovsky et al, 2004]; [Nopp et al, 2004]; [van Hoesel, 2004]; [Litovsky et al, 2006]; [Litovsky et al, 2009]; [Balkany, 2012]), many individuals with severe–profound bilateral hearing loss continue to have
limited access to treatment ([Blamey et al, 2001]; [Chen et al, 2014]). Studies have estimated a maximum of 36% of bilateral PSNHL patients are bilateral
implant recipients ([Peters et al, 2010]). This suggests that more than 60% of current CI recipients with bilateral PSNHL
function as MLs, thereby subject to the deficits inherent to monaural listening. We
sought to investigate the benefits of rerouting of signal from the impaired ear to
the better ear using CROS technology of two monaural listening groups: (a) unilateral
PSNHL and (b) bilateral PSNHL unilaterally implanted with a CI. Standard treatment
for MLs uses the normal cochlea to deliver sound from the impaired side to the better
hearing ear via CROS technology. Our data provide strong evidence that benefits gained
in traditional MLs through CROS technology are achievable in unilateral CI recipients.
Studies have shown that CROS technology successfully lifts the head shadow, allowing
for spatial separation of speech from background noise but is not successful in improving
auditory processing abilities that require binaural inputs such as localization ([Niparko et al, 2003]; [Lin et al, 2006]; [Snapp, Holt, et al, 2017]). Our data are consistent with previous reports in traditional MLs, suggesting benefit
from lifting of the head shadow but not in tasks requiring binaural function ([Niparko et al, 2003]; [Lin et al, 2006]; [Snapp, Holt, et al, 2017]).
For tasks of localization, there was no observable benefit in the horizontal plane
for either traditional MLs or unilateral CI users when the CROS technology was applied.
There was no significant difference between participant groups in aided performance
for any of the three localization stimuli. Localization is a binaural phenomenon requiring
input from two ears. As with traditional MLs ([Harford and Barry, 1965]), CROS microphone serves to transmit the acoustic signal arriving to the nonimplanted
ear to the CI, thereby only providing unilateral input. Although individuals gain
access to sounds arriving to the contralateral (nonimplanted) ear, they do not regain
interaural timing and level cues provided through binaural hearing required to restore
localization.
For measures of speech perception in noise, significant improvement from the unaided
to aided condition for both traditional MLs and unilateral CI users was observed when
speech is directed at the deaf ear and noise is masking the monaural ear, with no
significant difference for aided performance between participant groups. These findings
demonstrate that unilateral CI users display similar deficits for listening in noise
([Figure 4]) and localization ability ([Snapp, Hoffer, et al, 2017]) ([Table 1]) to traditional MLs. Likewise, our findings support that unilateral CI users are
capable of achieving similar gains in speech perception to that of traditional MLs
with wireless CROS. Unilateral CI users’ speech perception in noise ability improves
across most listening conditions with the exception of noise presented to the contralateral
ear. This is expected, as the CROS microphone transmits noise to the CI. Although
significant, the decrement in performance is minimal (∼3 dB) in comparison to the
notable gain (∼9 dB) in performance observed in the most debilitating listening condition,
when speech is presented to the nonimplanted ear.
This is the first investigation of contemporary wireless CROS technology in unilateral
CI users. [Grewal et al (2015)] investigated the potential benefits of CROS technology in unilateral CI users by
coupling a hearing aid via a wired input to the CI. Their findings suggested limited
benefit of CROS input to the CI. The design of this study differed in a number of
ways. The investigators looked at overall percent correct for speech when presented
in front of the listener. As expected, a decrease in performance was observed when
noise was masking the CROS. However, they also observed a decrease in performance
for the CROS condition when noise was lateralized to the CI. It is unclear how application
of the CROS in this condition would significantly reduce performance. It is possible
that the discrepancy in results is due to differing test designs. Specifically, [Grewal et al (2015)] used overall percent correct performance on the Arizona Biomedical Institute (AzBIO)
sentence test to assess CROS benefit, whereas our study used an adaptive measure to
determine the participant’s threshold for hearing in noise. Given the overall low
performance reported by the authors in this condition (45% correct), it is possible
that the change of 10% as reported by the authors is simply a factor of test–retest.
The present study looked to identify the SNR-50 with a lower dB representing better
performance. Review of [Figure 5] suggests that even though unilateral CI users improve in most situations with CROS
input, they still consistently require a +6-dB SNR to achieve 50% correct on measures
of speech perception in noise regardless of the listening configuration. [Grewal et al (2015)], assessed speech perception in noise at a fixed +5-dB SNR for all listening configurations.
It is possible that this condition (+5 SNR) is below the threshold for which the CROS
benefit may be observed in the unilateral CI user. In addition, the present study
required participants to demonstrate a greater threshold of speech perception performance
before enrollment, which may also account in part for some of the variance in findings.
[Taal et al (2016)] sought to quantify the effect of CROS for listening in unilateral CI users as function
of speaker location. In accordance with the present findings, they concluded that
use of a CROS results in a maximum gain in speech reception threshold of 7.9 dB when
speech comes from the CROS side compared with a maximum loss in speech reception threshold
of 2.1 dB when speech comes from the implanted side in the presence of diffuse noise.
A model of speech intelligibility suggested that in the case of directional noise,
the effect of the CROS is symmetric and the maximum loss or gain in speech reception
threshold was around 9 dB. Our findings were consistent with the predicted gain of
9 dB for speech directed at the CROS but only found an approximate decrement of 3
dB when noise was directed at the CROS. [Weder et al (2015)] investigation of wired CROS input via hearing aid to the CI also reported this contrast
in performance. Results of traditional MLs under the CROS condition paralleled that
of CI + CROS ([Figure 4]). Similarly, [Lin et al (2006)] demonstrated in NH + CROS that the advantages of reducing the head shadow for speech
directed at the better ear outweigh the disadvantages inherent in head shadow resulting
from introducing noise to the impaired ear. The subjective cost-benefit of CROS input
in real-world listening scenarios for unilateral CI users has yet to be studied. It
is possible that transmission of noise from the CROS device to the CI will result
in aversion to CI + CROS input.
Arguably, the most interesting finding is not in the benefit gained or lost, but in
the absolute performance across listening conditions. Review of [Figures 5A and B] demonstrates a high degree of variability in performance in the unaided condition
as a function of speaker and masker location. These results suggest that when the
listener is reliant on the unilateral input via the CI alone, they must continually
adapt or modify their environment to improve the SNR to allow for improved speech
perception. A listener would need to adjust their positioning, so the better hearing
ear, in this case the CI ear, is always directed at the speaker allowing for the most
optimal hearing condition. This may not be realistic in highly adaptive listening
situations or in those who require sustained listening. Addition of the CROS microphone
clearly reduces this variability and allows equal performance across all listening
conditions regardless of the listening configuration ([Figures 5A and B]). Although a small decrement in performance is observed when the noise is sent from
the CROS microphone to the CI, it is questionable whether or not the listener is even
aware of this negative effect. Consider the ML engaged in group discussion in a noisy
environment who must continually adjust their head position to remain engaged in the
conversation. The CROS user, on the other hand, has consistent speech perception performance
without having to adjust to the speaker. [van Loon et al (2014)] compared bilateral CI users to unilateral CI users with CROS and demonstrated similar
results. Although they concluded that bilateral CI resulted in the greatest degree
of benefit, a significant improvement of >6 dB was obtained in the CI + CROS group
for speech directed at the impaired ear. Like our observation, the unilateral CI users
displayed highly variable speech reception thresholds as a function of location of
the speaker, with performance equalizing across the three listening conditions for
CI + CROS ([van Loon et al, 2014]). Collectively, these observations raise intriguing questions regarding the limitations
of MLs and suggest that a reduction in listening effort may be an additional benefit
of the CROS. Significant gains in speech perception in noise have the potential to
positively impact unilateral CI patients in substantial ways. The ability to process
complex auditory signals is essential for effective communication and daily acts of
living. Likewise, the inability to process these signals results in considerable reduction
of quality of life, social exclusion, vocational limitations, and a high disability
and handicap index ([Gatehouse and Noble, 2004]; [Araujo et al, 2010]; [Augustine et al, 2013]; [Sano et al, 2013]).
The primary limitation of this study is that all participants were tested acutely,
precluding any adaptation to the CROS hearing modality. However, all CI participants
were experienced users and the CI processor was set to a 50/50 split between the CI
and the wireless CROS microphone. The resulting input is not novel as no changes to
the processing of the signal occurred, only the addition of the CROS microphone input.
The wireless CROS prototype (Advanced Bionics, LLC) was limited to the omnidirectional
mode. As demonstrated here, unilateral CI users require increased SNRs compared with
normal hearers. The potential for even greater CI + CROS benefit with the application
of directional microphone technology warrants further study. Our data suggest that
MLs must develop listening strategies to improve the SNR as the location of the speaker
changes. With CROS input, these strategies are undoubtedly modified, requiring some
adaptation for real-world listening in MLs. One such example is listening in diffuse
noise environments, which was not studied here. The addition of the CROS in unilateral
CI reduced the variability in performance, allowing for equal performance across all
listening conditions regardless of the location of the speaker or noise. Although
wireless CROS technology is a promising solution, it is unclear if unilateral CI recipients
will adopt and accept CROS as a treatment option. CI + CROS does not provide binaural
stimulation, or the improved performance on tasks requiring these inputs that can
be achieved through bilateral CI implantation. Although CI + CROS does not replace
true bilateral hearing, it could serve as an option for those who cannot obtain a
second implant for insurance purposes, health reasons, etc. Although many unilateral
CI recipients do not have access to a second implant ([Blamey et al, 2001]; [Bond et al, 2009]; [Chen et al, 2014]), there are those individuals who simply choose not to pursue a second implant.
Questions remain regarding the hearing handicap in unilateral CI users. CI + CROS
may offer a bridge to bilateral implantation in those individuals who have yet to
realize the benefit of bilateral input. It is also possible that the decrease in performance
that occurs when noise is directed to the CROS may result in poor acceptance of CROS
technology in the unilateral CI population. Future studies will focus on investigating
the adoption and acceptance of wireless CROS technology for unilateral CI recipients
through chronic studies of objective and subjective CROS performance.
CONCLUSIONS
Hearing loss is a global public health issue. The economic burden of hearing loss
weighs heavily on both the individual and society. Yet, access to treatment continues
to be limited, only serving about 10% of the global need ([Blamey et al, 2001]). In countries with a low per capital gross national income, the principal barrier
to acquiring a hearing device is cost ([Olusanya, 2009]). Bilateral CIs have improved outcomes over unilateral CIs, specifically for tasks
that require binaural input such as localization ([Litovsky et al, 2004]; [Litovsky et al, 2006]; [Balkany, 2012]; [Lammers et al, 2014]). However, convincing evidence that the benefit provided by adding the second implant
warrants the associated cost is lacking ([Kral and O'Donoghue, 2010]). Considering these factors, it is unsurprising that those individuals who have
access to CIs are often limited to a single implant. With the continued rise in health-care
cost constraints, the number of patients who will not have access to binaural processing
through bilateral implantation is expected to increase ([Chen et al, 2014]). When applied to unilateral CI users, this novel approach can overcome a key limitation
of current treatment by restoring access to sound from the nonimplanted side and improve
speech perception in noise. Although tasks requiring binaural input, such as localization,
are not improved with CI + CROS, the access to sound awareness on the unimplanted
side with wireless CROS is encouraging. The results of this study suggest that wireless
CROS decreases the functional deficits for listening in noise. Wireless CROS offers
an innovative approach to resolving one of the auditory deficits associated with monaural
listening by lifting the head shadow effect. Furthermore, it provides a noninvasive
alternative for patients who are unable or unwilling to undergo a second surgery,
thereby expanding access to rehabilitation in this population.
Abbreviations
CI:
cochlear implant
CROS:
contralateral routing of signal
MLs:
monaural listeners
NH:
normal hearing
PSNHL:
profound sensorineural hearing loss
SIN:
speech-in-noise
SNR:
signal-to-noise ratio
