Key Words
aging - hearing aids - hearing loss - speech perception
INTRODUCTION
The advantages of listening with two ears as opposed to one have long been established
([Koenig, 1950]; [Akeroyd, 2006]). They include improved sound localization ([Middlebrooks and Green, 1991]), improved speech perception in background noise due to the squelch effect ([Kock, 1950]), and loudness summation ([Keys, 1947]). It is generally assumed that listening with two ears is always preferable, and
consequently, that listeners with bilateral hearing loss should be fit with two hearing
aids. However, this common belief was challenged in the mid-1990s by case reports
of binaural interference. Binaural interference in the context of the present investigation
refers to the situation wherein performance with two ears is poorer than with the
better ear alone (note that the term “binaural interference” has also been used with
regard to spectral interference effects on binaural cues used for sound localization
[[McFadden and Pasanen, 1976]]). Those small-scale studies used mostly speech perception (in quiet and in noise)
and electrophysiological measures to explore underlying reasons for unsuccessful bilateral
hearing aid use in spite of fairly symmetric hearing thresholds ([Jerger et al, 1993]; [Chmiel et al, 1997]; [Carter et al, 2001]; [Holmes, 2003]).
This first documentation of the existence of binaural interference fueled interest
in further exploration of the phenomenon in larger groups of listeners. [Allen et al (2000)] reported better bilateral speech reception thresholds (SRTs) than the better unilateral
SRT in all but the older hearing-impaired group (no interference reported). Speech
perception with W-22 words in the bilateral and unilateral conditions showed no evidence
of binaural advantage or interference. Within-subject analyses using confidence intervals
revealed that two participants had binaural interference and one had advantage (all
belonging to one of the older age groups). The authors argued that the number of individuals
with binaural interference in their sample is close to what would be expected by chance.
However, testing speech perception in quiet as opposed to in background noise may
not have been conducive to binaural processing.
In contrast, [Walden and Walden (2005)] found better performance in noise with amplification in the better-performing ear
than with amplification in the poorer-performing ear and bilaterally (i.e., binaural
interference). In 82% of their elderly veterans, bilateral amplification yielded greater
signal-to-noise ratio (SNR) loss (i.e., poorer performance) than amplification in
either ear alone. No differences between the ears were found on the Dichotic Digits
Test (DDT). One potential issue with this study was that their presentation levels
of 70 dB HL (82 dB SPL) in the sound field could have been too high, possibly causing
the hearing aids to distort. Also, because the contralateral ear canal was open during
unilateral testing, for many participants the speech and noise levels could have been
audible to that ear too (as later confirmed by [McArdle et al, 2012]).
Similar binaural interference results were reported by [Henkin et al (2007)] for monosyllabic word recognition in noise at +10 dB SNR (speech at 0°, noise at
180°), with 71% of the listeners having better word recognition with one hearing aid
as opposed to two (significance not reported). Contrary to the findings of [Walden and Walden (2005)], a dichotic sentence test revealed a significant right-ear advantage, but the investigators
found no relationship between the right-ear advantage and the binaural reduction in
performance.
Given the unexpected results found by [Walden and Walden (2005)], [McArdle et al (2012)] replicated that study, verifying that the hearing aids were not distorting for the
inputs used and obtaining subjective loudness ratings for the high-level inputs. They
found no evidence of binaural interference (or advantage) with group results. Only
20% of the listeners performed better with one hearing aid than bilaterally (as opposed
to 82% in the original study). In a second experiment, speech perception in noise
was assessed via earphones at 70 dB HL, eliminating potential confounds from the hearing
aids. The two bilateral conditions yielded better performance than with either ear
alone. This was also true for 65% of the participants, with individual data analysis.
Still, the authors note that in both their experiments, ∼20% of the participants performed
better in noise with a single ear.
A potential confounder in some of the studies reviewed could be the use of hearing
aids by the participants during testing ([Walden and Walden, 2005]; [Henkin et al, 2007]). While providing an ecologically valid testing condition, digital processing in
hearing aids can introduce subtle time delays to the auditory signal delivered to
the ear. If the processing delay is not the same in the two hearing aids worn by a
given listener, binaural hearing may be hindered. It is known that binaural processing
relies on very precise timing/phase differences between the sounds that arrive at
the two ears ([Middlebrooks and Green, 1991]). Another potential confounder in previous studies is the lack of spatial separation
between speech and noise, by presenting both through a single loudspeaker ([Walden and Walden, 2005]). To afford participants the possibility of binaural hearing during speech perception
in noise, spatial separation between the two is important.
Finally, the use of veterans as the participant population could also have potentially
impacted previous findings ([Walden and Walden, 2005]; [McArdle et al, 2012]). Besides consisting of mostly males, those listeners in general have hearing loss
that is at least partially due to noise exposure. The configuration of noise-induced
hearing loss is similar to what would be expected from other etiologies such as genetics
or aging; however, the effects of noise exposure in the auditory system seem to go
beyond the loss of cochlear hair cells. Evidence from animal models suggests that
the effects of noise exposure (especially at an early age) continue for years after
the end of exposure in that auditory nerve fibers are lost, unlike observations in
aged control animals ([Kujawa and Liberman, 2006]).
Still, although previous studies have had limitations and thus provided weak support
for binaural interference with group analyses, the evidence suggests that binaural
interference exists in at least a proportion of listeners. Thus, considering the possibility
of binaural interference is important for hearing aid fitting. Recent data show that
nearly 85% of hearing aid fittings in the United States are bilateral ([Strom, 2014]). However, there is evidence that many users prefer wearing only one hearing aid.
A large retrospective survey of hearing aid users who were likely to benefit from
bilateral hearing aids showed that 31% of them preferred a single hearing aid ([Boymans et al, 2009]). Likewise, in a prospective study involving a structured 3-mo trial with bilateral
amplification, a striking 46% of listeners with symmetric hearing loss preferred using
one hearing aid ([Cox et al, 2011]). This has prompted researchers to examine which factors or tests, if any, can predict
future unilateral hearing aid use in listeners with symmetric hearing loss. While
a few significant predictors of unilateral/bilateral hearing aid use have been reported
([Köbler et al, 2010]; [Cox et al, 2011]), one study failed to find such predictors among a test battery administered before
fitting ([Boymans et al, 2008]). Several other factors such as cosmetics, cost, and poor manual dexterity may underlie
the preference for one hearing aid and pose a confounding variable in those studies.
Yet, better performance with a single hearing aid as opposed to two seems to be an
important factor for unilateral hearing aid use ([Cox et al, 2011]), suggesting that there may be a physiologic mechanism such as binaural interference
in play.
It is noteworthy that many of the binaural interference cases reported seem to be
found in the elderly population. Older adults in general have difficulty understanding
speech in challenging listening situations such as in background noise or in the presence
of reverberation ([CHABA, 1988]). In addition, older adults seem to have diminished temporal processing abilities,
as demonstrated in studies of gap detection and temporal order discrimination/identification
([Humes et al, 2012]). However, many highly functioning older adults are able to compensate for speech
perception difficulties by using contextual support ([Pichora-Fuller et al, 2007]), and/or by activating additional areas in the brain ([Wong et al, 2009]). Thus, it might be expected that many older adults would have similar binaural
processing to their younger counterparts.
This study was conducted in an effort to further explore the phenomenon of binaural
interference, which is one possible underlying reason for subjective preference for
a single hearing aid. However, due to potential confounds introduced by the hearing
aids themselves, testing conditions in this study did not involve hearing aids. The
aim of the present study was to investigate the occurrence of binaural interference
in groups of younger and older adults, and the effects of hearing loss in the older
age group. Prior noise exposure was ruled out. Because previous studies suggest that
the prevalence of binaural interference may not be high enough to be evident with
group analyses, within-subject analyses were also performed. It was hypothesized that
binaural interference would be found for a portion of the participants in the within-subject
analyses, mostly in the older age groups.
METHODS
Participants
Thirty-three listeners participated in this study, divided into three groups: younger
with normal hearing (YNH; n = 12, age range = 18–28 yr, mean = 22 yr, seven females),
older with normal hearing for their age (ONH; n = 9, age range = 73–87 yr, mean =
80.2 yr, seven females), and older with hearing impairment (OHI; n = 12, age range
= 78–94 yr, mean = 83.2, five females). Pure-tone thresholds were obtained for the
octave frequencies between 250 and 8000 Hz, including the interoctave frequencies
of 3000 and 6000 Hz. The average of normal-hearing thresholds for age (in reference
to [ISO, 2000]) for the older age group is depicted in [Figure 1] by the long dashed line. Hearing loss was primarily sensorineural, that is, no air-bone
gaps >10 dB were allowed. Normal middle-ear function was confirmed with immittance
measures.
Figure 1 Average hearing thresholds for the ONH and OHI groups are represented by symbols
connected by solid and dotted lines, respectively. The average of normal-hearing thresholds
for the elderly participants is depicted by the dashed line (in reference to [ISO, 2000]). (This figure appears in color in the online version of this article.)
[Figure 1] shows the average hearing thresholds for the elderly groups; symbols connected by
solid lines represent the ONH group, and symbols connected by dashed lines represent
the OHI group. Thresholds for the YNH group were 10 dB or better across all frequencies
(not shown). All participants exhibited symmetric thresholds, defined as no more than
a 15-dB difference between the two ears at any frequency. Participants were recruited
from clinic records, department personnel, and retirement residences. All participants
were paid for their participation. Data collection lasted ∼2.5 hr, over one or two
sessions. This study was approved by the University of Iowa Institutional Review Board.
Procedures
Following informed consent procedures, participants underwent a brief assessment of
their medical and otological history. Individuals presenting with conditions that
could potentially interfere with hearing and auditory processing, such as noise exposure,
head trauma, ototoxicity, and/or known genetic factors, were excluded from this study.
Four participants were excluded based on their medical history and/or audiometric
configuration.
The Mini-Mental State Examination ([Folstein et al, 1975]) was administered to all participants as a screening tool of cognitive functioning.
Mini-Mental State Examination scores decrease with age, which is consistent with a
greater incidence of cognitive impairments in the elderly. Therefore, passing criteria
were based on the 25th percentile scored by the normative population in each age group
([Crum et al, 1993]). All participants scored at or above the cutoff score.
Speech Perception
All testing was carried out in a double-walled, sound-treated IAC booth (Industrial
Acoustics Company, North Aurora, IL). Speech recognition was tested in three conditions:
right ear only, left ear only, and bilaterally. Speech perception materials were presented
through Grason-Stadler 1700-2002 loudspeakers (Grason-Stadler, Eden Prairie, MN),
with speech coming from 0° azimuth and the competing noise from 180°. When testing
unilaterally, the opposite ear was plugged with a compressible foam earplug. Participants
were seated equidistantly from both loudspeakers at 0.8 m and were instructed to keep
their heads straight facing the front loudspeaker. Practice items were presented to
participants before testing began. The presentation order of tests and conditions
across participants was counterbalanced.
The Hearing in Noise Test (HINT) ([Nilsson et al, 1994]) is an adaptive test seeking the signal-to-noise ratio that yields 50% correct speech
recognition (SNR-50). Speech materials consist of sentences in a spectrally matched
noise background. The participant’s task was to repeat each of 20 sentences (two lists),
as spoken by a male talker. The noise remained fixed at 65 dBA, while the speech level
was varied in 4-dB steps (for the first five sentences) and 2-dB steps (for the remaining
sentences), according to the participant’s performance. To derive the SNR-50, presentation
levels for sentences 5–20 were averaged and subtracted from the presentation level
of the noise.
The Connected Speech Test (CST; [Cox et al, 1987]) was used to assess speech intelligibility in noise. It consists of a large number
of passages about familiar topics, with ten sentences per passage topic. The speech
is presented in a background of multitalker babble, at a fixed signal-to-babble ratio
(SBR). In this study, two different SBRs were used: +2 dB, which was deemed to be
a fairly easy listening environment for both normal-hearing and hearing-impaired participants,
and −2 dB, which was intended to be more difficult. The speech was fixed at 63 dB
SPL at ear level for normal-hearing listeners and at 30 dB SL relative to the average
of 1000- and 2000-Hz thresholds for hearing-impaired listeners. Presentation levels
were slightly adjusted based on subjective report, and were kept constant throughout
testing. The participants’ task was to repeat as much of each sentence as possible.
Key words (25 per passage) were scored for a total percent correct. For each ear condition,
4 passages were presented, totaling 24 passages (4 passages × 2 SNRs × 3 ear conditions—bilaterally
and unilaterally left and right). To avoid further frustration, whenever participants
scored ≤25% correct, only two passages were presented per ear condition. When this
happened at the +2 dB SBR, passages were not presented at −2 dB SBR. As a result,
the −2 dB SBR condition was not presented to 3 of the 12 older hearing-impaired individuals.
The HINT and the CST were chosen due to their high reliability and small training
effects. In addition, the CST has abundant contextual cues that are representative
of everyday communication.
DDT
The DDT ([Musiek, 1983]) was used to assess the ability of binaural separation, which can lead to the expression
of binaural interference. In this test, different digits from 1 to 10 (excluding 7,
as it is the only nonmonosyllable digit) are presented simultaneously to both ears.
Two paradigms can be used with the DDT: “free recall,” when the listener is instructed
to repeat all the digits heard and “directed recall,” when the listener is to repeat
only the digits presented to the precued ear. The free-recall paradigm can be more
affected by cognitive functioning, as it places a heavier load on memory. Therefore,
as auditory processing was the main focus of this study, only the directed-recall
paradigm was used.
The DDT has high intertest reliability, both for younger and elderly listeners ([Strouse and Wilson, 1999a]), and the results do not appear to be significantly affected by hearing loss. Although
brain-processing asymmetries would predict a slight right-ear advantage on the DDT
(and on dichotic tests in general), larger asymmetries have been taken as a sign of
binaural interference. A right-ear asymmetry of 2.9–4% has been reported in younger
listeners in dichotic listening tests with verbal stimuli, while in older listeners,
this asymmetry seems to grow larger, up to 40% ([Jerger, 2001]).
DDT materials consisted of tracks 7 and 8 from the compact disc “Tonal and Speech
Materials for Auditory Perceptual Assessment, Disc 2.0” ([Department of Veterans Affairs, 1998]). Each track has 54 interleaved one-, two-, and three-pair digits that are presented
to each ear through earphones, in quiet. Digits were presented at 50 dB SL relative
to the SRT with spondees (as suggested by [Musiek and Pinheiro, 1985]; [Bellis, 2003]). For hearing-impaired individuals, when this level was reported to be too loud,
it was slightly reduced.
RESULTS
Group Analyses
Group mean scores were compared between the left unilateral, right unilateral, and
bilateral conditions across the three groups. HINT SNR-50 values were analyzed with
a repeated-measures analysis of variance (ANOVA) in a mixed-model framework, with
group as the between-subjects factor and ear condition (left, right, bilateral) as
the within-subjects factor. In this test, larger SNR-50 values indicate poorer performance.
Results revealed a significant main effect of group [F
(2,60) = 46.29; p < 0.0001], with the older hearing-impaired group having the poorest average SNR-50
across the three ear conditions (5 dB), followed by the ONH group (0.6 dB) and the
YNH group (−2 dB). The follow-up comparisons with Bonferroni corrections for multiple
tests suggested that each of the three groups was significantly different from the
other two. The main effect of the test ear was not significant [F
(2,60) = 0.68; p = 0.51], nor was there a significant interaction between ear and group [F
(4,60) = 0.27; p = 0.89], suggesting that the absence of an ear effect holds for all three groups.
HINT scores per group and ear condition are shown in [Figure 2A].
Figure 2 Group mean scores and standard errors are shown, for each ear condition as appropriate
(right, left, bilateral). (A) Refers to results of the HINT (in dB SNR-50), where
lower numbers represent better performance. (B) Depicts CST results at the +2 SBR
condition (in rau). (C) Depicts DDT results (in % correct). In these two tests, higher
numbers represent better performance.
The distribution of CST raw percentage scores was first normalized with the logit
transformation. In this procedure, percentages are converted into proportions, which
are then used to calculate the logarithm of the odds ratio. Transformed scores were
analyzed in a mixed-model, repeated-measures ANOVA, with group as the between-subjects
factor and test ear (left, right, bilateral) and SBR (+2 dB SBR, −2 dB SBR) as within-subjects
factors. The three main effects were significant: group [F
(2,30) = 90.30; p < 0.0001], ear [F
(2,60) = 6.08; p = 0.004], and SBR [F
(1,27) = 69.74; p < 0.0001]. Follow-up testing with Bonferroni corrections revealed that the mean logit-transformed
score for each group across all ear conditions and SBRs was significantly different
from that of the other two groups (from worst to best performance: OHI = −1.41, ONH
= 0.94, YNH = 2.73). Follow-up tests on the ear effect showed that the bilateral scores
were significantly better than either right or left ear (right = 0.66, left = 0.71,
bilateral = 1.14), while the difference between the two unilateral conditions was
not significant. The SBR effect refers to the logit-transformed scores being significantly
better at +2 SBR (1.22) than at −2 SBR (0.42), as expected. No interactions were significant,
suggesting that the within-subject effects were consistent across the three groups.
[Figure 2B] displays mean CST scores and standard errors, at the +2 SBR condition. Critical
differences for the CST, which were used in the individual data analysis (outlined
in “Within-subject Analyses”), are available from the test developers in rationalized
arcsine units (rau; [Studebaker, 1985]). Thus, although the logit transformation was used for group data analysis, the
raw CST percentage scores also had to be transformed into rau for comparison with
pre-established critical differences. For the sake of consistency, CST scores in [Figure 2B] are also shown in rau.
The distribution of DDT percentage scores was also normalized with the logit transformation.
Scores for left and right ears were pooled across groups and compared with a t test. A significant right-ear advantage was found [t
(32) = 4.83; p < 0.0001]. A one-way ANOVA on the ear difference score (right ear minus left ear)
revealed no significant differences between the three groups [F
(2,30) = 1.54; p = 0.23], suggesting that the right-ear advantage was present in all groups. For the
sake of clarity, DDT scores and standard errors are depicted as percent correct in
[Figure 2C].
Within-Subject Analyses
Given that the prevalence of binaural interference has been estimated to be as low
as 10% ([Jerger et al, 1993]), it might be expected that its effects would not be evident in group means. Thus,
a within-subject analysis was performed, comparing the difference between each participant’s
bilateral and better unilateral scores to pre-established 95% critical differences
for each test.
HINT individual bilateral minus the better unilateral scores are displayed in [Figure 3], for each group. The dashed lines represent the critical difference (±1.5 dB) ([Nilsson et al, 1994]). Recall that on the HINT, higher SNR values indicate poorer performance. [Figure 3] shows that the difference scores of 9 of 33 participants (27%) exceeded the critical
difference in the direction of interference (top part of the graph), while 6 participants
(18%) had binaural advantage (bottom part of the graph).
Figure 3 Individual bilateral minus better unilateral HINT scores (in dB SNR), for participants
in each group. Pre-established critical differences for this test are represented
by the dotted line.
The individual CST difference scores (in rau) were also compared to critical differences
of 14 rau for the normal-hearing participants ([Cox et al, 1987]) and 15.5 rau for hearing-impaired participants ([Cox et al, 1988]). [Figure 4] depicts the bilateral minus better unilateral scores for individual participants
in each group, at +2 dB SBR. Here, higher numbers indicate better performance. Participants
whose scores exceeded the critical difference above the dashed line had binaural advantage,
while those in the bottom part had binaural interference. At +2 dB SBR, a significant
binaural advantage was found in 12% (n = 4) of the participants, and significant interference
in 3% (n = 1). This individual belonged to the OHI group. Of the participants tested
at −2 dB SBR, binaural advantage was seen in 13% (n = 4) and interference in 3.3%
(n = 1) (condition not shown). This individual belonged to the YNH group.
Figure 4 Individual bilateral minus better unilateral CST scores (in rau), for participants
in each group, at the +2 SBR condition. Pre-established critical differences for this
test are represented by the different dotted lines, for normal-hearing and hearing-impaired
listeners.
Finally, critical differences were not available for the directed-recall presentation
of the DDT. There are published normative data from 30 participants in each group
spanning a 10-yr age range from 20 to 79 yr ([Strouse and Wilson, 1999b]). However, the fact that 39% of those participants showed below-normal performance
in the free-recall condition while having normal performance in the directed-recall
condition suggests that their deficit is possibly in the cognitive domain. Therefore,
this participant group did not seem to provide an adequate basis for comparison with
the present groups. Thus, no statistical analysis of individual DDT data was performed.
However, individual right- minus left-ear DDT scores can be inspected in [Figure 5]. It is evident that at least some participants display a large right-ear advantage.
A right-ear advantage on the DDT, although expected as a result of hemispheric specialization
([Kimura, 1961]), has also been taken as a sign of binaural interference ([Carter et al, 2001]; [Jerger, 2001]).
Figure 5 Individual right- minus left-ear DDT scores (in % correct) for participants in each
group.
DISCUSSION
The purpose of this study was to examine the occurrence of binaural interference in
groups of younger adults with normal hearing, older adults with normal hearing for
their age, and older adults with symmetric hearing loss. Participants were tested
using speech perception tasks in the sound field, each ear separately (with the contralateral
ear plugged) and bilaterally, without the use of hearing aids. Results varied across
test. On the HINT, the group analysis showed no effect of ear tested, while within-subject
analysis showed that 27% of the participants had binaural interference (18% had binaural
advantage) when considering critical difference values. A different pattern of results
emerged with the CST, which showed a significant binaural advantage across all groups
in the means analysis. Individual results showed that, similar to the HINT, a small
proportion of participants had binaural advantage (12%) at each SBR. However, only
one participant had binaural interference at each SBR. Finally, on the DDT, a significant
right-ear advantage was found with group data, and for at least some participants
when inspecting individual data.
The HINT and CST reveal somewhat contradicting results in the proportion of binaural
interference cases (but yet they show similarly low proportions of binaural advantage).
It should be noted, however, that the purpose of each test is different. For instance,
the HINT investigates the SNR-50. Recall that [Walden and Walden (2005)] found binaural interference for 82% of their participants using an SNR-50 test,
although in replicating that study, [McArdle et al (2012)] found that only 20% of participants had binaural interference on the Quick Speech-in-Noise
Test both with and without hearing aids. This is similar to the present findings of
binaural interference in 27% of the participants with the HINT. In the studies that
attempted to separate out groups of successful unilateral and bilateral hearing aid
users, SRT in noise was a significant predictor in one study ([Köbler et al, 2010]), but that was not the case in a different report ([Boymans et al, 2008]). The speech recognition tests used in previous studies of binaural interference
were also different from our use of the CST in that the speech materials consisted
of single words ([Allen et al, 2000]; [Henkin et al, 2007]). Results of those studies did not agree with each other or with the present findings,
in that [Allen et al (2000)] found no binaural interference or advantage with W-22 words in quiet, and [Henkin et al (2007)] found better unilateral scores on a Hebrew version of the AB test (an open-set test
of phonemically balanced monosyllabic words) in +10 dB SNR. In summary, the SNR-50
tests suggest that ∼25% of the listeners exhibit binaural interference, while audibility-based
tests appear to be dependent on level, background noise, context, and so on.
The present study found a significant right-ear advantage on the DDT across the groups.
Individual scores were not statistically analyzed, but visual inspection of the data
shows a right-ear advantage for at least some listeners. Dichotic tests were employed
in most of the early case reports in support of binaural interference. In larger samples
testing dichotic performance, results have been mixed, with reports of no ear difference
([Walden and Walden, 2005]; with the DDT) and a right-ear advantage ([Henkin et al, 2007]; with the threshold-of-interference test). When attempting to predict future unilateral/bilateral
hearing aid use, conflicting results have also been reported ([Köbler et al, 2010]; [Cox et al, 2011]). A right-ear advantage for speech signals is supported by structural models of
cerebral asymmetries; as the contralateral pathways to the hemispheres are stronger
than the ipsilateral one, and speech is known to be processed in the left hemisphere,
a right-ear advantage should be expected ([Nicholls, 1998]). In the context of binaural interference investigations, a right-ear advantage
for speech signals has been considered as a sign of interference, because it indicates
that when attention is focused on the right ear, the participant is able to ignore
the contralateral stimulus; however, that does not happen when the listener is to
attend to the left ear (i.e., there is interference) ([Carter et al, 2001]; [Jerger, 2001]). Despite these theories, there have been no published criteria for “normal” and
“abnormal” right-ear advantage.
When comparing individual performance across the different tests, of the participants
who exceeded the critical differences in the HINT or CST (at either SBR), nine showed
interference in one test only (one showed interference in both tests), and five showed
advantage in one test only (four showed advantage in both tests). Interestingly, of
the two participants with the largest right-ear advantage on the DDT, one also showed
significant interference on the HINT, and the other showed interference on both the
HINT and CST. Both were older participants, one with normal hearing and one with hearing
loss.
Regarding the effects of age, although the groups did not differ on the ear condition
(i.e., ear effects, when present, were constant across groups), within-subject HINT
results showed that more participants in the elderly groups had binaural interference
(33.3%, n = 7) than in the younger group (16.7%, n = 2). The individual CST and DDT
analyses are less conclusive on the age effects. On the CST, one participant in the
OHI group had interference at +2 SBR and one in the YNH group had interference at
−2 SBR. On the DDT, two older participants had a large right-ear advantage, but that
was also the case for other participants in each group (to a lesser extent). The means
analysis showed a significant right-ear advantage across all groups on the DDT.
The presence of hearing loss in elderly participants produced poorer speech perception
scores; however, when comparing bilateral to unilateral performance, hearing loss
had no apparent effect on group data. A similar conclusion can be reached with within-subject
analyses. On the HINT, 33.3% of participants in the ONH group showed binaural interference
(n = 3) and 22.2% showed binaural advantage (n = 2). This was the case for 33.3% (n
= 4) and 25% (n = 3) of the participants in the OHI group, respectively. On the CST
+2 dB SBR, 55.6% (n = 5) of ONH participants and 75% (n = 9) of OHI participants had
significant binaural advantage. On the −2 SBR, the binaural advantage was seen in
66.7% (n = 6) of the ONH listeners and 50% (n = 6) of the OHI listeners. The DDT revealed
at least one case of right-ear advantage in each of the elderly groups.
CONCLUSIONS
Taken together, the present results support the occurrence of binaural interference
in at least 16.7% of listeners. Hearing loss does not seem to compound the presence
of binaural interference. Our findings do provide evidence that binaural interference
may be more prevalent in older adults; however, more research in this topic is clearly
needed. More direct clinical applications should also be investigated, testing the
predictive power of the HINT and DDT in uncovering binaural interference in listeners
with reported difficulties with two hearing aids.
The possibility of binaural interference need not change the general practice of bilateral
hearing aid fittings for listeners with symmetric hearing loss. However, it is imperative
that clinicians be aware of binaural interference and be attentive to its signs, such
as subjective reports of preference for one hearing aid. Although potentially deleterious
consequences of unilateral fittings such as auditory deprivation should be carefully
weighted, bilateral speech testing with adaptive levels of background noise such as
with the HINT may provide an objective confirmation of the patient’s testimony.
Abbreviations
ANOVA:
analysis of variance
CST:
Connected Speech Test
DDT:
Dichotic Digits Test
HINT:
Hearing in Noise Test
OHI:
older with hearing impairment
ONH:
older with normal hearing for their age
rau:
rationalized arcsine units
SBR:
signal-to-babble ratio
SNR:
signal-to-noise ratio
SNR-50:
signal-to-noise ratio that yields 50% correct speech recognition
SRT:
speech reception threshold
YNH:
younger with normal hearing
