Spatial attention in bilateral cochlear-implant users.

This study aimed to characterize horizontal plane sound localization in interfering noise at different signal-to-noise ratios (SNRs) and to compare performance across normal-hearing listeners and users of unilateral and bilateral cochlear implants (CIs). CI users report difficulties with listening in noisy environments. Although their difficulties with speech understanding have been investigated in several studies, the ability to localize sounds in background noise has not extensively been examined, despite the benefits of binaural hearing being greatest in noisy situations. Sound localization is a measure of binaural processing and is thus well suited to assessing the benefit of bilateral implantation. The results will inform clinicians and implant manufacturers how to focus their efforts to improve localization with CIs in noisy situations. Six normal-hearing listeners, four unilateral, and 10 bilateral CI users indicated the perceived location of sound sources using a light pointer method. Target sounds were noise pulses played from one of 11 loudspeakers placed between -80 and +80 degrees in the frontal horizontal plane in the free field. Localization was assessed in quiet and in diffuse background noise at SNRs between +10 and -7 dB. Speech reception thresholds were measured and their relation to the localization results examined. Localization performance declined with decreasing SNR: target sounds were perceived closer to the median plane and the standard deviation of responses increased. Localization performance across groups was compared using a measure of "Spatial Resolvability" (SR). This measure gives the angular separation between two sound sources that would enable an ideal observer to correctly distinguish them 69.1% of the time. For all participants SR increased with decreasing SNR, that is, at low SNRs the spatial separation between sound sources remained distinguishable only when it was larger. Normal-hearing participants performed best, with SR between 1.4 and 5.1 degrees in quiet. Bilateral CI users showed SR between 8.3 and 43.6 degrees in quiet, corresponding approximately to the spatial resolution of normal-hearing listeners at an SNR of -5 dB. Most bilateral CI users had lost the ability to correctly determine which side the sound came from at an SNR of -3 dB. Overall, the SNR had to be at least +7 dB to achieve localization performance near to that in quiet for all bilateral CI users. No significant correlation was found between spatial resolution and speech reception thresholds, but the speech processor sensitivity setting did significantly affect performance. Unilateral CI users showed the most severe localization problems, with only two of four participants being able to correctly determine which side sounds came from in quiet. This study is the first to examine sound localization with CIs at various SNRs and to compare it with normal hearing. The results confirm that localization with CIs is strongly disrupted in noisy situations. Bilateral CIs were shown to be clearly superior over unilateral CIs for localization in quiet and in noisy situations. With bilateral CIs, localization declined at moderately high absolute noise levels (>63 dB SPL), suggesting that an extension of the acoustic-dynamic range to higher levels would be beneficial. The absence of a relation between speech reception thresholds and spatial resolution highlights the need for additional clinical tests to assess the binaural benefit of a second implant.

In normal hearing (NH), the perception of the gender of a speaker is strongly affected by two anatomically related vocal characteristics: the fundamental frequency (F0), related to vocal pitch, and the vocal tract length (VTL), related to the height of the speaker. Previous studies on gender categorization in cochlear implant (CI) users found that performance was variable, with few CI users performing at the level of NH listeners. Data collected with recorded speech produced by multiple talkers suggests that CI users might rely more on F0 and less on VTL than NH listeners. However, because VTL cannot be accurately estimated from recordings, it is difficult to know how VTL contributes to gender categorization. In the present study, speech was synthesized to systematically vary F0, VTL, or both. Gender categorization was measured in CI users, as well as in NH participants listening to unprocessed (only synthesized) and vocoded (and synthesized) speech. Perceptual weights for F0 and VTL were derived from the performance data. With unprocessed speech, NH listeners used both cues (normalized perceptual weight: F0 = 3.76, VTL = 5.56). With vocoded speech, NH listeners still made use of both cues but less efficiently (normalized perceptual weight: F0 = 1.68, VTL = 0.63). CI users relied almost exclusively on F0 while VTL perception was profoundly impaired (normalized perceptual weight: F0 = 6.88, VTL = 0.59). As a result, CI users' gender categorization was abnormal compared to NH listeners. Future CI signal processing should aim to improve the transmission of both F0 cues and VTL cues, as a normal gender categorization may benefit speech understanding in competing talker situations.

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On the relationship between auditory cognition and speech intelligibility in cochlear implant users: An ERP study

Factors affecting binaural fusion in cochlear implant users.

Due to interaural frequency mismatch, bilateral cochlear-implant (CI) users may be less able to take advantage of binaural cues that normal-hearing (NH) listeners use for spatial hearing, such as interaural time differences and interaural level differences. As such, bilateral CI users have difficulty segregating competing speech even when the target and competing talkers are spatially separated. The goal of this study was to evaluate the effects of spectral resolution, tonotopic mismatch (the frequency mismatch between the acoustic center frequency assigned to CI electrode within an implanted ear relative to the expected spiral ganglion characteristic frequency), and interaural mismatch (differences in the degree of tonotopic mismatch in each ear) on speech understanding and spatial release from masking (SRM) in the presence of competing talkers in NH subjects listening to bilateral vocoder simulations. During testing, both target and masker speech were presented in five-word sentences that had the same syntax but were not necessarily meaningful. The sentences were composed of five categories in fixed order (Name, Verb, Number, Color, and Clothes), each of which had 10 items, such that multiple sentences could be generated by randomly selecting a word from each category. Speech reception thresholds (SRTs) for the target sentence presented in competing speech maskers were measured. The target speech was delivered to both ears and the two speech maskers were delivered to (1) both ears (diotic masker), or (2) different ears (dichotic masker: one delivered to the left ear and the other delivered to the right ear). Stimuli included the unprocessed speech and four 16-channel sine-vocoder simulations with different interaural mismatch (0, 1, and 2 mm). SRM was calculated as the difference between the diotic and dichotic listening conditions. With unprocessed speech, SRTs were 0.3 and -18.0 dB for the diotic and dichotic maskers, respectively. For the spectrally degraded speech with mild tonotopic mismatch and no interaural mismatch, SRTs were 5.6 and -2.0 dB for the diotic and dichotic maskers, respectively. When the tonotopic mismatch increased in both ears, SRTs worsened to 8.9 and 2.4 dB for the diotic and dichotic maskers, respectively. When the two ears had different tonotopic mismatch (e.g., there was interaural mismatch), the performance drop in SRTs was much larger for the dichotic than for the diotic masker. The largest SRM was observed with unprocessed speech (18.3 dB). With the CI simulations, SRM was significantly reduced to 7.6 dB even with mild tonotopic mismatch but no interaural mismatch; SRM was further reduced with increasing interaural mismatch. The results demonstrate that frequency resolution, tonotopic mismatch, and interaural mismatch have differential effects on speech understanding and SRM in simulation of bilateral CIs. Minimizing interaural mismatch may be critical to optimize binaural benefits and improve CI performance for competing speech, a typical listening environment. SRM (the difference in SRTs between diotic and dichotic maskers) may be a useful clinical tool to assess interaural frequency mismatch in bilateral CI users and to evaluate the benefits of optimization methods that minimize interaural mismatch.

Music plays an important role in the daily life of cochlear implant (CI) users, but electrical hearing and speech processing pose challenges for enjoying music. Studies of unilateral CI (UCI) users’ music perception have found that these subjects have little difficulty recognizing tempo and rhythm but great difficulty with pitch, interval and melody. The present study is an initial step towards understanding music perception in bilateral CI (BCI) users. The Munich Music Questionnaire was used to investigate music listening habits and enjoyment in 23 BCI users compared to 2 control groups: 23 UCI users and 23 normal-hearing (NH) listeners. Bilateral users appeared to have a number of advantages over unilateral users, though their enjoyment of music did not reach the level of NH listeners.

Auditory cortical activity to different voice onset times in cochlear implant users

Rate of speech can affect the durations of certain phonemes (Crystal & House, 1982), and listeners must normalize for speech rate to properly identify words (Miller, 1981). How speech rate affects perception of phonemic boundaries in users of cochlear implants (CIs) in comparison to normal-hearing (NH) listeners is unknown. The speech processing that occurs in these devices may obscure word and phoneme boundaries, providing less reliable durational information about the incoming speech signal. Less effective rate normalization in CI users could thus contribute to the degraded speech perception often observed in this population. Preliminary results with stop-consonant series show absolute differences between NH and CI listeners in the precise location of phonemic boundaries (with CI users’ boundaries typically occurring at shorter voice onset time durations), but not in the relative relationship of these boundaries as a function of speech rate. Data from NH listeners presented a CI simulation show effects of number of channels on both the location of phonemic boundaries and presence of rate normalization.

Bilateral cochlear-implant (BiCI) users are less accurate at localizing free-field (FF) sound sources than normal-hearing (NH) listeners. This performance gap is not well understood but is likely due to a combination of compromises in acoustic signal representation by the two independent speech processors and neural degradation of auditory pathways associated with a patient’s hearing loss. To exclusively investigate the effect of CI speech encoding on horizontal-plane sound localization, the present study measured sound localization performance in NH subjects listening to vocoder processed and nonvocoded virtual acoustic space (VAS) stimuli. Various aspects of BiCI stimulation such as independently functioning devices, variable across-ear channel selection, and pulsatile stimulation were simulated using uncorrelated noise (Nu), correlated noise (N0), or Gaussian-enveloped tone (GET) carriers during vocoder processing. Additionally, FF sound localization in BiCI users was measured in the same testing environment for comparison. Distinct response patterns across azimuthal locations were evident for both listener groups and were analyzed using a multilevel regression analysis. Simulated implant speech encoding, regardless of carrier, was detrimental to NH localization and the GET vocoder best simulated BiCI FF performance in NH listeners. Overall, the detrimental effect of vocoder processing on NH performance suggests that sound localization deficits may persist even for BiCI patients who have minimal neural degradation associated with their hearing loss and indicates that CI speech encoding plays a significant role in the sound localization deficits experienced by BiCI users.

Cochlear implants (CIs) restore speech perception in quiet but they also eliminate or distort many acoustic cues that are important for music enjoyment. Unfortunately, quantifying music enjoyment by CI users has been difficult because comparisons must rely on their recollection of music before they lost their hearing. Here, we aimed to assess music enjoyment in CI users using a readily interpretable reference based on acoustic hearing. The comparison was done by testing "single-sided deafness" (SSD) patients who have normal hearing (NH) in one ear and a CI in the other ear. The study also aimed to assess binaural musical enjoyment, with the reference being the experience of hearing with a single NH ear. Three experiments assessed the effect of adding different kinds of input to the second ear: electrical, vocoded, or unmodified. In experiment 1, music enjoyment in SSD-CI users was investigated using a modified version of the MUSHRA (MUltiple Stimuli with Hidden Reference and Anchor) method. Listeners rated their enjoyment of song segments on a scale of 0 to 200, where 100 represented the enjoyment obtained from a song segment presented to the NH ear, 0 represented a highly degraded version of the same song segment presented to the same ear, and 200 represented enjoyment subjectively rated as twice as good as the 100 reference. Stimuli consisted of acoustic only, electric only, acoustic and electric, as well as other conditions with low pass filtered acoustic stimuli. Acoustic stimulation was provided by headphone to the NH ear and electric stimulation was provided by direct audio input to the subject's speech processor. In experiment 2, the task was repeated using NH listeners who received vocoded stimuli instead of electric stimuli. Experiment 3 tested the effect of adding the same unmodified song segment to the second ear, also in NH listeners. Music presented through the CI only was very unpleasant, with an average rating of 20. Surprisingly, the combination of the unpleasant CI signal in one ear with acoustic stimulation in the other ear was rated more enjoyable (mean = 123) than acoustic processing alone. Presentation of the same monaural musical signal to both ears in NH listeners resulted with even greater enhancement of the experience compared with presentation to a single ear (mean = 159). Repeating the experiment using a vocoder to one ear of NH listeners resulted in interference rather than enhancement. Music enjoyment from electric stimulation is extremely poor relative to a readily interpretable NH baseline for CI-SSD listeners. However, the combination of this unenjoyable signal presented through a CI and an unmodified acoustic signal presented to a NH (or near-NH) contralateral ear results in enhanced music enjoyment with respect to the acoustic signal alone. Remarkably, this two-ear enhancement experienced by CI-SSD listeners represents a substantial fraction of the two-ear enhancement seen in NH listeners. This unexpected benefit of electroacoustic auditory stimulation will have to be considered in theoretical accounts of music enjoyment and may facilitate the quest to enhance music enjoyment in CI users.

Neural encoding of spectro-temporal cues at slow and near speech-rate in cochlear implant users

Objectives Cochlear implants (CI) have dramatically improved speech perception for patients with sensorineural hearing impairment. However, listening to music is a great challenge for them. This study examined the perception and appraisal of Iranian musical instruments comparing with similar Western instruments. Methods Eleven adult CI users and 25 normal hearing (NH) individuals participated in this study. Musical stimuli of three commonly heard instrument pairs were prepared. Participants were asked to identify the instruments and rate their appraisal on a ten-point Likert scale (0 = dislike very much, 10 = like very much). Results The instrument recognition rate was 40.6% among the CI users, and the mean appraisal score was 5.2 ± 2.7. NH listeners had none significant higher scores on both tasks with a recognition rate of 50.0% and the mean appraisal score of 6.9 ± 1.5. Iranian instruments were more recognized in both groups. Regarding their appraisal, the mean score for both types was almost equal in the NH group, while CI users more appraised Iranian instruments. Conclusion In addition to better recognition of Iranian instruments, they were particularly better appraised in the CI group. Iranian instruments provide suitable musical pieces for CI recipients that can be considered in rehabilitation programs.

The identity of a speech sound can be affected by the spectrum of a preceding stimulus in a contrastive manner. Although such aftereffects are often reduced in people with hearing loss and cochlear implants (CIs), one recent study demonstrated larger spectral contrast effects in CI users than in normal-hearing (NH) listeners. The present study aimed to shed light on this puzzling finding. We hypothesized that poorer spectral resolution leads CI users to rely on different acoustic cues not only to identify speech sounds but also to adapt to the context. Thirteen postlingually deafened adult CI users and 33 NH participants (listening to either vocoded or unprocessed speech) participated in this study. Psychometric functions were estimated in a vowel categorization task along the /I/ to /ε/ (as in "bit" and "bet") continuum following a context sentence, the long-term average spectrum of which was manipulated at the level of either fine-grained local spectral cues or coarser global spectral cues. In NH listeners with unprocessed speech, the aftereffect was determined solely by the fine-grained local spectral cues, resulting in a surprising insensitivity to the larger, global spectral cues utilized by CI users. Restricting the spectral resolution available to NH listeners via vocoding resulted in patterns of responses more similar to those found in CI users. However, the size of the contrast aftereffect remained smaller in NH listeners than in CI users. Only the spectral contrasts used by listeners contributed to the spectral contrast effects in vowel identification. These results explain why CI users can experience larger-than-normal context effects under specific conditions. The results also suggest that adaptation to new spectral cues can be very rapid for vowel discrimination, but may follow a longer time course to influence spectral contrast effects.

Top–down restoration of speech in cochlear-implant users