The Effect of Simulated Interaural Frequency Mismatch on Speech Understanding and Spatial Release From Masking

Review of Binaural Processing With Asymmetrical Hearing Outcomes in Patients With Bilateral Cochlear Implants

Bilateral cochlear implants (BiCIs) result in several benefits, including improvements in speech understanding in noise and sound source localization. However, the benefit bilateral implants provide among recipients varies considerably across individuals. Here we consider one of the reasons for this variability: difference in hearing function between the two ears, that is, interaural asymmetry. Thus far, investigations of interaural asymmetry have been highly specialized within various research areas. The goal of this review is to integrate these studies in one place, motivating future research in the area of interaural asymmetry. We first consider bottom-up processing, where binaural cues are represented using excitation-inhibition of signals from the left ear and right ear, varying with the location of the sound in space, and represented by the lateral superior olive in the auditory brainstem. We then consider top-down processing via predictive coding, which assumes that perception stems from expectations based on context and prior sensory experience, represented by cascading series of cortical circuits. An internal, perceptual model is maintained and updated in light of incoming sensory input. Together, we hope that this amalgamation of physiological, behavioral, and modeling studies will help bridge gaps in the field of binaural hearing and promote a clearer understanding of the implications of interaural asymmetry for future research on optimal patient interventions.

Effects of spectral smearing on speech understanding and masking release in simulated bilateral cochlear implants

Differences in spectro-temporal degradation may explain some variability in cochlear implant users' speech outcomes. The present study employs vocoder simulations on listeners with typical hearing to evaluate how differences in degree of channel interaction across ears affects spatial speech recognition. Speech recognition thresholds and spatial release from masking were measured in 16 normal-hearing subjects listening to simulated bilateral cochlear implants. 16-channel sine-vocoded speech simulated limited, broad, or mixed channel interaction, in dichotic and diotic target-masker conditions, across ears. Thresholds were highest with broad channel interaction in both ears but improved when interaction decreased in one ear and again in both ears. Masking release was apparent across conditions. Results from this simulation study on listeners with typical hearing show that channel interaction may impact speech recognition more than masking release, and may have implications for the effects of channel interaction on cochlear implant users' speech recognition outcomes.

Interaural frequency mismatch jointly modulates neural brainstem binaural interaction and behavioral interaural time difference sensitivity in humans

The binaural interaction component (BIC) of the auditory brainstem response (ABR) is the difference obtained after subtracting the sum of right and left ear ABRs from binaurally evoked ABRs. The BIC has attracted interest as a biomarker of binaural processing abilities. Best binaural processing is presumed to require spectrally-matched inputs at the two ears, but peripheral pathology and/or impacts of hearing devices can lead to mismatched inputs. Such mismatching can degrade behavioral sensitivity to interaural time difference (ITD) cues, but might be detected using the BIC. Here, we examine the effect of interaural frequency mismatch (IFM) on BIC and behavioral ITD sensitivity in audiometrically normal adult human subjects (both sexes). Binaural and monaural ABRs were recorded and BICs computed from subjects in response to narrowband tones. Left ear stimuli were fixed at 4000 Hz while right ear stimuli varied over a ∼2-octave range (re: 4000 Hz). Separately, subjects performed psychophysical lateralization tasks using the same stimuli to determine ITD discrimination thresholds jointly as a function of IFM and sound level. Results demonstrated significant effects of IFM on BIC amplitudes, with lower amplitudes in mismatched conditions than frequency-matched. Behavioral ITD discrimination thresholds were elevated at mismatched frequencies and lower sound levels, but also more sharply modulated by IFM at lower sound levels. Combinations of ITD, IFM and overall sound level that resulted in fused and lateralized percepts were bound by the empirically-measured BIC, and also by model predictions simulated using an established computational model of the brainstem circuit thought to generate the BIC.

Effect of experimentally introduced interaural frequency mismatch on sentence recognition in bilateral cochlear-implant listeners

Bilateral cochlear-implant users experience interaural frequency mismatch because of asymmetries in array insertion and frequency-to-electrode assignment. To explore the acute perceptual consequences of such mismatch, sentence recognition in quiet was measured in nine bilateral cochlear-implant listeners as frequency allocations in the poorer ear were shifted by ±1.5, ±3, and ±4.5 mm using experimental programs. Shifts in frequency allocation >3 mm reduced bilateral sentence scores below those for the better ear alone, suggesting that the poorer ear interfered with better-ear perception. This was not a result of fewer active channels; deactivating electrodes without frequency shifting had minimal effect.

Importance of ipsilateral residual hearing for spatial hearing by bimodal cochlear implant users

Bimodal cochlear implant (CI) listeners have difficulty utilizing spatial cues to segregate competing speech, possibly due to tonotopic mismatch between the acoustic input frequency and electrode place of stimulation. The present study investigated the effects of tonotopic mismatch in the context of residual acoustic hearing in the non-CI ear or residual hearing in both ears. Speech recognition thresholds (SRTs) were measured with two co-located or spatially separated speech maskers in normal-hearing adults listening to acoustic simulations of CIs; low frequency acoustic information was available in the non-CI ear (bimodal listening) or in both ears. Bimodal SRTs were significantly better with tonotopically matched than mismatched electric hearing for both co-located and spatially separated speech maskers. When there was no tonotopic mismatch, residual acoustic hearing in both ears provided a significant benefit when maskers were spatially separated, but not when co-located. The simulation data suggest that hearing preservation in the implanted ear for bimodal CI listeners may significantly benefit utilization of spatial cues to segregate competing speech, especially when the residual acoustic hearing is comparable across two ears. Also, the benefits of bilateral residual acoustic hearing may be best ascertained for spatially separated maskers.

Effect of interaural electrode insertion depth difference and independent band selection on sentence recognition in noise and spatial release from masking in simulated bilateral cochlear implant listening

PURPOSE

Inter-aural insertion depth difference (IEDD) in bilateral cochlear implant (BiCI) with continuous interleaved sampling (CIS) processing is known to reduce the recognition of speech in noise and spatial release from masking (SRM). However, the independent channel selection in the 'n-of-m' sound coding strategy might have a different effect on speech recognition and SRM when compared to the effects of IEDD in CIS-based findings. This study aimed to investigate the effect of bilateral 'n-of-m' processing strategy and interaural electrode insertion depth difference on speech recognition in noise and SRM under conditions that simulated bilateral cochlear implant listening.

METHODS

Five young adults with normal hearing sensitivity participated in the study. The target sentences were spatially filtered to originate from 0° and the masker was spatially filtered at 0°, 15°, 37.5°, and 90° using the Oldenburg head-related transfer function database for behind the ear microphone. A 22-channel sine wave vocoder processing based on 'n-of-m' processing was applied to the spatialized target-masker mixture, in each ear. The perceptual experiment involved a test of speech recognition in noise under one co-located condition (target and masker at 0°) and three spatially separated conditions (target at 0°, masker at 15°, 37.5°, or 90° to the right ear).

RESULTS

The results were analyzed using a three-way repeated measure analysis of variance (ANOVA). The effect of interaural insertion depth difference (F (2,8) = 3.145, p = 0.098, ɳ² = 0.007) and spatial separation between target and masker (F (3,12) = 1.239, p = 0.339, ɳ² = 0.004) on speech recognition in noise was not significant.

CONCLUSIONS

Speech recognition in noise and SRM were not affected by IEDD ≤ 3 mm. Bilateral 'n-of-m' processing resulted in reduced speech recognition in noise and SRM.

Effect of experimentally introduced interaural frequency mismatch on sentence recognition in bilateral cochlear-implant listeners

Bilateral cochlear-implant users experience interaural frequency mismatch because of asymmetries in array insertion and frequency-to-electrode assignment. To explore the acute perceptual consequences of such mismatch, sentence recognition in quiet was measured in nine bilateral cochlear-implant listeners as frequency allocations in the poorer ear were shifted by ±1.5, ±3 and ±4.5 mm using experimental programs. Shifts in frequency allocation >3 mm were found to reduce bilateral sentence scores below those for the better ear alone, suggesting that the poorer ear interfered with better-ear perception. This was not a result of fewer active channels; deactivating electrodes without frequency shifting had minimal effect.

Anatomical Variations of the Human Cochlea Using an Image Analysis Tool

Understanding cochlear anatomy is crucial for developing less traumatic electrode arrays and insertion guidance for cochlear implantation. The human cochlea shows considerable variability in size and morphology. This study analyses 1000+ clinical temporal bone CT images using a web-based image analysis tool. Cochlear size and shape parameters were obtained to determine population statistics and perform regression and correlation analysis. The analysis revealed that cochlear morphology follows Gaussian distribution, while cochlear dimensions A and B are not well-correlated to each other. Additionally, dimension B is more correlated to duct lengths, the wrapping factor and volume than dimension A. The scala tympani size varies considerably among the population, with the size generally decreasing along insertion depth with dimensional jumps through the trajectory. The mean scala tympani radius was 0.32 mm near the 720° insertion angle. Inter-individual variability was four times that of intra-individual variation. On average, the dimensions of both ears are similar. However, statistically significant differences in clinical dimensions were observed between ears of the same patient, suggesting that size and shape are not the same. Harnessing deep learning-based, automated image analysis tools, our results yielded important insights into cochlear morphology and implant development, helping to reduce insertion trauma and preserving residual hearing.

Defining functional spatial boundaries using a spatial release from masking task

The classic spatial release from masking (SRM) task measures speech recognition thresholds for discrete separation angles between a target and masker. Alternatively, this study used a modified SRM task that adaptively measured the spatial-separation angle needed between a continuous male target stream (speech with digits) and two female masker streams to achieve a specific SRM. On average, 20 young normal-hearing listeners needed less spatial separation for 6 dB release than 9 dB release, and the presence of background babble reduced across-listener variability on the paradigm. Future work is needed to better understand the psychometric properties of this adaptive procedure.

Development of Sound Localization in Infants and Young Children with Cochlear Implants

Cochlear implantation as a treatment for severe-to-profound hearing loss allows children to develop hearing, speech, and language in many cases. However, cochlear implants are generally provided beyond the infant period and outcomes are assessed after years of implant use, making comparison with normal development difficult. The aim was to study whether the rate of improvement of horizontal localization accuracy in children with bilateral implants is similar to children with normal hearing. A convenience sample of 20 children with a median age at simultaneous bilateral implantation = 0.58 years (0.42−2.3 years) participated in this cohort study. Longitudinal follow-up of sound localization accuracy for an average of ≈1 year generated 42 observations at a mean age = 1.5 years (0.58−3.6 years). The rate of development was compared to historical control groups including children with normal hearing and with relatively late bilateral implantation (≈4 years of age). There was a significant main effect of time with bilateral implants on localization accuracy (slope = 0.21/year, R2 = 0.25, F = 13.6, p < 0.001, n = 42). No differences between slopes (F = 0.30, p = 0.58) or correlation coefficients (Cohen’s q = 0.28, p = 0.45) existed when comparing children with implants and normal hearing (slope = 0.16/year since birth, p = 0.015, n = 12). The rate of development was identical to children implanted late. Results suggest that early bilateral implantation in children with severe-to-profound hearing loss allows development of sound localization at a similar age to children with normal hearing. Similar rates in children with early and late implantation and normal hearing suggest an intrinsic mechanism for the development of horizontal sound localization abilities.

Computed-Tomography Estimates of Interaural Mismatch in Insertion Depth and Scalar Location in Bilateral Cochlear-Implant Users

HYPOTHESIS

Bilateral cochlear-implant (BI-CI) users will have a range of interaural insertion-depth mismatch because of different array placement or characteristics. Mismatch will be larger for electrodes located near the apex or outside scala tympani, or for arrays that are a mix of precurved and straight types.

BACKGROUND

Brainstem superior olivary-complex neurons are exquisitely sensitive to interaural-difference cues for sound localization. Because these neurons rely on interaurally place-of-stimulation-matched inputs, interaural insertion-depth or scalar-location differences for BI-CI users could cause interaural place-of-stimulation mismatch that impairs binaural abilities.

METHODS

Insertion depths and scalar locations were calculated from temporal-bone computed-tomography scans for 107 BI-CI users (27 Advanced Bionics, 62 Cochlear, 18 MED-EL).

RESULTS

Median interaural insertion-depth mismatch was 23.4 degrees or 1.3 mm. Mismatch in the estimated clinically relevant range expected to impair binaural processing (>75 degrees or 3 mm) occurred for 13 to 19% of electrode pairs overall, and for at least three electrode pairs for 23 to 37% of subjects. There was a significant three-way interaction between insertion depth, scalar location, and array type. Interaural insertion-depth mismatch was largest for apical electrodes, for electrode pairs in two different scala, and for arrays that were both-precurved.

CONCLUSION

Average BI-CI interaural insertion-depth mismatch was small; however, large interaural insertion-depth mismatch-with the potential to degrade spatial hearing-occurred frequently enough to warrant attention. For new BICI users, improved surgical techniques to avoid interaural insertion-depth and scalar mismatch are recommended. For existing BI-CI users with interaural insertion-depth mismatch, interaural alignment of clinical frequency tables might reduce negative spatial-hearing consequences.

Frequency Fitting Optimization Using Evolutionary Algorithm in Cochlear Implant Users with Bimodal Binaural Hearing

Optimizing hearing in patients with a unilateral cochlear implant (CI) and contralateral acoustic hearing is a challenge. Evolutionary algorithms (EA) can explore a large set of potential solutions in a stochastic manner to approach the optimum of a minimization problem. The objective of this study was to develop and evaluate an EA-based protocol to modify the default frequency settings of a MAP (fMAP) of the CI in patients with bimodal hearing. Methods: This monocentric prospective study included 27 adult CI users (with post-lingual deafness and contralateral functional hearing). A fitting program based on EA was developed to approach the best fMAP. Generated fMAPs were tested by speech recognition (word recognition score, WRS) in noise and free-field-like conditions. By combining these first fMAPs and adding some random changes, a total of 13 fMAPs over 3 generations were produced. Participants were evaluated before and 45 to 60 days after the fitting by WRS in noise and questionnaires on global sound quality and music perception in bimodal binaural conditions. Results: WRS in noise improved with the EA-based fitting in comparison to the default fMAP (41.67 ± 9.70% versus 64.63 ± 16.34%, respectively, p = 0.0001, signed-rank test). The global sound quality and music perception were also improved, as judged by ratings on questionnaires and scales. Finally, most patients chose to keep the new fitting definitively. Conclusions: By modifying the default fMAPs, the EA improved the speech discrimination in noise and the sound quality in bimodal binaural conditions.

Effects of tonotopic matching and spatial cues on segregation of competing speech in simulations of bilateral cochlear implants

In the clinical fitting of cochlear implants (CIs), the lowest input acoustic frequency is typically much lower than the characteristic frequency associated with the most apical electrode position, due to the limited electrode insertion depth. For bilateral CI users, electrode positions may differ across ears. However, the same acoustic-to-electrode frequency allocation table (FAT) is typically assigned to both ears. As such, bilateral CI users may experience both intra-aural frequency mismatch within each ear and inter-aural mismatch across ears. This inter-aural mismatch may limit the ability of bilateral CI users to take advantage of spatial cues when attempting to segregate competing speech. Adjusting the FAT to tonotopically match the electrode position in each ear (i.e., increasing the low acoustic input frequency) is theorized to reduce this inter-aural mismatch. Unfortunately, this approach may also introduce the loss of acoustic information below the modified input acoustic frequency. The present study explored the trade-off between reduced inter-aural frequency mismatch and low-frequency information loss for segregation of competing speech. Normal-hearing participants were tested while listening to acoustic simulations of bilateral CIs. Speech reception thresholds (SRTs) were measured for target sentences produced by a male talker in the presence of two different male talkers. Masker speech was either co-located with or spatially separated from the target speech. The bilateral CI simulations were produced by 16-channel sinewave vocoders; the simulated insertion depth was fixed in one ear and varied in the other ear, resulting in an inter-aural mismatch of 0, 2, or 6 mm in terms of cochlear place. Two FAT conditions were compared: 1) clinical (200-8000 Hz in both ears), or 2) matched to the simulated insertion depth in each ear. Results showed that SRTs were significantly lower with the matched than with the clinical FAT, regardless of the insertion depth or spatial configuration of the masker speech. The largest improvement in SRTs with the matched FAT was observed when the inter-aural mismatch was largest (6 mm). These results suggest that minimizing inter-aural mismatch with tonotopically matched FATs may benefit bilateral CI users' ability to segregate competing speech despite substantial low-frequency information loss in ears with shallow insertion depths.

Impaired interaural correlation processing in people with schizophrenia

Detection of transient changes in interaural correlation is based on the temporal precision of the central representations of acoustic signals. Whether schizophrenia impairs the temporal precision in the interaural correlation process is not clear. In both participants with schizophrenia and matched healthy-control participants, this study examined the detection of a break in interaural correlation (BIC, a change in interaural correlation from 1 to 0 and back to 1), including the longest interaural delay at which a BIC was just audible, representing the temporal extent of the primitive auditory memory (PAM). Moreover, BIC-induced electroencephalograms (EEGs) and the relationships between the early binaural psychoacoustic processing and higher cognitive functions, which were assessed by the Repeatable Battery for the Assessment of Neuropsychological Status (RBANS), were examined. The results showed that compared to healthy controls, participants with schizophrenia exhibited poorer BIC detection, PAM and RBANS score. Both the BIC-detection accuracy and the PAM extent were correlated with the RBANS score. Moreover, participants with schizophrenia showed weaker BIC-induced N1-P2 amplitude which was correlated with both theta-band power and inter-trial phase coherence. These results suggested that schizophrenia impairs the temporal precision of the central representations of acoustic signals, affecting both interaural correlation processing and higher-order cognitions.

Effects of Spectral Resolution and Frequency Mismatch on Speech Understanding and Spatial Release From Masking in Simulated Bilateral Cochlear Implants

OBJECTIVES

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.

DESIGN

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.

RESULTS

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.

CONCLUSIONS

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.

Binaural Optimization of Cochlear Implants: Discarding Frequency Content Without Sacrificing Head-Shadow Benefit

OBJECTIVES

Single-sided deafness cochlear-implant (SSD-CI) listeners and bilateral cochlear-implant (BI-CI) listeners gain near-normal levels of head-shadow benefit but limited binaural benefits. One possible reason for these limited binaural benefits is that cochlear places of stimulation tend to be mismatched between the ears. SSD-CI and BI-CI patients might benefit from a binaural fitting that reallocates frequencies to reduce interaural place mismatch. However, this approach could reduce monaural speech recognition and head-shadow benefit by excluding low- or high-frequency information from one ear. This study examined how much frequency information can be excluded from a CI signal in the poorer-hearing ear without reducing head-shadow benefits and how these outcomes are influenced by interaural asymmetry in monaural speech recognition.

DESIGN

Speech-recognition thresholds for sentences in speech-shaped noise were measured for 6 adult SSD-CI listeners, 12 BI-CI listeners, and 9 normal-hearing listeners presented with vocoder simulations. Stimuli were presented using nonindividualized in-the-ear or behind-the-ear head-related impulse-response simulations with speech presented from a 70° azimuth (poorer-hearing side) and noise from 70° (better-hearing side), thereby yielding a better signal-to-noise ratio (SNR) at the poorer-hearing ear. Head-shadow benefit was computed as the improvement in bilateral speech-recognition thresholds gained from enabling the CI in the poorer-hearing, better-SNR ear. High- or low-pass filtering was systematically applied to the head-related impulse-response-filtered stimuli presented to the poorer-hearing ear. For the SSD-CI listeners and SSD-vocoder simulations, only high-pass filtering was applied, because the CI frequency allocation would never need to be adjusted downward to frequency-match the ears. For the BI-CI listeners and BI-vocoder simulations, both low and high pass filtering were applied. The normal-hearing listeners were tested with two levels of performance to examine the effect of interaural asymmetry in monaural speech recognition (vocoder synthesis-filter slopes: 5 or 20 dB/octave).

RESULTS

Mean head-shadow benefit was smaller for the SSD-CI listeners (~7 dB) than for the BI-CI listeners (~14 dB). For SSD-CI listeners, frequencies <1236 Hz could be excluded; for BI-CI listeners, frequencies <886 or >3814 Hz could be excluded from the poorer-hearing ear without reducing head-shadow benefit. Bilateral performance showed greater immunity to filtering than monaural performance, with gradual changes in performance as a function of filter cutoff. Real and vocoder-simulated CI users with larger interaural asymmetry in monaural performance had less head-shadow benefit.

CONCLUSIONS

The "exclusion frequency" ranges that could be removed without diminishing head-shadow benefit are interpreted in terms of low importance in the speech intelligibility index and a small head-shadow magnitude at low frequencies. Although groups and individuals with greater performance asymmetry gained less head-shadow benefit, the magnitudes of these factors did not predict the exclusion frequency range. Overall, these data suggest that for many SSD-CI and BI-CI listeners, the frequency allocation for the poorer-ear CI can be shifted substantially without sacrificing head-shadow benefit, at least for energetic maskers. Considering the two ears together as a single system may allow greater flexibility in discarding redundant frequency content from a CI in one ear when considering bilateral programming solutions aimed at reducing interaural frequency mismatch.

Dichotic listening performance with cochlear-implant simulations of ear asymmetry is consistent with difficulty ignoring clearer speech

There are an increasing number of bilateral and single-sided-deafness cochlear-implant (CI) users who hope to achieve improved spatial-hearing abilities through access to sound in both ears. It is, however, unclear how speech is processed when inputs are functionally asymmetrical, which may have an impact on spatial-hearing abilities. Therefore, functionally asymmetrical hearing was controlled and parametrically manipulated using a channel vocoder as a CI simulation. In Experiment 1, normal-hearing (NH) listeners performed a dichotic listening task (i.e., selective attention to one ear, ignoring the other) using asymmetrical signal degradation. Spectral resolution varied independently in each ear (4, 8, 16 channels, and unprocessed control). Performance decreased with decreasing resolution in the target ear and increasing resolution in the interferer ear. In Experiment 2, these results were replicated using a divided attention task (attend to both ears, report one after sentence completion) in both NH and bilateral CI listeners, although overall performance was lower than in Experiment 1. In Experiment 3, frequency-to-place mismatch simulated shallow CI insertion depths (0, 3, 6-mm shifts, and unprocessed control). Performance mostly decreased with increasing shift in the target ear and decreasing shift in the interferer ear; however, performance nonmonotonicities occurred. The worst performance occurred when the shift matched across ears, suggesting that pitch similarity increases difficulty. The results show that it is more difficult to attend an ear that is relatively degraded or distorted, which may set spatial-hearing limitations for CI users when trying to attend to a target in complex auditory scenes.

The Temporal Limits Encoder as a Sound Coding Strategy for Bilateral Cochlear Implants

The difference in binaural benefit between bilateral cochlear implant (CI) users and normal hearing (NH) listeners has typically been attributed to CI sound coding strategies not encoding the acoustic fine structure (FS) interaural time differences (ITD). The Temporal Limits Encoder (TLE) strategy is proposed as a potential way of improving binaural hearing benefits for CI users in noisy situations. TLE works by downward-transposition of mid-frequency band-limited channel information and can theoretically provide FS-ITD cues. In this work, the effect of choice of lower limit of the modulator in TLE was examined by measuring performance on a word recognition task and computing the magnitude of binaural benefit in bilateral CI users. Performance listening with the TLE strategy was compared with the commonly used Advanced Combinational Encoder (ACE) CI sound coding strategy. Results showed that setting the lower limit to ≥200 Hz maintained word recognition performance comparable to that of ACE. While most CI listeners exhibited a large binaural benefit (≥6 dB) in at least one of the conditions tested, there was no systematic relationship between the lower limit of the modulator and performance. These results indicate that the TLE strategy has potential to improve binaural hearing abilities in CI users but further work is needed to understand how binaural benefit can be maximized.

Spatial Release From Informational and Energetic Masking in Bimodal and Bilateral Cochlear Implant Users

Purpose Spatially separating speech and background noise improves speech understanding in normal-hearing listeners, an effect referred to as spatial release from masking (SRM). In cochlear implant (CI) users, SRM has often been demonstrated using asymmetric noise configurations, which maximize benefit from head shadow and the potential availability of binaural cues. In contrast, SRM in symmetrical configurations has been minimal to absent in CI users. We examined the interaction between two types of maskers (informational and energetic) and SRM in bimodal and bilateral CI users. We hypothesized that SRM would be absent or "negative" using symmetrically separated noise maskers. Second, we hypothesized that bimodal listeners would exhibit greater release from informational masking due to access to acoustic information in the non-CI ear. Method Participants included 10 bimodal and 10 bilateral CI users. Speech understanding in noise was tested in 24 conditions: 3 spatial configurations (S₀N₀, S₀N_45&315, S₀N_90&270) × 2 masker types (speech, signal-correlated noise) × 2 listening configurations (best-aided, CI-alone) × 2 talker gender conditions (different-gender, same-gender). Results In support of our first hypothesis, both groups exhibited negative SRM with increasing spatial separation. In opposition to our second hypothesis, both groups exhibited similar magnitudes of release from informational masking. The magnitude of release was greater for bimodal listeners, though this difference failed to reach statistical significance. Conclusions Both bimodal and bilateral CI recipients exhibited negative SRM. This finding is consistent with CI signal processing limitations, the audiologic factors associated with SRM, and known effects of behind-the-ear microphone technology. Though release from informational masking was not significantly different across groups, the magnitude of release was greater for bimodal listeners. This suggests that bimodal listeners may be at least marginally more susceptible to informational masking than bilateral CI users, though further research is warranted.

Sound Localization in Real-Time Vocoded Cochlear-Implant Simulations With Normal-Hearing Listeners

Bilateral cochlear-implant (CI) users and single-sided deaf listeners with a CI are less effective at localizing sounds than normal-hearing (NH) listeners. This performance gap is due to the degradation of binaural and monaural sound localization cues, caused by a combination of device-related and patient-related issues. In this study, we targeted the device-related issues by measuring sound localization performance of 11 NH listeners, listening to free-field stimuli processed by a real-time CI vocoder. The use of a real-time vocoder is a new approach, which enables testing in a free-field environment. For the NH listening condition, all listeners accurately and precisely localized sounds according to a linear stimulus–response relationship with an optimal gain and a minimal bias both in the azimuth and in the elevation directions. In contrast, when listening with bilateral real-time vocoders, listeners tended to orient either to the left or to the right in azimuth and were unable to determine sound source elevation. When listening with an NH ear and a unilateral vocoder, localization was impoverished on the vocoder side but improved toward the NH side. Localization performance was also reflected by systematic variations in reaction times across listening conditions. We conclude that perturbation of interaural temporal cues, reduction of interaural level cues, and removal of spectral pinna cues by the vocoder impairs sound localization. Listeners seem to ignore cues that were made unreliable by the vocoder, leading to acute reweighting of available localization cues. We discuss how current CI processors prevent CI users from localizing sounds in everyday environments.

Effect of channel separation and interaural mismatch on fusion and lateralization in normal-hearing and cochlear-implant listeners

Bilateral cochlear implantation has provided access to some of the benefits of binaural hearing enjoyed by normal-hearing (NH) listeners. However, a gap in performance still exists between the two populations. Single-channel stimulation studies have shown that interaural place-of-stimulation mismatch (IPM) due to differences in implantation depth leads to decreased binaural fusion and lateralization of interaural time and level differences (ITDs and ILDs, respectively). While single-channel studies are informative, multi-channel stimulation is needed for good speech understanding with cochlear implants (CIs). Some multi-channel studies have shown that channel interaction due to current spread can affect ITD sensitivity. In this work, we studied the effect of IPM and channel spacing, along with their potential interaction, on binaural fusion and ITD/ILD lateralization. Experiments were conducted in adult NH listeners and CI listeners with a history of acoustic hearing. Results showed that IPM reduced the range of lateralization for ITDs but not ILDs. CI listeners were more likely to report a fused percept in the presence of IPM with multi-channel stimulation than NH listeners. However, no effect of channel spacing was found. These results suggest that IPM should be accounted for in clinical mapping practices in order to maximize bilateral CI benefits.