A Mixed-Rate Strategy on a Bilaterally-Synchronized Cochlear Implant Processor Offering the Opportunity to Provide Both Speech Understanding and Interaural Time Difference Cues.

Background/Objective: Bilaterally implanted cochlear implant (CI) users do not consistently have access to interaural time differences (ITDs). ITDs are crucial for restoring the ability to localize sounds and understand speech in noisy environments. Lack of access to ITDs is partly due to lack of communication between clinical processors across the ears and partly because processors must use relatively high rates of stimulation to encode envelope information. Speech understanding is best at higher stimulation rates, but sensitivity to ITDs in the timing of pulses is best at low stimulation rates. Methods: We implemented a practical "mixed rate" strategy that encodes ITD information using a low stimulation rate on some channels and speech information using high rates on the remaining channels. The strategy was tested using a bilaterally synchronized research processor, the CCi-MOBILE. Nine bilaterally implanted CI users were tested on speech understanding and were asked to judge the location of a sound based on ITDs encoded using this strategy. Results: Performance was similar in both tasks between the control strategy and the new strategy. Conclusions: We discuss the benefits and drawbacks of the sound coding strategy and provide guidelines for utilizing synchronized processors for developing strategies.

Lateralization of binaural envelope cues measured with a mobile cochlear-implant research processora).

Bilateral cochlear implant (BICI) listeners do not have full access to the binaural cues that normal hearing (NH) listeners use for spatial hearing tasks such as localization. When using their unsynchronized everyday processors, BICI listeners demonstrate sensitivity to interaural level differences (ILDs) in the envelopes of sounds, but interaural time differences (ITDs) are less reliably available. It is unclear how BICI listeners use combinations of ILDs and envelope ITDs, and how much each cue contributes to perceived sound location. The CCi-MOBILE is a bilaterally synchronized research processor with the untested potential to provide spatial cues to BICI listeners. In the present study, the CCi-MOBILE was used to measure the ability of BICI listeners to perceive lateralized sound sources when single pairs of electrodes were presented amplitude-modulated stimuli with combinations of ILDs and envelope ITDs. Young NH listeners were also tested using amplitude-modulated high-frequency tones. A cue weighting analysis with six BICI and ten NH listeners revealed that ILDs contributed more than envelope ITDs to lateralization for both groups. Moreover, envelope ITDs contributed to lateralization for NH listeners but had negligible contribution for BICI listeners. These results suggest that the CCi-MOBILE is suitable for binaural testing and developing bilateral processing strategies.

Lateralization of interaural time differences with mixed rates of stimulation in bilateral cochlear implant listeners.

While listeners with bilateral cochlear implants (BiCIs) are able to access information in both ears, they still struggle to perform well on spatial hearing tasks when compared to normal hearing listeners. This performance gap could be attributed to the high stimulation rates used for speech representation in clinical processors. Prior work has shown that spatial cues, such as interaural time differences (ITDs), are best conveyed at low rates. Further, BiCI listeners are sensitive to ITDs with a mixture of high and low rates. However, it remains unclear whether mixed-rate stimuli are perceived as unitary percepts and spatially mapped to intracranial locations. Here, electrical pulse trains were presented on five, interaurally pitch-matched electrode pairs using research processors, at either uniformly high rates, low rates, or mixed rates. Eight post-lingually deafened adults were tested on perceived intracranial lateralization of ITDs ranging from 50 to 1600 µs. Extent of lateralization depended on the location of low-rate stimulation along the electrode array: greatest in the low- and mixed-rate configurations, and smallest in the high-rate configuration. All but one listener perceived a unitary auditory object. These findings suggest that a mixed-rate processing strategy can result in good lateralization and convey a unitary auditory object with ITDs.

Asymmetric temporal envelope sensitivity: Within- and across-ear envelope comparisons in listeners with bilateral cochlear implants.

For listeners with bilateral cochlear implants (BiCIs), patient-specific differences in the interface between cochlear implant (CI) electrodes and the auditory nerve can lead to degraded temporal envelope information, compromising the ability to distinguish between targets of interest and background noise. It is unclear how comparisons of degraded temporal envelope information across spectral channels (i.e., electrodes) affect the ability to detect differences in the temporal envelope, specifically amplitude modulation (AM) rate. In this study, two pulse trains were presented simultaneously via pairs of electrodes in different places of stimulation, within and/or across ears, with identical or differing AM rates. Results from 11 adults with BiCIs indicated that sensitivity to differences in AM rate was greatest when stimuli were paired between different places of stimulation in the same ear. Sensitivity from pairs of electrodes was predicted by the poorer electrode in the pair or the difference in fidelity between both electrodes in the pair. These findings suggest that electrodes yielding poorer temporal fidelity act as a bottleneck to comparisons of temporal information across frequency and ears, limiting access to the cues used to segregate sounds, which has important implications for device programming and optimizing patient outcomes with CIs.

Investigating effortful speech perception using fNIRS and pupillometry measures.

The current study examined the neural mechanisms for mental effort and its correlation to speech perception using functional near-infrared spectroscopy (fNIRS) in listeners with normal hearing (NH). Data were collected while participants listened and responded to unprocessed and degraded sentences, where words were presented in grammatically correct or shuffled order. Effortful listening and task difficulty due to stimulus manipulations was confirmed using a subjective questionnaire and a well-established objective measure of mental effort - pupillometry. fNIRS measures focused on cortical responses in two a priori regions of interest, the left auditory cortex (AC) and lateral frontal cortex (LFC), which are closely related to auditory speech perception and listening effort, respectively. We examined the relations between the two objective measures and behavioral measures of speech perception (task performance) and task difficulty. Results: demonstrated that changes in pupil dilation were positively correlated with the self-reported task difficulty levels and negatively correlated with the task performance scores. A significant and negative correlation between the two behavioral measures was also found. That is, as perceived task demands increased and task performance scores decreased, pupils dilated more. fNIRS measures (cerebral oxygenation) in the left AC and LFC were both negatively correlated with the self-reported task difficulty levels and positively correlated with task performance scores. These results suggest that pupillometry measures can indicate task demands and listening effort; whereas, fNIRS measures using a similar paradigm seem to reflect speech processing, but not effort.

Pitch Perception With the Temporal Limits Encoder for Cochlear Implants.

The temporal-limits-encoder (TLE) strategy has been proposed to enhance the representation of temporal fine structure (TFS) in cochlear implants (CIs), which is vital for many aspects of sound perception but is typically discarded by most modern CI strategies. TLE works by computing an envelope modulator that is within the temporal pitch limits of CI electric hearing. This paper examines the TFS information encoded by TLE and evaluates the salience and usefulness of this information in CI users. Two experiments were conducted to compare pitch perception performance of TLE versus the widely-used Advanced Combinational Encoder (ACE) strategy. Experiment 1 investigated whether TLE processing improved pitch discrimination compared to ACE. Experiment 2 parametrically examined the effect of changing the lower frequency limit of the TLE modulator on pitch ranking. In both experiments, F0 difference limens were measured with synthetic harmonic complex tones using an adaptive procedure. Signal analysis of the outputs of TLE and ACE strategies showed that TLE introduces important temporal pitch cues that are not available with ACE. Results showed an improvement in pitch discrimination with TLE when the acoustic input had a lower F0 frequency. No significant effect of lower frequency limit was observed for pitch ranking, though a lower limit did tend to provide better outcomes. These results suggest that the envelope modulation introduced by TLE can improve pitch perception for CI listeners.

Sound source localization patterns and bilateral cochlear implants: Age at onset of deafness effects.

The ability to determine a sound's location is critical in everyday life. However, sound source localization is severely compromised for patients with hearing loss who receive bilateral cochlear implants (BiCIs). Several patient factors relate to poorer performance in listeners with BiCIs, associated with auditory deprivation, experience, and age. Critically, characteristic errors are made by patients with BiCIs (e.g., medial responses at lateral target locations), and the relationship between patient factors and the type of errors made by patients has seldom been investigated across individuals. In the present study, several different types of analysis were used to understand localization errors and their relationship with patient-dependent factors (selected based on their robustness of prediction). Binaural hearing experience is required for developing accurate localization skills, auditory deprivation is associated with degradation of the auditory periphery, and aging leads to poorer temporal resolution. Therefore, it was hypothesized that earlier onsets of deafness would be associated with poorer localization acuity and longer periods without BiCI stimulation or older age would lead to greater amounts of variability in localization responses. A novel machine learning approach was introduced to characterize the types of errors made by listeners with BiCIs, making them simple to interpret and generalizable to everyday experience. Sound localization performance was measured in 48 listeners with BiCIs using pink noise trains presented in free-field. Our results suggest that older age at testing and earlier onset of deafness are associated with greater average error, particularly for sound sources near the center of the head, consistent with previous research. The machine learning analysis revealed that variability of localization responses tended to be greater for individuals with earlier compared to later onsets of deafness. These results suggest that early bilateral hearing is essential for best sound source localization outcomes in listeners with BiCIs.

The Impact of Synchronized Cochlear Implant Sampling and Stimulation on Free-Field Spatial Hearing Outcomes: Comparing the ciPDA Research Processor to Clinical Processors.

OBJECTIVES: Bilateral cochlear implant (BiCI) listeners use independent processors in each ear. This independence and lack of shared hardware prevents control of the timing of sampling and stimulation across ears, which precludes the development of bilaterally-coordinated signal processing strategies. As a result, these devices potentially reduce access to binaural cues and introduce disruptive artifacts. For example, measurements from two clinical processors demonstrate that independently-running processors introduce interaural incoherence. These issues are typically avoided in the laboratory by using research processors with bilaterally-synchronized hardware. However, these research processors do not typically run in real-time and are difficult to take out into the real-world due to their benchtop nature. Hence, the question of whether just applying hardware synchronization to reduce bilateral stimulation artifacts (and thereby potentially improve functional spatial hearing performance) has been difficult to answer. The CI personal digital assistant (ciPDA) research processor, which uses one clock to drive two processors, presented an opportunity to examine whether synchronization of hardware can have an impact on spatial hearing performance. DESIGN: Free-field sound localization and spatial release from masking (SRM) were assessed in 10 BiCI listeners using both their clinical processors and the synchronized ciPDA processor. For sound localization, localization accuracy was compared within-subject for the two processor types. For SRM, speech reception thresholds were compared for spatially separated and co-located configurations, and the amount of unmasking was compared for synchronized and unsynchronized hardware. There were no deliberate changes of the sound processing strategy on the ciPDA to restore or improve binaural cues. RESULTS: There was no significant difference in localization accuracy between unsynchronized and synchronized hardware (p = 0.62). Speech reception thresholds were higher with the ciPDA. In addition, although five of eight participants demonstrated improved SRM with synchronized hardware, there was no significant difference in the amount of unmasking due to spatial separation between synchronized and unsynchronized hardware (p = 0.21). CONCLUSIONS: Using processors with synchronized hardware did not yield an improvement in sound localization or SRM for all individuals, suggesting that mere synchronization of hardware is not sufficient for improving spatial hearing outcomes. Further work is needed to improve sound coding strategies to facilitate access to spatial hearing cues. This study provides a benchmark for spatial hearing performance with real-time, bilaterally-synchronized research processors.

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.

Auditory Attention and Spatial Unmasking in Children With Cochlear Implants.

The ability to attend to target speech in background noise is an important skill, particularly for children who spend many hours in noisy environments. Intelligibility improves as a result of spatial or binaural unmasking in the free-field for normal-hearing children; however, children who use bilateral cochlear implants (BiCIs) demonstrate little benefit in similar situations. It was hypothesized that poor auditory attention abilities might explain the lack of unmasking observed in children with BiCIs. Target and interferer speech stimuli were presented to either or both ears of BiCI participants via their clinical processors. Speech reception thresholds remained low when the target and interferer were in opposite ears, but they did not show binaural unmasking when the interferer was presented to both ears and the target only to one ear. These results demonstrate that, in the most extreme cases of stimulus separation, children with BiCIs can ignore an interferer and attend to target speech, but there is weak or absent binaural unmasking. It appears that children with BiCIs mostly experience poor encoding of binaural cues rather than deficits in ability to selectively attend to target speech.

Development of Binaural Sensitivity: Eye Gaze as a Measure of Real-time Processing.

Children localize sounds using binaural cues when navigating everyday auditory environments. While sensitivity to binaural cues reaches maturity by 8-10 years of age, large individual variability has been observed in the just-noticeable-difference (JND) thresholds for interaural time difference (ITD) among children in this age range. To understand the development of binaural sensitivity beyond JND thresholds, the "looking-while-listening" paradigm was adapted in this study to reveal the real-time decision-making behavior during ITD processing. Children ages 8-14 years with normal hearing (NH) and a group of young NH adults were tested. This novel paradigm combined eye gaze tracking with behavioral psychoacoustics to estimate ITD JNDs in a two-alternative forced-choice discrimination task. Results from simultaneous eye gaze recordings during ITD processing suggested that children had adult-like ITD JNDs, but they demonstrated immature decision-making strategies. While the time course of arriving at the initial fixation and final decision in providing a judgment of the ITD direction was similar, children exhibited more uncertainty than adults during decision-making. Specifically, children made more fixation changes, particularly when tested using small ITD magnitudes, between the target and non-target response options prior to finalizing a judgment. These findings suggest that, while children may exhibit adult-like sensitivity to ITDs, their eye gaze behavior reveals that the processing of this binaural cue is still developing through late childhood.

Evaluating the Impact of Age, Acoustic Exposure, and Electrical Stimulation on Binaural Sensitivity in Adult Bilateral Cochlear Implant Patients.

Deafness in both ears is highly disruptive to communication in everyday listening situations. Many individuals with profound deafness receive bilateral cochlear implants (CIs) to gain access to spatial cues used in localization and speech understanding in noise. However, the benefit of bilateral CIs, in particular sensitivity to interaural time and level differences (ITD and ILDs), varies among patients. We measured binaural sensitivity in 46 adult bilateral CI patients to explore the relationship between binaural sensitivity and three classes of patient-related factors: age, acoustic exposure, and electric hearing experience. Results show that ILD sensitivity increased with shorter years of acoustic exposure, younger age at testing, or an interaction between these factors, moderated by the duration of bilateral hearing impairment. ITD sensitivity was impacted by a moderating effect between years of bilateral hearing impairment and CI experience. When age at onset of deafness was treated as two categories (<18 vs. >18 years of age), there was no clear effect for ILD sensitivity, but some differences were observed for ITD sensitivity. Our findings imply that maximal binaural sensitivity is obtained by listeners with a shorter bilateral hearing impairment, a longer duration of CI experience, and potentially a younger age at testing. 198/200.

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.

Asymmetric temporal envelope encoding: Implications for within- and across-ear envelope comparison.

Separating sound sources in acoustic environments relies on making ongoing, highly accurate spectro-temporal comparisons. However, listeners with hearing impairment may have varying quality of temporal encoding within or across ears, which may limit the listeners' ability to make spectro-temporal comparisons between places-of-stimulation. In this study in normal hearing listeners, depth of amplitude modulation (AM) for sinusoidally amplitude modulated (SAM) tones was manipulated in an effort to reduce the coding of periodicity in the auditory nerve. The ability to judge differences in AM rates was studied for stimuli presented to different cochlear places-of-stimulation, within- or across-ears. It was hypothesized that if temporal encoding was poorer for one tone in a pair, then sensitivity to differences in AM rate of the pair would decrease. Results indicated that when the depth of AM was reduced from 50% to 20% for one SAM tone in a pair, sensitivity to differences in AM rate decreased. Sensitivity was greatest for AM rates near 90 Hz and depended upon the places-of-stimulation being compared. These results suggest that degraded temporal representations in the auditory nerve for one place-of-stimulation could lead to deficits comparing that temporal information with other places-of-stimulation.

Auditory motion tracking ability of adults with normal hearing and with bilateral cochlear implants.

Adults with bilateral cochlear implants (BiCIs) receive benefits in localizing stationary sounds when listening with two implants compared with one; however, sound localization ability is significantly poorer when compared to normal hearing (NH) listeners. Little is known about localizing sound sources in motion, which occurs in typical everyday listening situations. The authors considered the possibility that sound motion may improve sound localization in BiCI users by providing multiple places of information. Alternatively, the ability to compare multiple spatial locations may be compromised in BiCI users due to degradation of binaural cues, and thus result in poorer performance relative to NH adults. In this study, the authors assessed listeners' abilities to distinguish between sounds that appear to be moving vs stationary, and track the angular range and direction of moving sounds. Stimuli were bandpass-filtered (150-6000 Hz) noise bursts of different durations, panned over an array of loudspeakers. Overall, the results showed that BiCI users were poorer than NH adults in (i) distinguishing between a moving vs stationary sound, (ii) correctly identifying the direction of movement, and (iii) tracking the range of movement. These findings suggest that conventional cochlear implant processors are not able to fully provide the cues necessary for perceiving auditory motion correctly.

Temporal dynamics and uncertainty in binaural hearing revealed by anticipatory eye movements.

Accurate perception of binaural cues is essential for left-right sound localization. Much literature focuses on threshold measures of perceptual acuity and accuracy. This study focused on supra-threshold perception using an anticipatory eye movement (AEM) paradigm designed to capture subtle aspects of perception that might not emerge in behavioral-motor responses, such as the accumulation of certainty, and rapid revisions in decision-making. Participants heard interaural timing differences (ITDs) or interaural level differences in correlated or uncorrelated narrowband noises, respectively. A cartoon ball moved behind an occluder and then emerged from the left or right side, consistent with the binaural cue. Participants anticipated the correct answer (before it appeared) by looking where the ball would emerge. Results showed quicker and more steadfast gaze fixations for stimuli with larger cue magnitudes. More difficult stimuli elicited a wider distribution of saccade times and greater number of corrective saccades before final judgment, implying perceptual uncertainty or competition. Cue levels above threshold elicited some wrong-way saccades that were quickly corrected. Saccades to ITDs were earlier and more reliable for low-frequency noises. The AEM paradigm reveals the time course of uncertainty and changes in perceptual decision-making for supra-threshold binaural stimuli even when behavioral responses are consistently correct.

Restoration of spatial hearing in adult cochlear implant users with single-sided deafness.

In recent years, cochlear implants (CIs) have been provided in growing numbers to people with not only bilateral deafness but also to people with unilateral hearing loss, at times in order to alleviate tinnitus. This study presents audiological data from 15 adult participants (ages 48 ± 12 years) with single sided deafness. Results are presented from 9/15 adults, who received a CI (SSD-CI) in the deaf ear and were tested in Acoustic or Acoustic + CI hearing modes, and 6/15 adults who are planning to receive a CI, and were tested in the unilateral condition only. Testing included (1) audiometric measures of threshold, (2) speech understanding for CNC words and AzBIO sentences, (3) tinnitus handicap inventory, (4) sound localization with stationary sound sources, and (5) perceived auditory motion. Results showed that when listening to sentences in quiet, performance was excellent in the Acoustic and Acoustic + CI conditions. In noise, performance was similar between Acoustic and Acoustic + CI conditions in 4/6 participants tested, and slightly worse in the Acoustic + CI in 2/6 participants. In some cases, the CI provided reduced tinnitus handicap scores. When testing sound localization ability, the Acoustic + CI condition resulted in improved sound localization RMS error of 29.2° (SD: ±6.7°) compared to 56.6° (SD: ±16.5°) in the Acoustic-only condition. Preliminary results suggest that the perception of motion direction, whereby subjects are required to process and compare directional cues across multiple locations, is impaired when compared with that of normal hearing subjects.

Comparative effectiveness of beta-interferons and glatiramer acetate for relapsing-remitting multiple sclerosis: systematic review and network meta-analysis of trials including recommended dosages.

BACKGROUND: We systematically reviewed the comparative effectiveness of injectable beta-interferons (IFN-ß) and glatiramer acetate (GA) on annualised relapse rate (ARR), progression and discontinuation due to adverse events (AEs) in RRMS, using evidence from within the drugs' recommended dosages. METHODS: We updated prior comprehensive reviews, checked references of included studies, contacted experts in the field, and screened websites for relevant publications to locate randomised trials of IFN-ß and GA with recommended dosages in RRMS populations, compared against placebo or other recommended dosages. Abstracts were screened and assessed for inclusion in duplicate and independently. Studies were appraised using the Cochrane risk of bias tool. Rate ratios for ARR, hazard ratios for time to progression, and risk ratios for discontinuation due to AEs were synthesised in separate models using random effects network meta-analysis. RESULTS: We identified 24 studies reported in 42 publications. Most studies were at high risk of bias in at least one domain. All drugs had a beneficial effect on ARR as compared to placebo, but not compared to each other, and findings were robust to sensitivity analysis. We considered time to progression confirmed at 3 months and confirmed at 6 months in separate models; while both models suggested that the included drugs were effective, findings were not consistent between models. Discontinuation due to AEs did not appear to be different between drugs. CONCLUSIONS: Meta-analyses confirmed that IFN-ß and GA reduce ARR and generally delay progression as defined in these trials, though there was no clear 'winner' across outcomes. Findings are additionally tempered by the high risk of bias across studies, and the use of an impairment/mobility scale to measure disease progression. Future research should consider more relevant measures of disability and, given that most trials have been short-term, consider a longitudinal approach to comparative effectiveness. REVIEW REGISTRATION: PROSPERO CRD42016043278 .

Improving Speech Recognition in Bilateral Cochlear Implant Users by Listening With the Better Ear.

For patients with bilateral cochlear implants (BiCIs), understanding a target talker in a noisy situation can be difficult. Current efforts for improving speech-in-noise understanding have focused on improving signal-to-noise ratio by using multiple microphones or signal processing, with only moderate improvements in speech understanding performance. However, BiCI users typically report having a better ear for listening which can lead to an asymmetry in speech unmasking performance. This work proposes a novel listening strategy for improving speech-in-noise understanding by combining (a) a priori knowledge of a better ear and having a BiCI user selectively attend to a target talker in that ear with (b) signal processing that delivers the target talker to the better ear and the noisy background to the opposite ear. This strategy is different from traditional noise reduction strategies because it maintains situational awareness (background sounds are delivered to the ear contralateral to the better ear) while improving speech understanding. Speech recognition performance was evaluated with and without the better ear strategy in a speech-in-noise listening test using a virtual auditory space created from individualized head-related transfer functions. Listeners showed an average improvement of 4.4 dB signal-to-noise ratio in their speech reception threshold when using the better ear strategy with no listener showing a decrement in performance. This implies that the strategy has the potential to boost speech-in-noise recognition in BiCI users and may be useful in other hearing assistance devices such as hearing aids.

Mixed stimulation rates to improve sensitivity of interaural timing differences in bilateral cochlear implant listeners.

Normal hearing listeners extract small interaural time differences (ITDs) and interaural level differences (ILDs) to locate sounds and segregate targets from noise. Bilateral cochlear implant listeners show poor sensitivity to ITDs when using clinical processors. This is because common clinical stimulation approaches use high rates [∼1000 pulses per-second (pps)] for each electrode in order to provide good speech representation, but sensitivity to ITDs is best at low rates of stimulation (∼100-300 pps). Mixing rates of stimulation across the array is a potential solution. Here, ITD sensitivity for a number of mixed-rate configurations that were designed to preserve speech envelope cues using high-rate stimulation and spatial hearing using low rate stimulation was examined. Results showed that ITD sensitivity in mixed-rate configurations when only one low rate electrode was included generally yielded ITD thresholds comparable to a configuration with low rates only. Low rate stimulation at basal or middle regions on the electrode array yielded the best sensitivity to ITDs. This work provides critical evidence that supports the use of mixed-rate strategies for improving ITD sensitivity in bilateral cochlear implant users.