Bilateral cochlear implants (CIs) have provided some success in improving spatial hearing abilities to patients, but with large variability in performance. One reason for the variability is that there may be a mismatch in the place-of-stimulation arising from electrode arrays being inserted at different depths in each cochlea. Goupell et al. [(2013b). J. Acoust. Soc. Am. 133(4), 2272–2287] showed that increasing interaural mismatch led to non-fused auditory images and poor lateralization of interaural time differences in normal hearing subjects listening to a vocoder. However, a greater bandwidth of activation helped mitigate these effects. In the present study, the same experiments were conducted in post-lingually deafened bilateral CI users with deliberate and controlled interaural mismatch of single electrode pairs. Results show that lateralization was still possible with up to 3 mm of interaural mismatch, even when off-center, or multiple, auditory images were perceived. However, mismatched inputs are not ideal since it leads to a distorted auditory spatial map. Comparison of CI and normal hearing listeners showed that the CI data were best modeled by a vocoder using Gaussian-pulsed tones with 1.5 mm bandwidth. These results suggest that interaural matching of electrodes is important for binaural cues to be maximally effective.

This report highlights research projects relevant to binaural and spatial hearing in adults and children. In the past decade we have made progress in understanding the impact of bilateral cochlear implants (BiCIs) on performance in adults and children. However, BiCI users typically do not perform as well as normal hearing (NH) listeners. In this article we describe the benefits from BiCIs compared with a single cochlear implant (CI), focusing on measures of spatial hearing and speech understanding in noise. We highlight the fact that in BiCI listening the devices in the two ears are not coordinated; thus binaural spatial cues that are available to NH listeners are not available to BiCI users. Through the use of research processors that carefully control the stimulus delivered to each electrode in each ear, we are able to preserve binaural cues and deliver them with fidelity to BiCI users. Results from those studies are discussed as well, with a focus on the effect of age at onset of deafness and plasticity of binaural sensitivity. Our work with children has expanded both in number of subjects tested and age range included. We have now tested dozens of children ranging in age from 2 to 14 yr. Our findings suggest that spatial hearing abilities emerge with bilateral experience. While we originally focused on studying performance in free field, where real world listening experiments are conducted, more recently we have begun to conduct studies under carefully controlled binaural stimulation conditions with children as well. We have also studied language acquisition and speech perception and production in young CI users. Finally, a running theme of this research program is the systematic investigation of the numerous factors that contribute to spatial and binaural hearing in BiCI users. By using CI simulations (with vocoders) and studying NH listeners under degraded listening conditions, we are able to tease apart limitations due to the hardware/software of the CI systems from limitations due to neural pathology.

Although bilateral cochlear implantation has the potential to improve sound localization and speech understanding in noise, obstacles exist in presenting maximally useful binaural information to bilateral cochlear-implant (CI) users. One obstacle is that electrode arrays may differ in cochlear position by several millimeters, thereby stimulating different neural populations. Effects of interaural frequency mismatch on binaural processing were studied in normal-hearing (NH) listeners using band-limited pulse trains, thereby avoiding confounding factors that may occur in CI users. In experiment 1, binaural image fusion was measured to capture perceptual number, location, and compactness. Subjects heard a single, compact image on 73% of the trials. In experiment 2, intracranial image location was measured for different interaural time differences (ITDs) and interaural level differences (ILDs). For larger mismatch, locations perceptually shifted towards the ear with the higher carrier frequency. In experiment 3, ITD and ILD just-noticeable differences (JNDs) were measured. JNDs increased with decreasing bandwidth and increasing mismatch, but were always measurable up to 3 mm of mismatch. If binaural-hearing mechanisms are similar between NH and CI subjects, these results may explain reduced sensitivity of ITDs and ILDs in CI users. Large mismatches may lead to distorted spatial maps and reduced binaural image fusion.

Objectives: In bilateral cochlear implant users, electrodes mapped to the same frequency range in each ear may stimulate different places in each cochlea due to an insertion depth difference of electrode arrays. This interaural place of stimulation mismatch can lead to problems with auditory image fusion and sensitivity to binaural cues, which may explain the large localization errors seen in many patients. Previous work has shown that interaural place of stimulation mismatch can lead to off-centered auditory images being perceived even though interaural time and level differences (ITD and ILD, respectively) were zero. Large interaural mismatches reduced the ability to use ITDs for auditory image lateralization. In contrast, lateralization with ILDs was still possible but the mapping of ILDs to spatial locations was distorted. This study extends the previous work by systematically investigating the effect of interaural place of stimulation mismatch on ITD and ILD sensitivity directly and examining whether “centering” methods can be used to mitigate some of the negative effects of interaural place of stimulation mismatch. Design: Interaural place of stimulation mismatch was deliberately introduced for this study. Interaural pitch-matching techniques were used to identify a pitch-matched pair of electrodes across the ears approximately at the center of the array. Mismatched pairs were then created by maintaining one of the pitch-matched electrodes constant, and systematically varying the contralateral electrode by two, four, or eight electrode positions (corresponding to approximately 1.5, 3, and 6 mm of interaural place of excitation differences). The stimuli were 300 msec, constant amplitude pulse trains presented at 100 pulses per second. ITD and ILD just noticeable differences (JNDs) were measured using a method of constant stimuli with a two-interval, two-alternative forced choice task. The results were fit with a psychometric function to obtain the JNDs. In experiment I, ITD and ILD JNDs were measured as a function of the simulated place of stimulation mismatch. In experiment II, the auditory image of mismatched pair was centered by adjusting the stimulation level according to a lateralization task. ITD and ILD JNDs were then remeasured and compared with the results of experiment I. Results: ITD and ILD JNDs were best (lowest thresholds) for pairs of electrodes at or near the pitch-matched pair. Thresholds increased systematically with increasing amounts of interaural mismatch. Deliberate and careful centering of auditory images did not significantly improve ITD JNDs but did improve ILD JNDs at very large amounts of simulated mismatch. Conclusions: Interaural place of stimulation mismatch decreases sensitivity to binaural cues that are important for accurate sound localization. However, deliberate and careful centering of auditory images does not seem to significantly counteract the effects of mismatch. Hence, to obtain maximal sound localization benefits of bilateral implantation, clinical and surgical techniques are needed that take into account differences in electrode array insertion depths across the ears.