Effect of cochlear implant processing, interaural place mismatch, and spread of excitation on interaural coherence of speech signals and its implications on binaural fusion.

OBJECTIVE: Binaural fusion - the ability to perceptually group signals presented to the two ears into a single auditory percept - can be challenging for some cochlear implant (CI) users. Binaural fusion relies, in part, on interaural coherence, or the statistical similarity of the signal across the two ears. However, aspects of how cochlear implants process sound can reduce interaural coherence, as can interaural place mismatch, in which electrodes encoding the same frequency region stimulate different cochlear regions across ears. This study examines the effects of these factors on interaural coherence and, in turn, on binaural fusion. METHODS: In Experiment 1, electrodograms were recorded in response to speech presented in a low-to-moderately reverberant environment, simulating interaural place mismatch and spread of excitation. To determine the perceptual implications of the resulting changes in interaural coherence, thirteen bilateral CI users completed a binaural fusion task in Experiment 2, in which the interaural coherence of the signal was directly manipulated. RESULTS: Findings from Experiment 1 showed that CI processing significantly increased interaural coherence. Interaural place mismatch decreased interaural coherence, though this decrease was mitigated by simulated spread of excitation. Consistent with prior studies, behavioral results from Experiment 2 showed a gradual decrease in binaural fusion as interaural coherence decreased. Critically, the coherence values associated with unfused percepts were well below those observed across the electrodogram manipulations in Experiment 1. CONCLUSIONS: These findings suggest that most cochlear implant users are likely to retain the ability to binaurally fuse sounds in low-to-moderate reverberant environments, even with substantial interaural place mismatch.