Neural processing of interaural time differences between acoustic and electrical stimulation in an animal model for bimodal stimulation

Unilateral cochlear implants (CIs) have emerged as a treatment option for subjects with single-sided deafness (’SSD-CI’ users). Nevertheless, directional hearing in SSD-CI users is typically poorer than that in subjects with normal hearing. This finding indicates suboptimal support of binaural integration by contemporary unilateral CIs. To characterize the limitations of binaural-bimodal processing, we compared the neural encoding of interaural time differences (ITDs) between unimodal (bilateral acoustic) and bimodal (electrical/acoustic) stimulation in gerbil auditory midbrain. Normal-hearing gerbils were implanted bilaterally with round-window electrodes to preserve hearing in the implanted ears. The incidence of ITD-sensitive neurons was similar in response to unimodal and bimodal stimulation. However, responses to bimodal stimulation showed higher ITD discrimination thresholds. Intraaural matching of neural response strengths and latency differences between acoustic and electric stimulation led to a shift of bimodal rate ITD functions into the physiological range. In modern human CIs, peripheral latency differences between acoustic and electrical stimulation are compensated by applying a single delay. At the neuronal population level, an optimal latency match was estimated by a Fisher information (FI) analysis. Unimodal acoustic and bimodal stimulations showed similar FI profiles when a delay of about 0.5 ms was added to the electrical stimulation. Our results suggest that balancing the relative latencies and interaural levels between both modes of stimulation can improve directional hearing in SSD-CI users.

DFG VO 640/2-2

Publication History

Article published online:
12 May 2023