Binaural-bimodal stimulation degrades neural coding of interaural time differences

Unilateral cochlear implants (CIs) allow the functional restoration of binaural hearing in subjects with single-sided deafness ('SSD-CI' users). Nevertheless, directional hearing in SSD-CI users is typically poorer than that in normal hearing subjects. This finding indicates suboptimal support of binaural integration by contemporary unilateral CIs. To identify the limitations of binaural-bimodal processing, we compared neural ITD coding between bimodal (electrical/acoustic) stimulation and two kinds of unimodal stimulation, namely bilateral acoustic and bilateral electric stimulation, in the inferior colliculus of the Mongolian gerbil. Normal-hearing gerbils were bilaterally implanted with round-window electrodes to preserve hearing in the implanted ears. Neural responses to bimodal stimulation showed higher ITD discrimination thresholds and lower ITD coding efficacy (Fisher information) than those to unimodal acoustic and unimodal electric stimulation. Temporal offsets between tuning functions in response to bimodal and unimodal ITDs were matched to elucidate the effects of different delays in the encoding of acoustic and electric stimuli on bimodal coding efficacy. Such adjustments in temporal offsets shifted averaged bimodal ITD tuning towards the physiological range of ITDs. This shift was associated with an increase in Fisher information. However, maximum Fisher information in response to bimodal stimulation remained lower than that to unimodal acoustic or unimodal electric stimulation. Our results suggest that balancing the temporal offsets between both modes of stimulation can improve directional hearing in SSD-CI users. Nevertheless, the efficacy of bimodal ITD coding remained poor when compared to unimodal acoustic or unimodal electric stimulation.

Funding information Supported by DFG VO 640/2-2.

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Artikel online veröffentlicht:
19. April 2024