Modeling sound localization with cochlear implants

This chapter describes a model framework for evaluating the precision of as to which interaural time differences, ITD, are represented in the left- and right-ear auditory-nerve responses. This approach is very versatile, as it allows not only for the evaluation of spiking neuronal responses from models of intact inner ears but also of responses of the deaf ears of cochlear implantees. The model framework delivers quantitative data and, therefore, enables comparisons between different cochlear-implant coding strategies. As the model of electric excitation of the auditory nerve also includes effects such as channel crosstalk, neuronal adaptation and mismatch of electrode positions between left and right ears, its predictive power is much higher than an analysis of the electrical impulses delivered to the electrodes. Evaluation of a novel fine-structure-coding strategy as used by a major implant manufacturer, revealed that, in a best case scenario, sophisticated strategies should be able to provide ITD cues with sufficient precision for sound localization. However, whether these cues can actually be exploited by cochlear implant users has yet to be determined by listening tests. Nevertheless, the model framework introduced here is a valuable tool for the development and pre-evaluation of bilateral cochlear implant coding strategies.