Lateralization of Sounds based on Interaural Time Differences in Cochlear Implant Listeners

Dissertation (pdf 1000KB)

Interaural time differences (ITD) occur when a binaural signal is delayed at one ear. They arise when a sound is presented outside of the median plane of a subject. In modulated sounds like speech, ITD is present in both the rapidly varying fine structure and the slowly varying envelope. There is evidence that ITD in the fine structure of a sound is most important for sound localization and for understanding speech in noise. Cochlear implants (CI), neural prosthetic devices that restore hearing in the profoundly deaf, are increasingly implanted to both ears to provide implantees with the advantages of binaural hearing. Bilateral CI listeners currently use devices with stimulation strategies which encode ITD in the temporal envelope but do not transmit ITD in the fine structure. The reason for this is that it is not clear if CI listeners are sensitive to the fine structure ITD for stimulation pulse rates which are required for a satisfactory perception of speech.

This thesis investigates the fine structure ITD sensitivity in CI listeners in three different studies. The results show that CI listeners are sensitive to the fine structure ITD up to a pulse rate of 800 pps. In a fourth study, a new method is investigated which improves the lateralization based on the fine structure ITD for pulse rate up to 1600 pps. The method introduces a binaurally-synchronized jitter to the regular pulse train and thus, reduces the binaural adaptation effect. With this method the CI listeners were able to lateralize sounds for high pulse rates as commonly used in current stimulation strategies.