Interaural time difference sensitivity at higher pulse rates in an early deafened auditory system

Sound localization is one of the major challenges for bilateral cochlear implant (CI) users. To date they have a limited perception of binaural cues, especially of interaural time difference (ITD) sensitivity. While studies on human implantees conclude that the issue lies in the lack of early sensory input, our recent study on neonatally deafened, CI-implanted rats has shown that ITD sensitivity can be developed independent from early sensory input if the bilateral CIs are synchronized. Here, we investigate to what extent the pulse rate influences the ITD performance of early deaf CI users.
We used the adult Wistar rat as a new model to investigate binaural hearing with electrical intracochlear stimulation. Deafness was induced neonatally by kanamycin and verified by measuring auditory brainstem responses. In young adulthood, CI-electrodes were inserted into the middle turn of both cochleae. Sensitivity for ITDs at different pulse rates (50, 300, 900 Hz) was studied by training CI-implanted rats on a two-alternative forced choice stimulus lateralization task. During training, CI-rats were connected to an external stimulator and trained on ITD discrimination by using binaural, biphasic stimuli.
In a matter of a few weeks of training, all neonatally deafened CI-rats showed microsecond ITD sensitivity when provided with precise ITD cues right from the start of first stimulation. Within the rats’ physiological range of +/- 160 μs, surprisingly good ITD discrimination was found independent from the pulse rate although the performance was slightly better for lower rates. This ITD performance compares with that achieved by normal hearing rats.
Using synchronized CI stimulation immediately after bilateral CI implantation allows an inexperienced, adult auditory system to develop ITD discrimination within the normal range. Thereby, electric stimulation rates of up to 900 Hz do not prevent the development of ITD discrimination. These findings suggest that, given synchronized CI stimulation from the time of implantation, even early deafened CI patients with clinical stimulation rates should be able to use ITDs for sound localization.