Sensitivity of inferior colliculus to interaural time and level differences in neonatally deafened rats.
Shiyi FANG, Fei Peng, Bruno Castellaro, Muhammad Zeeshan, Nicole Rosskothen-Kuhl, Jan Schnupp
Abstract:
Binaural cues, such as interaural time difference (ITD), play a crucial role in localizing sound sources in the auditory system. However, contemporary cochlear implant (CI) processors use a coding strategy that only conveys the ITD information contained in the envelope of the sound (envelope ITD) to the cochlear implant (CI) user. As a result, the ITD information contained in the temporal fine structure of the sound (pulse-timing ITD) is not transmitted, which may contribute to the poor spatial hearing perception of CI users.
To investigate the sensitivity of CI-implanted rats to envelope and pulse timing ITD, we designed a stimulus comprising a 900pps pulse train modulated by a 20 Hz sine envelope in which pulse timing ITD (PT_ITD) and envelope ITD (ENV_ITD) could vary independently from the values {-0.1, 0, 0.1 ms}. We recorded neural activity from the inferior colliculus (IC) of anesthetized neonatal deafened rats using a multi-channel silicon probe. For each multi-unit, we first applied a Wiener filter method to remove electrical artifacts, and then computed the analog multi-unit activity (AMUA) over the onset response window (0-50 ms) and the baseline window (150-200 ms). Any multi-unit with a peak amplitude in the onset window AMUA larger than the average plus 5 times the standard deviation of the baseline window AMUA was identified as responsive to the CI stimulation. For every responsive multi-unit, the proportion of variance explained by PT_ITD and ENV_ITD was computed to reveal the effect of envelope and pulse timing ITD on AMUA intensity.
Our study recorded a total of 332 responsive multi-units, with 83% of them being sensitive to PT_ITD, while only one multi-unit was found to be sensitive to ENV_ITD. This indicates that CI-implanted rats exhibit far greater sensitivity to pulse timing ITD than envelope ITD. These findings suggest that the current CI stimulus strategy is not providing effective ITD information that CI users are sensitive to, and that CI users have the potential for better sound localization ability.