Sensitivity of Inferior Colliculus to Interaural Time and Level Differences in Neonatally Deafened Rats
Muhammad Zeeshan, Fei Peng, Bruno Castellaro, Shiyi Fang, Nicole Rosskothen-Kuhl, Jan W.H. Schnupp
Cochlear implants (CIs) are an advanced treatment for patients with profound sensorineural hearing loss. Over the last few decades, bilateral CIs are increasingly used to restore binaural hearing. However, human bilateral CI users usually exhibit poor sensitivity to interaural time differences (ITDs) in particular. The reasons for this poor ITD sensitivity are still only poorly understood. We suspect that the manner in which ITDs interact with Interaural Level Difference (ILD) cues may play a role, given that conventional CI stimulation provides CI users with impoverished ITD cues, so that they become increasingly reliant on ILD and insensitive to ITD. To investigate this possibility, we first need to document ITD and ILD sensitivity and their interactions in the naive auditory pathway.
We deafened the rats neonatally with kanamycin injections intraperitoneally (from day 9 to 20) and verified by measuring the auditory brainstem responses. Biphasic pulse train stimuli at rates of 1, 100, and 900 pps with different ITD (±0, ±0.04, ±0.08, ± 0.12 ms) and ILD (±0, ±1, ±4 dB) combinations were delivered through bilaterally implanted CIs. Inferior colliculus (IC) multiunit responses were recorded extracellularly, and analyzed for statistically significant ITD or ILD sensitivity using Kruskal-Wallis tests. At pulse rates of 1, 100 and 900 Hz, 85.6%, 99.7% and 97.2% respectively of multiunits were found to be ITD sensitive, 88.5%, 96.4% and 88% were ILD sensitive, and 76.8%, 96.1%, 85.5% were sensitive to both. We conclude that sensitivity to both ITDs and ILDs was very widespread in the IC of our naive, neonatally deafened rats, that it can be observed at a wide range of pulse rates, and that multiunits sensitive to one type of cue are invariably sensitive to the other type of cue too.