AUDITORY EXPERIENCE WITH BILATERAL COCHLEAR IMPLANTS SHAPES INTERAURAL TIME DIFFERENCE TUNING

Sound localization relies mainly on two binaural cues: interaural level differences (ILDs) and interaural time differences (ITDs). Recent work of our group has shown that in normal hearing rats, neurons of the inferior colliculus (IC), an auditory midbrain region, exhibit > 90 % contralateral tuning for ITDs. Meanwhile, in neonatally deafened (ND) rats, contralateral dominance only reaches ~50 %. Here, we investigate whether training ND rats to lateralize pulse timing ITDs delivered via bilateral cochlear implants (biCI) alters neural tuning patterns in the auditory midbrain.

Recordings were performed in five anesthetized ND biCI rats with 2-6 months of electrical hearing experience. We presented ITDs of ±{120, 40, 0} µs randomly interleaved with ILDs of ±{4, 1, 0} dB using biphasic biCI pulses presented at 1 Hz. Multi-unit activity was recorded from the IC using 32-channel silicon probes, electrical artifacts were removed, and analog multi-unit activity (AMUA) was computed. ITD and ILD tuning curves for each unit were normalized (z-score) and clustered via principal component analysis.

The analysis showed that, in these rats with extensive experience in discriminating ITDs delivered in the timing of biCI pulses, > 80 % of IC units were contralaterally tuned. These are substantially more than in the auditory midbrain of inexperienced ND rats, and almost as many as in the acoustically hearing cohort. Thus, while auditory experience does not seem to be required for ITD tuning to emerge in the IC, precise biCI stimulation seems to push IC tuning curves towards a contralaterally tuned shape.