Onset Weighting of Temporal Spatial Cues with Cochlear Implant Stimulation in Early Onset Deafness

Cochlear implants (CIs) are undeniably useful, but the perceptual performance they permit in complex, real-life environments still falls short. One particular shortcoming is the inability to provide CI users with adequate temporal spatial cues, interaural time differences (ITDs). Early deaf users of current CIs are entirely insensitive to ITD cues over the physiological range. However, we have previously shown that neonatally deafened (ND) rats are capable of discriminating ITDs as small as 50 μs, no worse than normal hearing (NH) rats (RosskothenKuhl et al., 2019). Better ITD sensitivity may therefore be achievable with better devices, and rats may be a great model to develop improved technology. We therefore determined whether rats show similar onset weighting for ITDs as humans (Brown and Stecker 2010), and whether that onset weighting is similar in NH and ND CI animals.
ND-CI rats were prepared by neonatal injection of kanamycin, followed by bilateral implantation as young adults. Age matched NH litter mates were also tested. Animals learned a two-alternative forced choice task to lateralize pulse trains consisting of eight binaural pulses (for ND rats: biphasic electric stimuli delivered via CIs; for NH rats: acoustic clicks delivered over tube phones). ITD values for individual pulses varied independently and uniformly across the rat’s physiological range (+/- 130 µs). Pulse rates were 50, 300 or 900 Hz. Two types of trials were presented for each session: Honesty trials comprised either all left- or all right-ear leading pulses, while for probe trials pulse ITDs were unconstrained. Temporal Weighting Functions (TWFs) were calculated using multiple regression analysis to determine the perceptual weight of each pulse in the train in shaping the behavioral response to probe trials.
NH animals showed TWFs across all frequencies that were comparable with the TWFs of human listeners. At lower pulse rates (≤300 Hz), CI rats also showed TWFs with strong onset responses. However, this up-weighting was not as pronounced as in NH listeners, and was lost for higher stimulation rates (900 Hz).
Rat TWFs are fundamentally very similar to those found in humans, illustrating the suitability of rats as a model for human binaural hearing. The pulse rate dependence of onset ITD processing we observed in CI rats may also help explain the poor ITD perception of early deafened CI users, given that current clinical CI processors are running at ≥900 pps.