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Hearing and Donuts (Brain and Bagels) Seminar
October 22, 2021 @ 8:30 am - 9:30 am
Presenter: Alexa Buck, MD, PhD (Faculty of Neuroscience) City University of Hong Kong
Topic: Binaural Cochlear Implants Must Encode Interaural Time Difference Cues through Pulse Timing, Not Envelopes
Interaural time difference (ITD) discrimination with binaural cochlear implants (biCIs) is notoriously difficult. Particularly prelingually deaf patients exhibit ITD thresholds too large to provide any useful information, which led to the hypothesis that a lack of binaural experience during an early critical period may be responsible for the poor biCI ITD sensitivity. However, most late deaf biCI patients also have quite poor ITD thresholds, and recent work from our research groups has demonstrated that neonatally deafened (ND), adult implanted rats can be trained to lateralize ITDs with remarkably low thresholds (~50 μs) comparable to their normal hearing peers, suggesting that technical issues are probably the main limitation for biCI ITD perception.
An obvious problem is that practically all clinical CI processors employ pulsatile stimulation in which pulse timing is not synchronized between ears, so that biCI users can only experience ITDs in pulse-train envelopes. However, how useful these cues are to biCI users is controversial. Here we tested whether ND adult bilaterally implanted rats would preferentially use ITDs carried on the individual pulses, when synchronized bilaterally, or ITDs carried on the amplitude envelopes, as has previously been assumed. We found, without exception, that pulse timing ITDs completely dominated the animals’ lateralization judgments emphasizing the need for manufacturers to change the delivery of these cues.