Interaural time difference sensitivity under binaural cochlear implant stimulation persists at high pulse rates up to 900 pps

Abstract

Spatial hearing remains one of the major challenges for bilateral cochlear implant (biCI) users, and early deaf patients in particular are often completely insensitive to interaural time differences (ITDs) delivered through biCIs. One popular hypothesis is that this may be due to a lack of early binaural experience. However, we have recently shown that neonatally deafened rats fitted with biCIs in adulthood quickly learn to discriminate ITDs as well as their normal hearing litter mates, and perform an order of magnitude better than human biCI users. Our unique behaving biCI rat model allows us to investigate other possible limiting factors of prosthetic binaural hearing, such as the effect of stimulus pulse rate and envelope shape. Previous work has indicated that ITD sensitivity may decline substantially at the high pulse rates often used in clinical practice. We therefore measured behavioral ITD thresholds in neonatally deafened, adult implanted biCI rats to pulse trains of 50, 300, 900 and 1800 pulses per second (pps), with either rectangular or Hanning window envelopes. Our rats exhibited very high sensitivity to ITDs at pulse rates up to 900 pps for both envelope shapes, similar to those in common clinical use. However, ITD sensitivity declined to near zero at 1800 pps, for both Hanning and rectangular windowed pulse trains. Current clinical cochlear implant (CI) processors are often set to pulse rates ≥ 900 pps, but ITD sensitivity in human CI listeners has been reported to decline sharply above ~ 300 pps. Our results suggest that the relatively poor ITD sensitivity seen at > 300 pps in human CI users may not reflect the hard upper limit of biCI ITD performance in the mammalian auditory pathway. Perhaps with training or better CI strategies good binaural hearing may be achievable at pulse rates high enough to allow good sampling of speech envelopes while delivering usable ITDs.

Does age protect against loss of tonotopy after acute deafness in adulthood?

“…ABRs were recorded by averaging scalp potentials measured with subcutaneous needle electrodes between mastoids and the vertex of the rat’s head over 300 click presentations. While normal hearing rats typically exhibited click ABR thresholds near 0 dB SL, deafened rats showed increased hearing thresholds ≥95 dB SL to broadband click stimuli as well as pure tones (Rosskothen-Kuhl et al, 2021; Buck et al, 2023). Figures 1 A and B show ABRs of an adult-deafened rat before and after deafening by EA and KM injection.…”

Evaluating Cochlear Implant Stimulation Strategies Through Wide-field Calcium Imaging of the Auditory Cortex

“…However, CIS solves the issue of pulse collision avoidance in neighboring channels by forcing all channels in the array to conform to the same temporal grid of regular, fixed pulse intervals, sacrificing the possibility to encode temporal fine structure features of incoming sounds with submilisecond accuracy. Recent developments in CI research strongly suggest that leveraging the full potential of precisely timed pulses could result in clinically relevant improvements in binaural hearing (Rosskothen-Kuhl et al 2021;Buck A. et al 2023) and periodicity pitch perception (Goldsworthy and Shannon 2014;Goldsworthy and Bissmeyer 2023) for CI users, but to deliver such advantages in a multichannel implant capable of effective speech encoding may require lifting the restriction that each channel must operate at the same, fixed, underlying pulse rate regardless of the nature of the input signal, even if this makes it much harder to avoid occasional simultaneous pulses in more than one channel.…”

“…However, animal experiments too have their difficulties. Behavioral testing of experimental animals with CIs is technically difficult and time consuming, so although animal behavior studies have led to perhaps surprising and interesting insights (Miller et al 1995;Rosskothen-Kuhl et al 2021;Buck A. et al 2023), the use of this approach remains relatively rare. Electrophysiological recordings from neurons of the auditory pathway of experimental animals are somewhat easier to achieve and more commonly performed, but despite recent technical advances, this approach remains plagued by difficulties in removing the electrical stimulation artifacts generated by the CI stimuli, particularly when stimulation parameters are to be compared in the context of rich, multichannel stimulation paradigms which generate dense and complex electrical artifacts.…”