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Hearing and Donuts (Brain and Bagels) Seminar
February 11, 2022 @ 8:30 am - 9:30 am
Presenter: Nicole Roßkothen-Kuhl , Ph.D., Department of Oto-Rhino-Laryngology, Section of Experimental and Clinical Otology, Neurobiological Research Laboratory, Medical Center – University of Freiburg, Germany
Topic: Enabling Good Spatial Hearing After Early Hearing Loss: The Role of Precise Interaural Timing
The central question of this talk is how we can improve spatial hearing in early deafened patients. In particular, we seek to understand why such patients are often insensitive to ITDs even if they are fitted with bilateral cochlear implants (biCIs) at a young age. It is widely suspected that this issue arises at least in part from lack of sensory input during early development (the “critical period hypothesis”), we suspect the reason lies in the missing synchronization of today´s speech processors. In fact, current CI processors are unable to synchronize the timing of their stimulus pulses binaurally, resulting in inconsistent and therefore uninformative delivery of interaural time difference (ITD) cues to the electrically stimulated auditory system. This might prevent biCI recipients from learning to use ITDs normally even if there was no strong critical period causing permanent deficits by the time the devices are fitted.
To investigate this hypothesis, early deafened, adult rats were implanted with biCIs and tested in a new behavioral setup. Despite their early auditory deprivation, they were able to lateralize ITDs for pulse rates up to 900 pps and achieved thresholds of around 50 μs, which correspond to those of normally hearing litter mates. Even for clinical rates of 900 pps, good ITD sensitivity was observed for stimuli with rectangular envelopes or even more “speech-like” waveforms. Good ITD sensitivity can therefore be developed even in the absence of early sensory input if the biCIs provide useful ITD cues from right after implantation. In electrophysiological experiments we could furthermore show that IC neurons of the hearing inexperienced system are highly ITD sensitive from immediately after implantation. Based on this result, we propose that, at least in rats, there is no “strong” critical period for the development of ITD sensitivity, and we now investigate to what extent varying, uninformative presentation of ITDs interferes with the development of ITD sensitivity in hearing inexperienced CI rats. Our behavioral studies in neonatally deafened, adult CI-supplied rats showed that ITDs were only used for sound lateralization if the animals were presented with informative ITDs from the beginning of stimulation. In contrast, when animals were exposed to stimulation with varying, uninformative ITDs, mimicking the stimulation that patients fitted with asynchronous CIs would experience, the animals did not learn to use ITD cues for lateralization. Overall, our results suggest that ITD sensitivity appears to be little affected by the absence of early auditory experience in very early life but may be perturbed by uninformative ITD cues provided by unsynchronised CI stimulation after binaural CIs are fitted. Under synchronized CI stimulation with useful ITDs, rats achieved good sensitivities even for clinical pulse rates up to 900 pps. Consequently, ITDs do not necessarily require slower rates with appropriate binaural stimulation. We hope that these findings will stimulate the development of new biCI speech processing algorithms that enable better temporal-spatial perception.