Bilateral cochlear implantation provides benefits over unilateral implantation for sound localization and speech reception in noise, yet binaural performance of bilateral cochlear implant (CI) users is still well below normal, especially in task involving interaural time differences (ITD), and especially in pre-lingually deaf subjects. The present collaboration between neurophysiologists and psychophysicists aims at gaining a basic understanding of the stimulus, neural, perceptual and developmental factors that influence ITD sensitivity with bilateral CIs in order to devise innovative stimulation and rehabilitation strateges that improve binaural performance. Psychophysical experiments in bilaterally-implanted human subjects will be conducted in parallel with single-unit recordings from the auditory midbrain in a novel, awake rabbit model of bilateral CIs that allows neural recordings over several months.
Specific Aim 1 is to investigate whether the abnormalities in neural ITD sensitivity previously observed in neonatally deaf animals can be reversed by appropriate chronic electric stimulation through bilateral CIs. We will compare neural ITD sensitivity in neonatally-deafened rabbits that receive various types of chronic electric stimulation from an early age with the sensitivity in dea rabbits that receive minimal stimulation. Some of the rabbits will receive stimulation with an experimental strategy that was specifically designed to deliver ITD information effectively with bilateral CIs, and some rabbits will only receive unilateral stimulation as occurs during the perio of unilateral deafness between the two cochlear implantations when these are performed in separate operations. The combination of neonatal deafening, controlled electric stimulation through CIs and longitudinal neural recording in awake animals offers a powerful new approach for studying the developmental plasticity of the binaural system.
Specific Aim 2 is to test a new approach for overcoming the severe degradation in ITD sensitivity observed at the high carrier pulse rates used in today's speech processors by introducing additional carrier pulses so as to create short interpulse intervals (IPIs). This approach is motivated by our finding that neurons that short IPIs in irregular pulse trains can restore firing and ITD sensitivity in neurons that ony respond to the onset of high-rate periodic pulse trains. Neural and perceptual ITD sensitivities will be compared with and without short IPIs for both simple pulse trains and modulated pulse trains resembling the current waveforms produced by CI processors with speech syllable inputs. Together, these aims will increase our basic understanding of how stimulation parameters and early auditory experience shape performance with bilateral CIs. They will likely lead to new processing strategies and rehabilitation procedures for bilateral CIs that work better in everyday acoustic environments with spatially separated sound sources, and that are adapted to the history of auditory experience and deprivation of individual deaf patients.
Many profoundly deaf people wearing cochlear implants (CI) still face challenges understanding conversations in noise, especially if they lost hearing early i life. Through parallel perceptual studies in CI users and neurophysiological studies in animal models of CI, we will devise and test new approaches for delivering binaural information effectively, and test whether chronic CI stimulation can help the brain to develop the circuitry fo binaural processing in animals with early-onset deafness. Results from this study will stimulate development of new sound processors and rehabilitation strategies that improve sound localization and speech reception in noise with CIs and will help to define what kind of stimulation is required for the development of binaural hearing in children with severe to profound hearing loss.
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