Bilateral cochlear implants:Physiology and psychophysics

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Binaural hearing is essential for accurate sound localization and for hearing out sounds of interest among competing sound sources, and these binaural advantages are an important motivating factor for the increasing number of bilateral cochlear implants. In this application, investigators from three laboratories at the Massachusetts Eye and Ear Infirmary, MIT, and Boston University join together to propose closely- integrated psychophysical, neural and modeling studies that seek to identify the best stimulus configurations for effectively encoding binaural information in bilateral implants. The primary focus is on interaural time differences (ITD) because ITD is a dominant cue for sound localization, and because binaural advantages in detection depend on ITD. The psychophysical experiments will be conducted in human patients implanted bilaterally with Clarion devices. The neural experiments will be conducted in deafened, anesthetized cats implanted bilaterally with intracochlear electrode arrays. Single and multi-unit recordings will be made from the inferior colliculus (IC), for which a great deal of information is available on binaural properties of neurons. In order to explicitly and quantitatively test our understanding of the empirical results, we will also use models of binaural processing to predict both IC responses and psychophysical abilities from ch_¿cr,inticm_ nf anditorv nerve activity in responm to electric stimulation. The s_cific aims are to (1) letermine the relative effectiveness of temporal envelope and fine structure for ITD coding in electric hearing, (2) determine how 1TD sensitivity depends on interaural disparities in cochlear locations of the stimulating electrodes, (3) determine whether there is a binaural advantage for signal detection in noise with bilateral stimulation. An important goal of these experiments will be to test a novel processing strategy that seeks to improve the representation of the stimulus waveform in temporal discharge patterns of auditory neurons by using an ongoing, high-rate, desynchronizing pulse train as the carrier waveform. Overall, these studies will provide basic knowledge about the stimulus parameters that influence binaural interactions in electric hearing and are likely to lead to new processor designs specifically adapted to bilateral cochlear implants.