Bilateral cochlear implantation aims at restoring the functional benefits of binaural hearing to the profoundly deaf. While wearers of bilateral implants show improved sound localization and speech reception in noise, their ability to process interaural time differences (ITD) is still far from normal, resulting in smaller binaural benefits than normal in everyday acoustic environments. In this application, investigators from the Massachusetts Eye and Ear Infirmary, MIT, and Boston University combine their expertise in psychophysics, neurophysiology and neural computation in order to gain a basic understanding of the ITD processing with bilateral cochlear implants. Psychophysical experiments in bilaterally-implanted human subjects will be conducted in parallel with single-unit recordings from the inferior colliculus of deaf cats implanted bilaterally with intracochlear electrode arrays. The specific aims are to (1) compare neural ITD sensitivity in animals differing in age of onset of deafness and duration of deafness in order to test whether deprivation of binaural experience has an effect on ITD sensitivity and whether there is a critical period for these effects; (2) characterize the effects of electrode interactions on both neural and behavioral ITD sensitivity in order to understand how ITD is processed with multi-channel stimulation as occurs when listening through a processor and in the presence of multiple sound sources; (3) develop models for binaural brainstem neurons to understand the mechanisms underlying neural ITD sensitivity with bilateral implants, and models for activity patterns in populations of neurons to predict psychophysical performance, and compare the effectiveness of various strategies for estimating the stimulus ITD from the population activity. These studies will increase our basic understanding of how neural, psychophysical and electrode factors interact with binaural experience in shaping performance with bilateral cochlear implants. They are also likely to lead to new binaural sound processors for bilateral cochlear implants that work better in everyday acoustic environments comprising multiple sound sources and reverberation, and that are adapted to specific types of patients depending on their history of binaural experience and deprivation. Finally, the combination of controlled deafening, bilateral cochlear implantation and neural recording offers a novel approach for studying the plasticity of the binaural system. The profoundly deaf increasingly receive cochlear implants in both ears in the hope that such bilateral implantation will improve their ability to localize sounds and understand speech in noise. Our studies combine expertise from neurophysiology, psychology and biophysics in order to gain a basic understanding of the challenges wearers of bilateral implants face in everyday situations and how performance interacts with their history of deafness. These studies will likely lead to new electronic sound processors for bilateral implants that work better in noise and are better adapted to specific types of deafness histories.