This program of research is designed to evaluate our knowledge of hearing capabilities provided by cochlear prostheses and to determine the physiological and anatomical features upon which these capabilities are based. The research will be performed on non-human primates and guinea pigs. These animals will be used as models for behavioral, electrophysiological, biophysical and histological investigations, the results of which we hope to generalize to man. The program consists of four overlapping projects: The first is directed at evaluating the dependency of hearing capabilities on form and site of middle and inner ear stimulation, and pathology of the temporal bone. The second project is directed at defining and evaluating the neuro-physiological mechanisms that underlie prosthesis function and the utility of neural measures in prosthesis candidate selection and evaluation of prosthesis function. This project, with project 2 will also evaluate various encoding schemes for providing complex information to the prosthesis user. The third project will define and analyze the current paths generated in the inner ear by prosthesis stimulation, define points of optimal stimulation and (with Project 2) determine first elements excited by prosthesis stimulation. The fourth project will evaluate histological changes that accompany prosthesis use, evaluate the mechanisms by which changes occur, define the safe limits for prosthesis use, and attempt to determine ways to minimize potential damage. These data, hopefully, will aid in the development and appropriate application of cochlear prosthesis in humans and provide a base for forming realistic expectations of the risks and benefits of cochlear prostheses.