This proposal will study a recently recognized form of hearing disorder called auditory neuropathy (AN) that is due to a disorder of auditory nerve functions in the presence of normal cochlear receptor activities. AN subjects have normal measures of cochlear outer hair cell activities but abnormal measures of the central auditory pathway functions beginning with auditory nerve. The hearing disorder typically affects speech comprehension out-of-proportion to the pure tone loss, particularly speech recognition in noise. AN is not rare and accounts for 10 percent of newborns identified as having hearing loss and also develops in childhood and in adults. In adults the disorder of auditory nerve is commonly associated with a peripheral neuropathy. In addition, AN and sensory hearing loss occur together. Loss of neural synchrony and decreased auditory nerve input are proposed as cardinal mechanisms underlying the hearing disorder. The sites of abnormality along auditory nerve have recently been identified at auditory ganglion cells and their axons (Type I AN) or distally in the auditory periphery where auditory nerve terminals, inner hair cells, and the synapses between inner hair cells with nerve terminals are in close approximation (Type II AN). Our long-term goals are to understand the underlying mechanisms of AN and the accompanying hearing impairments to provide a scientific basis for alleviating the hearing deficit in AN subjects. We propose four experiments using both psychophysical and electrophysiological techniques that will help us achieve these goals. We will characterize (1) the reorganization of central auditory pathway occurring in response to altered auditory nerve input; (2) the contributions of dysynchronization and decreased auditory input as pathophysiological mechanisms underlying AN; (3) the objective physiological and psychophysical measures of improved auditory temporal processes in cochlear implanted AN subjects, and (4) the improvement of speech recognition using new auditory processing techniques. The results of our studies could have major impact on the diagnosis, classification, understanding, and treatment of auditory neuropathy.
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