Temporal Resolution of Cochlear and Auditory Nerve Responses in Older Adults

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DESCRIPTION (provided by applicant): Aging impairs the ability of listeners to process temporally complex acoustic stimuli, such as speech. Sources of this decline in temporal processing ability include age-related changes in the central auditory nervous system. Auditory nerve fiber activity within the peripheral auditory system also becomes more asynchronous with age. Furthermore, many older adults suffer from cochlear hearing loss, which may effectively limit temporal processing ability by reducing cochlear nonlinearity. The long-term goal of this research is to determine the auditory basis of speech understanding deficits imposed by aging. The objective of this application is to determine the extent to which abnormal perception of time-varying speech cues results from age-related changes at or before the level of the auditory nerve. The central hypothesis is that aging alters cochlear mechanical and auditory nerve responses, thereby interfering with the ability to understand speech. This hypothesis is based on evidence that nonlinear cochlear responses and synchronized neural responses are involved in the encoding of temporal speech features. Evaluation of peripheral (cochlear mechanical and auditory nerve) temporal processing in older adults is necessary because it is not known how peripheral timing deficits contribute to abnormal perception of temporal speech cues. The approach is to measure physiological responses, modifying recently-published psychophysical techniques used to assess temporal resolution and basilar membrane compression. These measures will then be compared to performance on temporal speech tasks in the same groups of elderly and young adults with normal or impaired hearing. Specific Aim 1 will measure temporal resolution based on auditory nerve compound action potentials (CAPs) obtained using a forward-masking paradigm. Specific Aim 2 will estimate cochlear mechanical compression and travel time from stimulus-frequency otoacoustic emissions (SFOAEs). Comparisons of SFOAE and CAP responses will determine the extent that the amplitude and timing of auditory nerve responses is altered by outer hair cell function in the auditory systems of older adults. Specific Aim 3 will determine whether performance on a temporal speech task depends on cochlear mechanical and auditory nerve responses. The rationale for this work is that once sources of reduced temporal processing are known, hearing aid fitting and rehabilitative training strategies can be devised that remediate the temporal processing problems of individual patients. Proposed physiological measures are free from confounding by age-related changes in cognition or memory. Therefore, this research has additional significance in that it may lead to development of clinical tools that will target sources of reduced temporal processing.