The goal of this project is to test in two animal models of presbycusis, the C57Bl/6J and the CBA/CaJ mouse, and in human listeners, the hypothesis that presbycusis listeners are unable to process acoustic transients as quickly or as efficiently as normal-hearing listeners, and that the extent of their deficit caries systematically across the spectrum. We will use reflex modification audiometry to determine in the middle aged C57Bl (with aged-related peripheral degeneration, plus central sequela) and old CBA mice (with modest peripheral damaged but an aged central auditory system), and also in aged humans, compared to young normal subjects, if differences in the time course and asymptomatic levels of reflex modification of the acoustic startle reaction by pure-tone combinations and band-limited noise prepulses vary: (a) for types of reflex modification that depend on the inferior colliculus and cortex versus the cochlear nucleus; (b) for a transient shift in the spatial location of the prepulse; or (c) for the difference in masking noise presented at the same or at different spatial locations as the prepulse. Behavioral Effects will be compared to electophysiological data in the same animal, and related to age differences in anatomy and neurochemistry. Some mice will be treated with calcium blockers and buffers to see if this will retard the appearance of age related deficits, and with neuroactive compounds intended to repair neural function in the aged, or simulate aging in the young mouse. Positive findings in these studies would indicate that differential delays in encoding amplitude modulated stimuli may be less responsible for altering the timing of the neural response to fluctuation in speech, which would thus distort its neural code; and would also reduce the ability of the aged listener to group stimulus onsets and offsets on the basis of synchronicity, and to assign complex sounds to a common spatial location. This latter deficit would then limit the higher order perceptual grouping of sound objects that is important for speech perception, and contribute to the problem that presbycusis listeners have in perceiving speech in noisy or complex acoustic environments.