DYNAMIC PROCESSES IN THE AUDITORY SYSTEM

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A fundamental property of the auditory system is the ability to follow changes in amplitude and frequency over time. Although we understand a good deal about static properties of the auditory system, the dynamic properties, although extensively researched, are less well understood. There is evidence that the sensitivity and tuning of the auditory system may change as a function of prior stimulation. Experiments on the temporal development of auditory filtering have shown that turning appears to be sharper for a signal presented with a delay after masker onset, relative to a signal presented at masker onset. Evidence that both sensitivity and tuning may change comes from a phenomenon called overshoot. Under certain conditions of frequency and level, the threshold for the tone is higher on the onset of the masker than it is after a delay of 100 ms or more. One of the most commonly proposed mechanisms for overshoot is that it reflects short-term neural adaptation. Some recent evidence suggests that this 'adaptation' is not simple fatigue at the signal frequency, but may be mediated by a neural feedback (or efferent) loop in the auditory system. This would explain the fact that frequencies remote from the signal frequency are at least partially responsible for the overshoot effect. This evidence and other research suggest that the effects underlying overshoot may provide a window on the relationship between active cochlear filtering and the efferent system. Although change in filter shape by itself cannot account for the magnitude of the overshoot effect, using an overshoot-type paradigm would allow examination of adaptation ad auditory filtershape independently. Auditory filter shapes have not been directly measured under the same conditions present in overshoot. This project proposes to measure overshoot and auditory filter shapes I similar conditions, and determine whether auditory filter shapes appear to narrow with stimulation under conditions in which overshoot is seen, and not in conditions in which overshoot is not seen. This has implications for understanding of the active process in normal hearing and for the effects of hearing impairment.