The investigators propose a series of measurements to address many theoretically important issues at the level of cellular interactions in the intact cochlea using the action of medial olivocochlear efferents as a tool. A subsidiary goal is to understand the changes produced by efferents in stimulus coding in the auditory nerve. The data obtained should provide significant new insights relevant to such questions as: What events control the firing of auditory-nerve fibers? Does efferent activity act in part through a change in endocochlear potential? What factors make responses of auditory-nerve fibers with high spontaneous firing rates different from those with low spontaneous rates? What changes in firings of auditory-nerve fibers are produced by efferent activity when sounds are well above threshold and not at the fiber characteristic frequency (CF)? How is two-tone suppression produced, and how is it influenced by efferent activity? How are otacoustic emissions generated, and how does efferent activity affect them? How do the time courses of various efferent effects relate to one another and to the underlying efferent actions which give rise to these effects? How large are the effects of a single efferent fiber on auditory-nerve responses, and how do these effects summate? The investigators will address these questions by 1: comparing efferent-evoked effects on auditory-nerve fibers with different Sirs but the same CF from the same cat, 2: measuring both mechanical and neural effect of two-tone suppression and comparing these effects with those efferent stimulation. 3: measuring the time courses of medial-efferent effects (mechanical, electrical and neural) in response to trains of shock, and 4. determining the correspondence between the number of medial efferents that fire and the effects produced. Changes in cochlear mechanics will be monitored by measuring changes in stimulus frequency emissions and efferent-evoked electrical changes will be monitored by measuring changes in endocochlear potential.
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