For many years it was unknown how medial olivocochlear neurons, which terminate only on outer hair cells, can strongly suppress activity of myelinated auditory-nerve fibers, which originate only from inner hair cells. Recently there has been a fundamental change in thinking about the role of outer hair cells in cochlear mechanics, and the demonstration that cochlear efferents can produce mechanical changes within the cochlea. These have led to the hypothesis that the suppression of auditory-nerve fibers by medial olivocochlear efferent activity is mediated through mechanical coupling at the hair cell level. There are almost no data, however, which indicate the mechanisms which produce this coupling or the properties of the coupling. Our goals are to determine these properties and to understand how the coupling is achieved. Another goal is to determine the conditions under which efferent activity might be functionally significant. Through the combination of these, we seek to learn how the central nervous system is able to influence cochlear function. Work in two areas is proposed. The first project is aimed at measuring the macromechanical changes in the cochlea produced by activity in medial olivocochlear efferents, using a methodology developed during the past two years. To do this, efferent-induced changes in ear-canal sound pressure will be measured. Specific experiments will determine the basic properties of these mechanical changes, the extent along the length of the cochlea over which the changes occur, and the degree to which they provide a measure of sound amplification within the cochlea. The second project seeks to determine the effects on the cochlea of sound-evoked activity of olivocochlear efferents. This will be done by measuring sound-evoked changes in ear-canal sound pressure and in the compound action potential of the auditory nerve.