The long-range goal of this project is to understand the descending neural systems that modify information output from the cochlea and cochlear nucleus to higher brain centers. Existence of descending innervation to hair-cell systems in most vertebrate species examined thus far is strong presumptive evidence for the importance of descending systems. There is evidence that the function of descending systems may be: 1) to provide anti-masking for transient signals in a continuous background of noise, 2) to expand the limited dynamic range of auditory-nerve fibers, 3) to enable selective attention to certain signals, and 4) to protect the inner ear from overstimulation by intense sound. These functions are essential in everyday hearing; their interruption would severely damage important processes like speech discrimination. This project will concentrate on one particular descending system, the olivocochlear system, in guinea pigs and in mice. Shock-evoked activity in neurons of the medial olivocochlear (MOC) group can greatly affect the responses of the cochlea, but most studies of the responses of these neurons have used monaural sound in a background of quiet. We will study the responses of MOC neurons to binaural sounds including noise, which are the types of stimuli normally encountered by an animal in the free field. We will also use "priming" stimuli that can change the neuron's responses even after the sound has been turned off. These experiments will answer questions like, "What kinds of sounds cause the MOC reflex to be activated?" and "What kinds of sounds change the MOC reflex?" MOC neurons have branches to the cochlear and vestibular nuclei in the brainstem. These branches may inform the brain about how much and what kind of feedback is to the cochlea, which would be essential information in coding for sound intensity. We will identify the targets of these branches and study their role in the MOC reflex. We will also study small neurons involved in descending pathways. These neurons include the lateral olivocochlear (LOC) neurons and cochlear- nucleus small stellate and granule cells. Electrophysiological responses from small neurons have been difficult to measure. These neurons produce Fos, the protein product of the immediate-early gene, c-fos. Studying their Fos expression will identify what types of sounds activate these neurons. This knowledge would greatly increase our understanding of both the descending and the ascending auditory pathways, by answering questions like, "What kinds of sounds activate the LOC neurons nd small neurons of the cochlear nucleus?"