Neural Encoding of Sound Intensity in the Ventral Cochlear Nucleus of Awake and Behaving Cats

Abstract

Changes in the average discharge rate of most auditory neurons saturate over a 35-dB range of sound levels when experiments are performed on anesthetized animals. Nevertheless, human listeners are sensitive to small intensity differences over a 110-dB range of sound levels. We have tested the hypothesis that the neural encoding of sound intensity is enhanced in awake subjects relative to anesthetized animals by recording unit activity in the ventral cochlear nucleus (VCN) of awake, behaving cats. Data obtained in the presence of continuous background noise show a shift to higher stimulus levels and dynamic range compression. These noise effects limit a unit's ability to encode changes in stimulus intensity with changes in discharge rate. Units in awake cats exhibited larger shifts and less compression in noise (i.e., better dynamic range adjustment) than were observed in previous studies of anesthetized cats (Costalupes et al., 1984). Dynamic range adjustment to noise in awake cats is similar to effects produced in anesthetized cats by stimulating olivocochlear efferent fibers (Winslow and Sachs, 1987).