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Auditory nerve fiber responses following chronic cochlear de‐efferentation

The aim of the present study was to examine the role of the olivocochlear system in auditory processing by examining the long‐term effects of cochlear de‐efferentation on auditory nerve response properties in adult chinchillas. Spontaneous rates, response thresholds, tuning curves, discharge rate‐level functions, and adaptation of single auditory nerve fibers were measured in chinchillas with complete cochlear de‐efferentation produced by sectioning the olivocochlear bundle in the internal auditory meatus. De‐efferentation was verified as successful on the basis of acetylcholinesterase staining of surface preparations of the organ of Corti. Following chronic de‐efferentation, there was a striking decrease in spontaneous rate, consistent with earlier observations in cats. In addition, the present study shows that complete de‐efferentation results in: (1) increased driven discharge rates and decreased dynamic range of discharge rate‐level functions, (2) larger onset‐to‐steady state ratio of discharge rate at moderate intensities, and (3) a hypersensitive tail of the tuning curve. These effects, largely confined to neurons that were most sensitive to frequencies between 2–8 kHz, indicate that the cochlear efferent system is important in maintaining normal function (e.g., frequency and intensity selectivity) of the auditory periphery by modulating auditory nerve fiber response properties. J. Comp. Neurol. 406:72–86, 1999. © 1999 Wiley‐Liss, Inc.