Studies of peripheral gating in the auditory system of cats

Abstract

Cochlear potentials were recorded from the round window in unanesthetized cats with unilaterally tenotomized middle ear muscles. The effects of repetitive stimulation and conditioning procedures on the cochlear microphonic (CM) and auditory-nerve action potential (AP) were examined. Experimental procedures provided control over arousal effects and acoustic input variability and enabled precise quantification and statistical analysis of the data. Repetitive stimulation parameters were selected to provide optimal conditions for the occurrence of olivocochlear bundle (OCB) effects. Responses to high and low frequency tone bursts, noise bursts, and near-threshold click stimulation were investigated. In no case was there any systematic change in either CM or AP as a function of repetitive stimulation. The effects of aversive conditioning procedures on cochlear potentials were examined by pairing a train of clicks with infra-orbital shock. Appropriate interstimulus and inter-trial intervals were utilized. There were no significant changes in either CM or AP as a consequence of these procedures. Simultaneous behavioral observations indicated that initial shock presentation resulted in sensitization of the eye-blink response to the acoustic CS, and that this sensitization effect dissipated as a function of repeated pairing. The stability of CM and AP under these conditions is contrary to the view that cochlear potential amplitude varies as a function of procedures assumed to change the significance of the acoustic stimulus. It is concluded that the OCB does not function as a peripheral gating mechanism in the auditory system under these conditions.