Effects of Bilateral Olivocochlear Lesions on Pure-Tone Intensity Discrimination in Cats.

Behavioral experiments examined the effects of olivocochlear efferent lesions on performance in an intensity discrimination task. Five cats were trained with food reinforcement to signal the detection of a change in the intensity of pure tones by releasing a response lever. Intensity cues were conveyed by 1 and 8-kHz tone bursts in quiet and in the presence of continuous broadband noise. After the collection of baseline behavioral data, the olivocochlear bundle (OCB) was sectioned with bilateral knife cuts on the floor of the IVth ventricle. The completeness of OCB lesions was evaluated at the conclusion of post-lesion behavioral testing by light microscopic examination of cochlear acetylcholinesterase staining and electrophysiological measures of contralateral noise suppression of compound action potentials (CAPs). Cats with OCB lesions showed greatest performance deficits for the discrimination of 8-kHz intensity changes in continuous background noise. The subjects' ability to discriminate 1-kHz intensity changes in noise was poor prior to OCB lesioning and did not change after the surgical procedure. Lesioning effects were not observed at either frequency when tests were conducted in quiet. These results suggest that olivocochlear feedback contributes to the auditory processing of mid-frequency acoustic signals in noisy backgrounds.