Date of Award

1990

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Animal Science (Animal, Dairy, and Poultry Sciences)

First Advisor

John K. Cullen, Jr

Abstract

A growing body of research suggests that the efferent innervation to the cochlea, the olivocochlear bundle, plays a role in modulating the mechanical function of the cochlea. Some of the more provocative findings supporting an hypothesis of OCB modulation have come from studies which assessed efferent influences on tuning curves and acoustic distortion products, two measures thought to be sensitive indicators of cochlear mechanical function. While the OCB appears to be capable of modifying cochlear mechanics, the functional role attributed to the efferents has been difficult to define. For example, some studies which have transected all or part of the OCB have found changes in cochlear tuning, while others have not. Similarly, investigations which have electrically or acoustically stimulated the OCB have yielded contradictory findings. Particularly little is known about the role of tonic OCB input in cochlear function. The purpose of this project was to determine the influence of tonic efferent input on cochlear mechanics, by transecting the entire olivocochlear bundle (OCB) in guinea pigs, and observing changes in tuning curves and acoustic distortion products. The olivocochlear bundle was transected by avulsing the inferior vestibular nerve, as it enters the internal auditory canal of the right bulla. Auditory brainstem response (ABR) thresholds, ABR tuning curves, and acoustic distortion products were measured before and after surgery. Successful transection of the OCB was verified histochemically. Results revealed no consistent changes in tuning curves or in the growth functions of the acoustic distortion products. It was therefore concluded that tonic olivocochlear input is not necessary for normal cochlear mechanical function. It appears that the OCB normally exerts an influence on cochlear mechanics only when activated by sound. Possible roles for the OCB in the forward masking phenomenon were discussed, as well as a putative OCB role in reducing the auditory input to cochlear nucleus and higher centers, to improve central auditory processing.

Pages

102

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