EVIDENCE OF A MEDIAL OLIVOCOCHLEAR INVOLVEMENT IN CONTRALATERAL SUPPRESSION OF OTOACOUSTIC EMISSIONS IN HUMANS

Otoacoustic emissions (OAEs) evoked by click stimuli were recorded in both ears of 20 normal human subjects, in the presence and absence of a contralateral masking broad band noise. No difference in the amplitu de of OAE suppression was noted between the first rested ear and the s econd one. In addition, 20 pathological subjects were tested according to the same protocol. Ten of them belonged to a group of patients who se vestibular nerve was sectioned on one side to relieve incapacitatin g vertigo and thus represented a group in whom olivocochlear efferents were severed. A great reduction of suppression observed in the operat ed ear suggested that olivocochlear efferent fibers are necessary to o btain a full suppressive effect. Three of the pathological subjects we re patients who had undergone a decompression of the facial nerve whic h necessitated the same surgical approach as vestibular neurotomy, but without any section of vestibular fibers. This surgical control group demonstrated that the surgical act by itself cannot explain the diffe rence observed in the neurotomized group. Finally, seven of the pathol ogical subjects were patients with Bell's palsy, which paralyses the f acial nerve and abolishes the stapedial reflex. No suppression differe nce was observed between healthy ears and ears without stapedial refle x. Therefore, it appeared that the stapedial reflex was not involved i n the contralateral suppression of EOAEs. However, as the tensor tympa ni muscle remained functional in these patients, its involvement in th e suppressive effect cannot be excluded.