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 amplitude of OAE suppression was noted between the first tested ear and the second one. In addition, 20 pathological subjects were tested according to the same protocol. Ten of them belonged to a group of patients whose vestibular nerve was sectioned on one side to relieve incapacitating vertigo and thus represented a group in whom olivocochlear efferents were severed. A great reduction of suppression observed in the operated ear suggested that olivocochlear efferent fibers are necessary to obtain a full suppressive effect. Three of the pathological subjects were patients who had undergone a decompression of the facial nerve which 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 difference observed in the neurotomized group. Finally, seven of the pathological subjects were patients with Bell's palsy, which paralyses the facial nerve and abolishes the stapedial reflex. No suppression difference was observed between healthy ears and ears without stapedial reflex. Therefore, it appeared that the stapedial reflex was not involved in the contralateral suppression of EOAEs. However, as the tensor tympani muscle remained functional in these patients, its involvement in the suppressive effect cannot be excluded.