ABSTRACT
The auditory system contains a large number of efferent fibers that terminate via the olivocochlear bundle at the hair cells of the cochlea. Two distinct groups of fibers have been identified - the lateral and medial efferents (Guinan, Warr, & Norris, 1983; Warr, Guinan, & White, 1986). The lateral fibers originate near the lateral superior olivary nucleus and innervate afferent dendrites at the inner hair cells, mostly in the ipsilateral cochlea; the medial fibers originate in the medial superior olivary nucleus and end directly on outer hair cells, mostly in the contralateral cochlea. Neurophysiological studies (cf. Wiederhold, 1986) have shown that activation of the efferents, usually by electrical stimulation, inhibits afferent activity in the eighth nerve. When the stimulus is tone plus noise, this inhibition may reduce the response to the noise, especially at higher stimulus levels, such that the tone evokes a relatively stronger response; accordingly, the effective neural signal-to-noise ratio would be greater than without efferent stimulation (Winslow & Sachs, 1987). Of course, electrical stimulation is not necessary to activate the efferents, and Liberman (1988) has shown that efferent discharge increases in the presence of broadband noise. These results suggested to Liberman (1988) "a potent role of the olivocochlear bundle in increasing the rate-response of auditory-nerve afferents to narrow-band stimuli embedded in broadband noise" (p. 1797), thereby possibly increasing "the detectability of narrow-band stimuli embedded in noise" (p. 1779). However, animal behavioral studies have not
Ideally, after the operation. This was possible for three patients. In those three and in the rest of our sampie we also compare detection in the operated ear with that in the healthy ear and with that reported in the literature for normally hearing subjects.
