Histochemical evidence that cochlear efferents are carried with the inferior vestibular nerve in guinea pigs

The role of the medial olivocochlear reflex in psychophysical masking and intensity resolution in humans: a review

This review addresses the putative role of the medial olivocochlear (MOC) reflex in psychophysical masking and intensity resolution in humans. A framework for interpreting psychophysical results in terms of the expected influence of the MOC reflex is introduced. This framework is used to review the effects of a precursor or contralateral acoustic stimulation on 1) simultaneous masking of brief tones, 2) behavioral estimates of cochlear gain and frequency resolution in forward masking, 3) the buildup and decay of forward masking, and 4) measures of intensity resolution. Support, or lack thereof, for a role of the MOC reflex in psychophysical perception is discussed in terms of studies on estimates of MOC strength from otoacoustic emissions and the effects of resection of the olivocochlear bundle in patients with vestibular neurectomy. Novel, innovative approaches are needed to resolve the dissatisfying conclusion that current results are unable to definitively confirm or refute the role of the MOC reflex in masking and intensity resolution.

Auditory efferents involved in speech-in-noise intelligibility

Following studies proposing that medial olivocochlear efferents might be involved in the processing of complex signals in noise, we tested the involvement of efferent feedback in speech-in-noise intelligibility. Two approaches were used: measures of speech-in-noise intelligibility in vestibular neurotomized patients with cut efferents and comparison with normal hearing subjects; and correlations between effectiveness of olivocochlear feedback, assessed by contralateral suppression of otoacoustic emissions and speech-in-noise intelligibility in normal subjects. Contralateral noise improved speech-in-noise intelligibility in normal ears. This improvement, which was almost absent in de-efferented ears of vestibular neurotomized patients, was correlated with the strength of the olivocochlear feedback. Together, these results suggest that olivocochlear efferents play an antimasking role in speech perception in noisy environments.

The role of the cochlear efferent system in acquired resistance to noise-induced hearing loss

Previous work has shown that the cochlear efferent system may play a role in protecting the ear from noise-induced temporary threshold shifts (TTS) following exposures to a single tone or series of moderate-level noises ('toughening'). However, whether the olivocochlear bundle (OCB) is important in decreasing noise-induced permanent threshold shifts (PTS) remains an open question. The importance of the OCB in decreasing the ear's susceptibility to noise, as reflected by 2f1-f2 distortion product otoacoustic emissions, was assessed by sectioning both the ipsilateral and contralateral divisions of the efferent system and exposing chinchillas while awake to an octave band noise (4 kHz) at a low level (85 dB SPL) for 10 days (6 h/day) and then at a high level (95 dB SPL) for 48 h. Complete de-efferentation was verified by cochlear acetylcholinesterase staining. The ears that were de-efferent showed substantially more TTS, greater PTS and larger cochlear lesions of outer hair cells. The results suggest that the efferent system may influence the ear's ability to develop resistance to noise trauma.