Medial olivocochlear efferent effects on auditory responses

Abstract

Medial Olivocochlear (MOC) inhibition in one ear can be elicited by sound in either ear. Curiously, the ratio of ipsilateral/contralateral inhibition depends on sound bandwidth; the ratio is ~2 for narrow-band sounds but ~1 for wide-band sounds. Reflex amplitude also depends on elicitor bandwidth and increases as bandwidth is increased, even when elicitor-sound energy is held constant. After elicitor onset (or offset), nothing changes for 20–30 ms and then MOC inhibition builds up (or decays) over 100–300 ms. MOC inhibition has typically been measured in humans by its effects on otoacoustic emissions (OAEs). Problems in such OAE studies include inadequate signal-to-noise ratios (SNRs) and inadequate separation of MOC effects from middle-ear-muscle effects. MOC inhibition reduces basilar-membrane responses more at low levels than high levels, which increases the response SNRs of higher-level signals relative to lower-level background noises, and reduces noise-induced adaptation. The net effect is expected to be increased intelligibility of sounds such as speech. Numerous studies have looked for such perceptual benefits of MOC activity with mixed results. More work is needed to determine whether the differing results are due to experimental conditions (e.g., the speech and noise levels used) or to methodological weaknesses. [Work supported by NIH-RO1DC005977.]