Abstract
Modulation of cochlear mechanics by the medial olivocochlear efferent system is characterized by a reduction in active, outer hair cell-mediated amplification of basilar membrane motion. This increases cochlear thresholds and linearizes basilar membrane input-output functions for low-to-moderate stimulus levels. Significant efferent effects have also been observed for responses to higher stimulus levels, potentially reflecting changes in the mechanical properties of the cochlear partition. In humans, sound activated changes in stimulus-frequency otoacoustic emissions have been used as a tool for investigating the dynamics of the medial olivocochlear reflex. However, the degree to which the amplitude and phase of otoacoustic emissions are related to those of basilar membrane motion is not entirely clear. For the purposes of comparison with invasive physiological measurements in animals, stimulus-frequency otoacoustic emission input-output functions were obtained from human subjects in the presence and absence of contralateral acoustic stimulation. Medial olivocochlear effects were quantified in terms of the absolute change in emission level as well as the vector change, which incorporates changes in emission phase. The extent to which efferent modulation of emissions in humans reflects that observed in previous reports of basilar membrane motion will be discussed.
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