MEDIAL OLIVOCOCHLEAR SYSTEM STABILIZES ACTIVE COCHLEAR MICROMECHANICAL PROPERTIES IN HUMANS

To investigate the involvement of the medial olivocochlear system (MOC S) in outer hair cell (OHC) motility stabilization, evoked otoacoustic emissions (EOAEs) were recorded in 20 normal-hearing subjects and in eight vestibular-neurotomized subjects, successively in the presence a nd absence of low-intensity contralateral acoustic stimulation. Intras ubject EOAE amplitude variability was assessed as the standard deviati on computed over several successive recordings. In normal-hearing subj ects, a significantly lower EOAE amplitude variability with contralate ral acoustic stimulation (CAS) was observed in subjects in whom the CA S induced the greatest EOAE amplitude reduction. This result could not be attributed to the EOAE amplitude reduction itself, since variabili ty was otherwise found to increase when EOAE amplitude decreased. More over, statistically significant correlations between EOAE amplitude at tenuation and EOAE amplitude variability under CAS were observed, In t he eight subjects operated for vestibular neurotomy, no such effect wa s found. Being sectioned in vestibular-neurotomized subjects, the MOCS can no longer exert its effects. These results strongly support the n otion that MOCS activity, as induced by GAS, elicits a reduction in EO AE amplitude variability in normal-hearing subjects. This finding and some of its possible implications for understanding the role of the MO CS in hearing in humans are discussed.