Activation of medial olivocochlear efferent system in humans: influence of stimulus bandwidth

The activity of the medial olivocochlear bundle (MOCB) can be studied in humans through variations in the level of evoked otoacoustic emissions (EOAEs) elicited by contralateral acoustic stimuli (CAS). The present study sought to investigate how the activity of the MOC system at a given frequency, as measured through the contralateral suppression of tone-pip EOAEs, depends on the bandwidth of the contralateral stimulus. EOAEs were recorded in 155 normal-hearing subjects, successively with and without contralateral stimuli whose bandwidth, center frequency and level were systematically varied. We showed a clear dependence of contralateral EOAE suppression on bandwidth demonstrating increased suppression with increased bandwidth over about two octaves around the center frequency of the noise. This effect was obtained irrespective of whether contralateral noise energy was kept constant independently of bandwidth or not, which indicates a role of bandwidth per se in contralateral EOAE suppression. Results are interpreted in terms of a simple model of MOCB activation mechanisms including peripheral bandpass filtering, within-channel compression and across-channel spatial summation by the afferent paths. Complementary experiments suggested a greater effectiveness of increases in bandwidth on the upper than on the lower side and of frequency components akin to or remote from the test frequency than of intermediate bands. Finally, these results were complemented by detailed spectrum analyses of the EOAE level variations induced by the different noises, which revealed that whilst noise components close to or remote from the center frequency generally attenuated EOAE level, intermediate components could in some cases lead to a relative increase in EOAE level. These results can further be explained by assuming different positive and negative weights on the inputs to the spatial summation process depending on their position relative to the center frequency.