Cochlear outer-hair-cell efferents and complex-sound-induced hearing loss:Protective and opposing effects

Centrifugal crossed and uncrossed medial olivocochlear systems (CMOCS and U MOCS) terminate on cochlear outer hair cells (OHCs) and exercise effects th rough a nicotinic cholinergic receptor. Hence their cochlear effects have n ot been differentiated. Recent work on protection from loud-sound-induced t emporary threshold shifts (TTSs) in hearing sensitivity suggest the two OHC efferent systems may act differently. This was tested, using traumatic com plex sound, to determine if such sound could activate both MOCS components and then reveal whether they exerted different effects on TTSs to such stim uli. Traumatic noise bands activated crossed and uncrossed MOCS efferents. Two different CMOCS effects were observed. For frequencies in the noise (wi thin-band frequencies), it protected hearing sensitivity as expected. Novel findings were that at frequencies higher than the noise band range (high-s ide frequencies), it acted to worsen hearing sensitivity and that this was opposed by a UMOCS effect generally targeted to these frequency regions. It is proposed that the two crossed MOCS actions are extensions of a contrast -enhancement action for low-level noise bands. It is also proposed that the UMOCS plays a state-restoration role to prevent an undesired CMOCS side-ef fect of exacerbation of high-side TTSs to high-level noise bands.