Binaural acoustic stimulation exercises protective effects at the cochlea that mimic the effects of electrical stimulation of an auditory efferent pathway

Abstract

Low-level acoustic stimulation of one (contralateral) ear reduced the neural desensitization caused by a simultaneous loud sound exposure in the other (ipsilateral) ear in a loss-related manner. Greatest reductions in the temporary threshold shifts (TTS) in the exposed ear were obtained when the exposure would have caused large amounts of TTS. Low-level exposures (reduced intensity or duration of exposure) which caused low levels of TTS, from which the cochlea could recover relatively quickly, were not affected by the contralateral stimulus. Intermediate levels of TTS showed intermediate levels of reduction for the same contralateral acoustic stimulus. These effects were similar to effects previously demonstrated with electrical stimulation of an efferent pathway to the cochlea, the crossed olivocochlear bundle (COCB); lesioning the COCB prevented the contralateral stimulus from having any effect on TTS due to an ipsilateral exposure. Like COCB stimulation, the contralateral acoustic stimulus had tonic effects, so that reductions in ipsilateral TTS could be obtained even when the contralateral stimulus was presented 5 min before the ipsilateral exposure. With 10 min delay no effect on TTS occured. The contralateral stimulus did not appear too cause any changes in responses in the ipsilateral cochlea prior to the loud sound exposure. These results are discussed as indicating an interaction between the two inputs at a central locus, leading to activation of the COCB fibres to the cochlea exposed to the loud sound.