Efferent-induced change in human cochlear compression and its influence on masking of tones.

Several lines of evidence suggest that medial olivocochlear (MOC) efferent neurons modify cochlear output to improve signal detection in noise. In animal models, stimulation of MOC efferents reduces the amount of compression in basilar membrane (BM) growth functions. Linearization of BM growth functions may assist in extending the neural response to the signal above that of noise, leading to a decrease in masking. In order to test this hypothesis, effects of MOC efferent neurons on BM compression were studied indirectly in humans by examining the effects of contralateral noise on distortion-product otoacoustic emission (DPOAE) input-output functions at 1.0 and 2.0kHz. Compression threshold estimates from a three-segment linear regression model applied to the DPOAE functions were derived in order to determine correlations with psychophysical measurements of masking of tones at 1.0 and 2.0kHz. Contralateral noise shifted the DPOAE compression threshold to a significantly higher level at 1.0kHz, but not at 2.0kHz. A significant negative correlation between the change in DPOAE compression threshold and the amount of masking at 1.0kHz was observed, but no correlation between these variables was detected at 2.0kHz. The results of this experiment at the lower test frequency indicated that contralateral noise linearized DPOAE input-output functions, and individuals with larger DPOAE compression threshold shifts tended to exhibit less masking. Under certain conditions, decreases in cochlear compression induced by MOC efferent neurons may lead to unmasking of tones presented in noise.