Influence of hearing sensitivity on mechano-electric transduction.

This study examined the relation between the extent of permanent hearing loss and the change in a third-order polynomial transducer function (PTF) representing mechano-electric transduction (MET). Mongolian gerbils were exposed to noise for 1 to 128 h. A control group received no exposure. The cochlear microphonic (CM) was recorded from a round-window electrode and stapes velocity was recorded with a laser Doppler vibrometer in response to Gaussian noise. A nonlinear systems identification procedure provided the frequency-domain coefficients of the PTF and their associated coherence functions. In the control group, the PTF in the high frequencies was dominated by linear and cubic terms. In noise-exposed animals, the magnitude of these terms decreased with increasing threshold, suggesting a progressive decrease in the receptor currents through basal hair cells. Moreover, the linear coherence increased and the cubic coherence decreased, indicating that MET in the cochlear base became linear. In the low frequencies, noise exposure altered the group delay of the CM, demonstrating a redistribution of hair-cell currents. The low-frequency PTF was characterized by an increase in the contribution in the quadratic term. With increasing threshold, the slope of the PTF decreased and the saturation for positive CM was eliminated.