Relations between cochlear fatigue and the asymmetrical nonlinearity of the cochlear microphonics

Changes in cochlear microphonic and neural sensitivity produced by acoustic trauma

The low-frequency (200 Hz) microphonic potentials at the round window and in the organ of Corti of the first turn of the guinea pig cochlea have been measured before and after acoustic overstimulation. Reductions in the amplitude of this microphonic after loud sound are highly correlated with neural threshold elevation in this region. The fall in the microphonic amplitude appears due to an inactivation of mechano-electrical transduction channels at the apex of the outer hair cells into a closed state. These results are consistent with the idea that the current through the outer hair cells controls the mechanical sensitivity of the organ of Corti, and that the temporary loss of mechanical and neural sensitivity following loud sound is due to a simple inactivation of the mechano-electrical transduction channels.

The active process is affected first by intense sound exposure

Evidence exists to suggest that intense sound releases excess neurotransmitter from the inner hair cells. However, it has been previously reported that intense sound affects the cochlear micromechanics by altering the stereocilia. Therefore, we tested the hypothesis that intense sound affects structures involved in transduction before it affects the nerve endings. In order to test this hypothesis, we examined the interaction of intense sound with kynurenate which blocks the action of the neurotransmitter on the afferent nerve endings. Intracochlear perfusion of artificial perilymph containing 5 mM kynurenate did not reduce the effect of intense sound when we compare the results with a control group perfused with artificial perilymph alone. These results show that blockade of afferent transmitter receptors did not reduce the effect of acoustic trauma, and the acoustic trauma used herein affected structures involved in transduction before it affected the postsynaptic structures. We speculate that the active process is affected first during acoustic trauma. This interpretation is consistent with the notion that stereocilia are structures that make up part of the active process.

Auditory-evoked potential correlates of susceptibility to noise-induced hearing loss

Correlations between changes in cochlear microphonics (CM) and auditory brainstem-evoked potentials (ABEP) resulting from transitory-threshold-shift (TTS)-inducing noise, in normally hearing subjects, and the eventual permanent threshold shift (PTS) which the same subjects developed after 9-14 months of well-quantified occupational noise were evaluated. In addition, the predictive value of pigmentation, as an indicator of eventual PTS, was assessed. Eleven CM and ABEP indices which showed significant correlation with the eventual hearing loss were identified. Eight of these indices reflect the effect of increased stimulus rate and experimental TTS-inducing noise. These results show that the difference in ABEP latencies as a result of increased stimulus rate is smaller in persons with large eventual PTS (high susceptibility to noise-induced hearing loss). The effect is larger in persons that eventually developed a small PTS (low susceptibility). Pigmentation, as reflected by iris and skin color, was found to have a negative correlation with susceptibility to noise, i.e. the more pigment the less PTS developed. The above indices may be useful to determine personal susceptibility to noise in normally hearing subjects.

Modifications of the nonlinearity of the cochlear microphonic responses produced by noise exposure in the guinea pig

Cochlear microphonics (CM) were recorded with differential electrodes from several locations in the guinea pig cochlea. Input-output curves and amplitude-frequency curves were plotted before and after short exposures to intense noise. In addition to amplitude losses, important changes in the intensity functions resulted in a decrease of the nonlinearity and in a modification of the frequency response. The mechanisms producing these alterations are discussed.