Altered susceptibility of the inner ear following repeated noise exposure

Abstract

The susceptibility of the inner ear to repeated noise exposure was investigated in the rabbit using tests of behavioral thresholds and distortion-product emissions (DPEs). Behavioral thresholds, measured by a classical-conditioning technique, were collected at 11 frequencies representative of the rabbit hearing range. Acoustic distortion products at the 2f1 − f2 frequency were measured as both DPE audiograms, generated by equilevel primaries at 45, 55, and 65 dB SPL, and input-output functions in 5-dB steps at nine frequencies. Following acquisition of control measures of DPEs and behavioral thresholds, DPEs were analyzed at regular intervals during both exposure and recovery periods and compared to corresponding behavioral measures. In four animals, following repeated episodes of exposure (octave band of noise centered at 1 kHz at 95 dB SPL) and recovery (3 weeks), both DPE amplitudes and behavioral audiograms revealed an increasing resistance to the effects of noise exposure. Comparison with the amount of amplitude reduction in a 4-kHz DP caused by a 5-min exposure during the preexposure periods to a 95-dB SPL tone at 4.215 kHz (1/2 octave below the geometric mean of the primaries) showed a poor correlation with the lessened susceptibility to repeated noise exposure. The concept describing a dynamic mechanism of repair of sensory elements proposed by a number of investigators to explain recovery from sound exposure may account for the increasing resistance to the effects of repeated noise exposure as detected by both DPEs and behavioral responses. [Work supported by NINCDS.]