Threshold shifts and enhancement of cortical evoked responses after noise exposure in rats

The effect of exposure to various types of noise (broadband, high-frequency or low-frequency) was studied in adult pigmented rats. Thresholds and ampl itudes of middle latency responses (MLR) recorded from electrodes implanted on the surface of the auditory cortex were analyzed before and after noise exposure. Exposure to noise with intensities ranging from 105 to 120 dB fo r 1 h produced only temporary threshold shifts (TTS). Exposure to broadband noise produced TTS throughout the whole frequency range of the rat's heari ng, mostly expressed at Frequencies of maximal hearing sensitivity (16-32 k Hz). Hearing loss produced by high- or low-frequency noise exposure was rel ated to the spectral characteristics of the noise. The exposure to high-int ensity noise may also result in amplitude enhancement of the MLR. This phen omenon was seen mainly after broadband noise exposure and occurred in respo nse to both low-frequency and high-frequency test stimuli. High-frequency a nd low-frequency noise produced amplitude enhancement mainly at frequencies which corresponded to the maximum exposure energy. In contrast to the rela tively similar values of TTS obtained in different rats under the same cond itions of noise exposure, great inter-individual variability was found in t he MLR amplitude enhancement. III all rats the dynamics of recovery functio ns for amplitude enhancement were different from those for MLR thresholds. The data indicate that whereas post-exposure TTS are related to peripheral changes, the post-exposure MLR amplitude enhancement is most probably conne cted with a change in the processing of auditory information in the central nervous system. (C) 2000 Elsevier Science B.V. All rights reserved.