Signs of noise-induced neural degeneration in humans
Animal studies demonstrated that noise exposure causes a primary and selective loss of auditory-nerve fibres with low spontaneous firing rate. This neuronal impairment, if also present in humans, can be assumed to affect the processing of supra-threshold stimuli, especially in the presence of background noise, while leaving the processing of low-level stimuli unaffected. The purpose of this study was to investigate if signs of such primary neural damage from noise-exposure could also be found in noise-exposed human individuals. It was investigated: (1) if noise-exposed listeners with hearing thresholds within the “normal” range perform poorer, in terms of their speech recognition threshold in noise (SRTN), and (2) if auditory brainstem responses (ABR) reveal lower amplitude of wave I in the noise-exposed listeners. A test group of noise/music-exposed individuals and a control group were recruited. All subjects were between 18-32 years of age and had pure-tone thresholds ≤ 15 dB HL from 250-8000 Hz. Despite normal pure-tone thresholds, the noise-exposed listeners required a significantly better signal-to-noise ratio to obtain SRTN, compared to the control group. The ABR results showed significantly lower amplitude of wave I, in the left-ear, of the test group listeners. Significantly higher wave III and normal wave V were also found in the left ear of the test group listeners suggesting a compensated neural gain in the brainstem. Overall, the results from this study seem to suggest that noise exposure affects supra-threshold processing in humans before pure-tone sensitivity, raising suspicion to the hypothesis of primary neural involvement.
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