Discrimination of envelope statistics reveals evidence of sub-clinical hearing damage in a noise-exposed population with ‘normal’ hearing thresholds

Noise exposure can affect the functioning of inner hair cells (IHC) within the cochlea, leading to a 'noisy' representation of the properties of sounds even when absolute thresholds are normal. This may lead to a reduced ability to discriminate sounds based on their statistical properties, especially for narrowband sounds presented at low levels, since such sounds excite only a small proportion of the IHCs. This idea was tested by requiring subjects to discriminate a Gaussian noise from a 'low-noise noise' with minimal envelope fluctuations. The noises were centred at 2, 3, or 4 kHz. The stimulus duration required for threshold was measured as a function of sensation level (SL) for two groups of normal-hearing subjects, one rarely exposed to high-noise events (control group, C) and one frequently exposed to high-noise events (experimental group, X). For group X, threshold consistently rose for SLs below about 20 dB, while for group C the threshold did not consistently rise at low SLs (although it did for a minority of subjects). The worsening in performance with decreasing level was significantly greater for group X than for group C.