Uncomfortable loudness levels in experienced unilateral and bilateral hearing aid users: evidence of adaptive plasticity following asymmetrical sensory input?

Munro and Trotter [Inter J Audiol 45, 684-688, 2006] reported an asymmetry in uncomfortable loudness levels (ULLs) in a preliminary study using 12 experienced unilateral hearing aids users. The aim of the present study was to investigate ULLs in a larger sample of listeners with unilateral and bilateral hearing aid experience. Participants were adults with symmetrical high-frequency hearing impairment. There were 48 listeners with unilateral experience, 13 listeners with bilateral experience and a control group of 47 listeners with no hearing aid experience. The minimum duration of hearing aid experience was 2 years. ULLs were measured at 0.5, 1, 2 and 4 kHz. The unilateral group showed a statistically significant inter-aural asymmetry of 3-5 dB. Compared to the controls, there was a non-significant trend of higher ULLs in the fitted ear and lower ULLs in the not-fitted ear. ULLs in the bilateral group were symmetrical and similar to the control group. The findings suggest that changes in ULL may be a feature of asymmetrical sensory input and are consistent with a central gain mechanism.