MRC Inst. of Hear. Res., Royal Infirmary, Glasgow G31 2ER, Scotland
Systematic effects on intensity discrimination occur in listeners who have experienced long-term unilateral amplification [Robinson and Gatehouse, Br. J. Audiol. (in press)]. For a bandlimited tone complex centered at 3 kHz, the normally aided ear was more sensitive than the unaided ear at levels greater than 85 dB SPL, yet was less sensitive at lower levels. No effects were found at 250 Hz. Intensity discrimination for these same stimuli was prospectively investigated for six listeners with bilateral symmetric sensorineural hearing impairment following provision of unilateral amplification. Weber functions for both ears were similar at both frequencies prior fitting, and 4 to 12 weeks post-fitting. From 4 to 12 weeks, the normally aided ear became more sensitive than the unaided ear for the 3-kHz stimulus presented at 95 dB SPL. No effects were observed at 250 Hz, or for the unaided ear. Hence, changes in intensity coding occur in the normally aided ear following unilateral amplification, and exhibit similar characteristics to the changes observed for speech identification in noise [Gatehouse, J. Acoust. Soc. Am. 92, 1258--1268 (1992)]. The effects may be due to long-term exposure to the frequency gain characteristic of the hearing aid, and can be explained by a perceptual relearning process that maximizes sensitivity across the dynamic range.