Effects of aspirin on psychophysical, measures of frequency selectivity, two-tone suppression, and growth of masking

Three psychophysical measures of nonlinearity were evaluated before and dur ing a course of aspirin ingestion to investigate the role of outer hair cel ls (OHCs) in these measures, as aspirin is thought to alter the functioning of OHCs. Six normal-hearing individuals received a moderate dose (3.9 g/da y) of aspirin for four days, producing essentially flat, temporary hearing losses that ranged from 5-20 dB. The losses were about 2 dB greater for a 3 00-ms signal than for a 15-ms signal, indicating reduced temporal integrati on with aspirin. On the final three days of aspirin use, three experiments were completed; each was designed to measure one aspect of nonlinear behavi or: (1) the effects of level on frequency selectivity in simultaneous maski ng using notched-noise maskers, (2) two-tone suppression using forward mask ers at the signal frequency (f(s)) and suppressor tones above f(s), and (3) growth-of-masking functions in forward masking using a masking tone below f(s). Signal frequencies of 750 and 3000 Hz were used to evaluate the effec ts of aspirin at relatively low- and high-frequency regions of the cochlea. In experiment I, aspirin broadened the auditory filters and reduced the ef fect of level on frequency selectivity. In experiment 2, aspirin reduced or eliminated two-tone suppression. And, in experiment 3, aspirin reduced the slopes of the growth-of-masking functions. Thus, the aspirin was effective in reducing nonlinearity in all three experiments, suggesting that these m easures reflect the same (or a similar) active, nonlinear mechanism, namely the compressive nonlinearity provided by the OHCs. In all experiments, asp irin tended to have larger detrimental effects on the nonlinear measures at 3000 Hz than at 750 Hz, which can be explained in terms of greater involve ment of nonlinear processing at higher frequencies. Finally, these effects of aspirin were found to be similar to those observed in preliminary measur ements in two subjects with mild, permanent hearing loss. (C) 1999 Acoustic al Society of America. [S0001-4966(99)04509-9].