Effects of masker frequency and duration in forward masking: further evidence for the influence of peripheral nonlinearity

Forward masking has often been thought of in terms of neural adaptation, wi th nonlinearities in the growth and decay of forward masking being accounte d for by the nonlinearities inherent in adaptation. In contrast, this study presents further evidence for the hypothesis that forward masking can be d escribed as a linear process, once peripheral, mechanical nonlinearities ar e taken into account. The first experiment compares the growth of masking f or on- and off-frequency maskers. Signal thresholds were measured as a func tion of masker level for three masker-signal intervals of 0, 10, and 30 ms. The brief 4-kHz sinusoidal signal was masked by a 200-ms sinusoidal forwar d masker which had a frequency of either 2.4 kHz (off-frequency) or 4 kHz ( on-frequency). As in previous studies, for the on-frequency condition, the slope of the function relating signal threshold to masker level became shal lower as the delay between the masker and signal was increased. In contrast , the slopes for the off-frequency condition were independent of masker-sig nal delay and had a value of around unity, indicating linear growth of mask ing for all masker-signal delays. In the second experiment, a broadband Gau ssian noise forward masker was used to mask a brief 6-kHz sinusoidal signal . The spectrum level of the masker was either 0 or 40 dB (re: 20 mu Pa). Th e gap between the masker and signal was either 0 or 20 ms. Signal threshold s were measured for masker durations from 5 to 200 ms. The effect of masker duration was found to depend more on signal level than on gap duration or masker level. Overall, the results support the idea that forward masking ca ll be modeled as a linear process, preceded by a static nonlinearity resemb ling that found on the basilar membrane. (C) 2000 Elsevier Science B.V. All rights reserved.