The development and decline of forward masking.

Three experiments investigated the forward masking of brief sinusoids by bursts of noise for masker durations of between 5 and 320 ms and for masker-signal delays between 5 and 40 ms. Experiment 1 showed that, for a 2-kHz signal and a 5-ms delay, long-duration maskers produced much more masking than brief maskers of the same spectrum level. As masker-signal delay was increased, thresholds dropped more rapidly for long than for brief maskers, so that at a delay of 40 ms the difference between thresholds produced by long and by brief maskers was reduced. Experiment 2 showed that, for a 2-kHz signal, the transition from simultaneous masking to forward masking was accompanied by a large drop of about 23 dB in threshold. For a 250-Hz signal this large drop did not occur until masker-signal delay was increased from 10 to 20 ms. For shorter masker-signal delays, brief 250-Hz maskers produced thresholds higher than those produced by longer maskers. The results of Experiment 2 are consistent with the idea that ringing on the basilar membrane causes the responses to masker and signal to overlap for delay times up to 10 ms. Experiment 3 compared the recovery functions for 3 markers of different durations, where the masker levels were adjusted so that each produced equal masking of a 2-kHz signal at a delay of 5 ms. Brief intense maskers produced slower recovery than longer weaker ones, indicating that the recovery from a given amount of forward masking depends on how that masking developed. The implications of this finding for attempts to explain forward masking in terms of short-term adaptation are discussed.