Mechanism and frequency distribution of two-tone suppression in forward masking

These experiments are concerned with the frequency distribution of two-tone suppression in forward masking. In each experiment the threshold of a 20-msec probe tone following a 300-msec masker was determined as a function of probe frequency. In experiments I and II this was done for two maskers (a single tone with frequency fm, and that tone with a suppressive tone added) which had previously been equated for their masking effectiveness at fm by adjusting the levels of the components at fm. The maskers were equally effective only in a restricted frequency region around fm, a result which is not consistent with the idea that suppression is equivalent to a simple reduction in level of the suppressed tone. In experiment III probe thresholds were measured for a fixed tone at 2 kHz with and without a suppressing tone whose frequency was systematically varied. The suppression, measured as the reduction in probe threshold produced by adding the suppressing tone, was found to have two components. One of these was limited to a narrow range of probe frequencies around 2 kHz and is attributed to a qualitative change in the cues available to the observer when a suppressing tone is added to the masker. The other component shifted in frequency as the suppressor frequency was altered and is explained as a reduction in level of only part of the excitation pattern of the suppressed tone.