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

Abstract

The frequency distribution of 2-tone suppression in forward masking was studied in humans. The threshold of a 20 ms probe tone following a 300 ms masker was determined as a function of probe frequency. In experiments 1 and 2 this was done for 2 maskers (a single tone with frequency fm, and that tone with a suppressing tone added) which were 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 3 probe thresholds were measured for a fixed tone at 2 kHz with and without a supprssing tone whose frequency was systematically varied. The suppression, measured as the reduction in probe threshold produced by adding the suppressing tone, had 2 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.