Forward and backward masking: interactions and additivity.

Abstract

Monaural masked thresholds for clicks were established on three listeners. Masking functions across selected Δt's (the time disparities between the masker and click) were determined for four masking conditions in which the 70 dB SPL masker was a 500-msec burst of random noise. These four conditions were (1) simultaneous masking, (2) forward masking (FM) at Δt's from 1 to 250 msec, (3) backward masking (BM) at Δt's from −1 to −250 msec, and (4) forward-backward masking at selected Δt's between two 500-msec noise bursts. In this last instance, the interval between bursts was varied to achieve seven interburst intervals (IBIs) ranging from 25 to 500 msec. Simultaneous masking was 3–5 dB less at the boundaries of the noise burst than it was in the middle of the noise burst. The functions for FM and for BM were not inversely symmetrical with one another, although both showed less threshold shift (TS) the further displaced in time the click was from the masker. The masking that appeared when the click was presented between two noise bursts was greater immediately adjacent to a noise burst than near the center of the IBI. During the longer IBIs (500–300 msec), the configuration of masking was equivalent to that of FM followed by BM, with slight excess appearing only in the interval 20–40 msec before the second noise burst. The configurations of FM and BM were preserved during the shorter IBIs (200–50 msec), but the TSs were greater than either type would have produced if operating alone. These TSs became larger as the IBI was shortened, so that the brief IBIs were permeated with large amounts of masking even in the middle of the IBI. This cumulation of masking was so great when the IBI was reduced to 25 msec that the masking function lost the configuration characterizing the longer IBIs. The TSs during shorter IBIs exceeded the sum of the residual effective powers of FM and BM.