Temporal factors and suppression effects in backward and forward masking.

We discuss several experiments examining the influence of temporal parameters on suppression effects in backward and forward masking. The signal is always a brief 10-ms 2-kHz sinusoid; the masker a narrow band of noise of 40-dB spectrum level, 200-Hz wide, centered at the signal frequency. In some conditions, the presence of a second band of noise of the same spectral level in the region of 2300--3700 Hz appears to suppress the effects of the masker. Changes in the amount of suppression are examined as functions of the delay and duration of the suppressor (experiments 1 and 2). Adding the suppressor during the 50-ms interval nearest the signal produces changes in the signal threshold that are similar to those produced by reducing the level of the masker during this interval for both backward and forward masking (experiment 4). The similarity of these results suggests the operation of peripheral processes common to both backward and forward masking. However, if one increases the duration of the suppressor beyond this 50-ms interval there is no effect on forward masking, but large additional reductions in backward masking. This difference, in conjunction with other recent experiments, suggests the operation of additional central processes in backward masking. For some observers, these additional processes appear to operate over fairly long periods of time (250-500 ms). Such long durations are inconsistent with the estimates of integration time (less than or equal to 200 ms) obtained for these same observers (experiment 3).