Backward, simultaneous, and forward masking as a function of signal delay and frequency.

Backward, simultaneous, and forward masking were investigated in 7 normal-hearing adults using a 1-kc/s sinusoid masker of 250 msec duration and 60 db SPL. 10-msec sinusoidal signals (probes) were varied in frequency (.4, .8, .9, .95, 1.05, 1.1, 1.2, 1.6 kc/s) and temporal location, delta t, prior to (-250, -150, -40, -20, -10, -2 msec re: masker onset), during (2, 10, 20, 40, 125, 250 msec re: masker onset), or following (2, 10, 20, 40, 125, 250 msec re: masker offset). The double random staircase psychophysical method (Cornsweet, 1962) was used in all conditions. Monotic thresholds were obtained for the backward and forward masking conditions. Dichotic and monotic thresholds were collected for the simultaneous condition. Results showed: (a) Backward masking. The greatest masking (up to 25 db) occurred between 0.0 and -40 msec when the signal probe was within +/- .2 cps of the masker. There was a linear decrease in masking as a function of delta t; masking effects extended to 250 msec. (b) Forward masking. The data revealed no masking beyond 125 msec delta t, an asymmetry in the masking pattern, relatively large threshold shifts (up to 33 db) when delta t was less than 40 msec and the signal was approximately +/- .2 kc/s within the masker. (c) Simultaneous masking. Overshoot occurred at onset and offset for both dichotic and monotic conditions; onset overshoot was larger in both conditions. The data revealed more forward masking than backward masking at equivalent absolute delta t values. Data were discussed in terms of cochlear vibration patterns and hypothesized central mechanisms.