Band-limiting experiments were performed at five spectrum level differences (10, 20, 30, 40 and 50dB) between the 1st and 2nd bandpass noise maskers for simultaneous, forward and backward masking. Masked threshold for 2kHz sinusoidal signal (duration 15ms and rise-fall time 5ms) was studied as a function the two bandpass maskers (duration 600ms and rise-fall time 10ms or 5ms), and its center was fixed at 2kHz, and the 1st masker level was fixed at 30dB SPL.
The results were as follows.
1) Masked threshold for a 2kHz sinusoidal signal was increased and reached its magnitude for the bandwidths greater than ten times of critical band at the delay time of 8ms in the simultaneous masking, and the critical bands estimated at the delay time 300ms in the simultaneous masking were described by a level-dependent critical band with upward spread.
2) The signal was always and immediately preceded (in the backward masking) or immediately followed (in the forward masking) the 1st masker. The 1st masker-plus-signal and the 2nd masker or the signal and 2nd masker were presented simultaneously. The suppression effect was increased at supracritical bands and the suppression effect at the masker band width 4kHz, level differences between the 1st and 2nd masker, from 10dB to 30dB, increased for the forward masking, but level differences between the 1st and 2nd masker, from 30dB to 50dB were increased rapidly for the backward masking.
3) These differences in the suppression effect observed in the forward and backward masking was consistent with an interpretation that the suppression in the forward masking was determind masking by a reduction of the effective level of the masker in the peripheral processes, and that the suppression in the backward masking was dominated by lateral inhibition of the central processes, possibly the dorsal cochlear nucleus.
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