Place synchrony and the masking-level difference

Detection threshold of a masked signal can be lowered in two ways: First, by varying the interaural configuration of the signal and masker, the MLD, and second, by a noise masker which has the same envelope at different frequencies, place synchrony. This experiment was performed to determine if these two processes are independent. The signal was a tone burst. The masker, a continuous 100-Hz-wide noise band centered at the signal frequency, was created by modulating a tone with a 50-Hz low-pass noise. A second noise band, centered at a different frequency, had either the same envelope as the first noise or a different envelope. The MLD was measured for two conditions; when the two noise bands in the homophasic condition were synchronous, and when they were not. MLDs measured in the synchronous noise were substantially smaller than those in the independent noise or in the 700-Hz noise alone. It is probable that the detection threshold shifts which are due to the MLD and those due to place synchrony are not independent. The MLD may be a manifestation of place synchrony. [Work supported by NIH.]