Effects of combining maskers in modulation detection interference.

The threshold for detecting 10-Hz amplitude modulation of a 2000-Hz carrier was measured in quiet, in the presence of an unmodulated masker, and in the presence of an amplitude-modulated masker. Two experiments were run; in each, the masker consisted of one or two sinusoidal carriers (chosen from among the frequencies of 800, 1600, 2400, and 3200 Hz). In experiment 1, the modulation rate of the masker ranged from 2 to 80 Hz. The "tuning" in the modulation domain was not affected much by the masker carrier frequency or the increase from one to two carriers. The amount of interference, however, was sometimes greater in the two-carrier condition, although this resulted primarily from the presence of the carriers and not from their modulation. In experiment 2, the modulation rate of each single-carrier masker ranged from 2 to 80 Hz (as in experiment 1), but for the two-carrier conditions, all possible combinations of two carriers (2400 and 3200 Hz) and three masker rates (5, 10, and 20 Hz) were evaluated. In general, the combination of two modulated carriers did not produce more interference than that produced by the more interfering carrier presented alone. Thus the results from both experiments provide little evidence for an additivity of modulation detection interference.