MODULATION DETECTION INTERFERENCE UNDER CONDITIONS FAVORING WITHIN-OR ACROSS-CHANNEL PROCESSING

A series of experiments was conducted to examine modulation detection interference (MDI) under two general conditions: one where the processing was likely to be across channel (2.0-kHz signal, 4.0-kHz masker), and the other where the possibility of within-channel processing was much more likely (2.0-kHz signal, 1.8-kHz masker). In the first experiment, MDI was measured as the modulation frequency of the signal and masker was increased from 5 to 100 Hz. The amount of MDI decreased as modulation frequency increased from 5 to 20 Hz, beyond which it remained approximately constant. This effect of common modulation frequency was somewhat smaller for the 1.8-kHz masker. The second experiment examined the effects of relative modulator phase between the masker and signal for modulation frequencies of 10 or 100 Hz. In general, there was not a consistent effect of phase. However, when the signal and masker modulators were in-phase, there was a tendency, on average, for the amount of MDI to be at a maximum when the masker frequency was 1.8 kHz and at a minimum when it was 4.0 kHz. In the third experiment, increases in masker modulation depth usually resulted in nearly proportional increases in signal modulation depth at threshold; this was true for both masker carriers, although the increase was slightly greater for the 1.8-kHz masker. The final experiment examined the effects of level for conditions where the signal and masker carriers were either equal or unequal in level. When they were equal in level, the amount of MDI increased somewhat with increases in level for both masker carriers. Comparing the amount of MDI in these conditions with those where the carriers were unequal in level revealed that the amount of MDI was considerably less when the signal was more intense than the masker. This occurred almost exclusively for the 1.8-kHz masker, however, suggesting that it may reflect a within-channel effect. Taken together, the results from the four experiments suggest that the processing underlying MDI was generally the same in the presence of both the 1.8- and 4.0-kHz masker carriers, although within-channel processing via spread of excitation probably influenced some of the results with the 1.8-kHz masker.