Modulation detection interference: some spectral effects.

Several experiments are described in which the threshold for detecting 10-Hz amplitude modulation (AM) of a 2000-Hz signal carrier was measured in the presence of a masker that consisted of one to ten carrier frequencies. The masker was either unmodulated or amplitude modulated at a depth of 0.5 and a frequency of 10 Hz. In experiment 1, threshold was measured as a function of the frequency of a single masker carrier. The patterns of the interference effect were similar whether the masker was modulated or not, suggesting that some of the interference was due to a within-channel effect (spread of excitation). In experiment 2, it was shown that a masker consisting of two carrier frequencies could produce more interference than either carrier alone. However, there was generally no additional interference when more than two carriers were presented, at least up to the ten tested here (experiments 3 and 4). In experiment 5, the masker carrier frequencies were either harmonically or nonharmonically related to the signal carrier frequency. There was little difference in the size of the interference effect between the two maskers, suggesting that harmonicity may not play an important role in modulation detection interference.