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 measu red in the presence of a masker that consisted of one to ten carrier f requencies. The masker was either unmodulated or amplitude modulated a t 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 mas ker 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 co uld produce more interference than either carrier alone. However, ther e 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 harmo nically or nonharmonically related to the signal carrier frequency. Th ere was little difference in the size of the interference effect betwe en the two maskers, suggesting that harmonicity may not play an import ant role in modulation detection interference.