Effect of amplitude modulation on profile detection

The effect of amplitude modulation on profile detection was examined in three experiments. The observer's task was to determine in which observation interval an increment was added to the 1000-Hz target component of a multitone complex in which the components were equally spaced on a logarithmic frequency scale from 200 to 5000 Hz. The target was unmodulated throughout the study. In some conditions, all nontarget components of the standard were modulated in phase; in other conditions, they were modulated with random phase. In experiment 1, the threshold was measured as a function of the modulation rate. The results show that, at low modulation rates, 5 Hz for example, modulation elevates threshold by about 13 dB. The threshold decreases as the modulation rate increases, with the threshold elevation being only 3 dB at 80 Hz. In experiment 2, threshold was measured as a function of modulation depth for both 21- and 5-component complexes. The results show that for a 5-Hz modulation rate the threshold decreases as the modulation depth decreases, and that the rate of decrease is greater for the 21-component complex than for the 5-component complex. In experiment 3, the effects of random-phase modulation were explored; the phase of the modulation waveform was randomly chosen for each component. The results show that there is no difference between in-phase and random-phase modulation when each component occupies a different critical band. If, however, two or more components occupy the same critical band, then randomizing the phase of modulation reduces the effective depth of modulation within that critical band, and the effect of modulation is thereby lessened.