Amplitude modulation rate discrimination with sinusoidal carriers.

Discrimination of the change in rate of sinusoidal amplitude modulation was investigated for sinusoidal carriers. The just-noticeable change in the modulation rate was measured as a function of (1) carrier frequency (fc = 500, 1000, 2000, or 4000 Hz), (2) initial modulation rate (fm = 10, 20, 40, 80, 160, or 320 Hz), and (3) stimulus duration (T = 62, 125, 250, 500, 1000, or 2000 ms). For modulation rates less than 320 Hz, the thresholds for a change in modulation rate were not affected by changing the carrier frequency. When fm = 320 Hz, the threshold for changes in the modulation rate ordered in the same way as the threshold for changes in the frequency of a pure tone, suggesting that the primary cue for discriminating a change in modulation rate was the frequency of either sideband. As stimulus duration increased to a certain T (critical duration), the thresholds decreased by more than a factor of 2. Above that duration, the thresholds for changes in modulation rate decreased only slightly or remained constant. The critical duration corresponded to about five cycles of modulation, at least for the slower modulation rates.