Previous study suggested that AM depth discrimination was influenced by the number of AM cycles, not by the duration of stimulus. AM detection and AM depth discrimination were measured as a function of the number of AM cycles for modulation rates of 10, 20, 40, 80, 125, 160, and 320 Hz. The carrier was a broadband-noise (10 kHz lowpass), and was either gated with the modulator (gated carrier) or presented in the middle of an unmodulated noise that began 250 ms earlier and ended 250 ms later than the modulator (continuous carrier). The overall level of each presentation was randomized within 6 dB range around 65 dB SPL. There was a significant improvement with the continuous carrier at smaller number of cycles (2 and 4 modulation cycles) as compared to the gated carrier condition for the lower modulation rates (10, 20, 40, and 80 Hz),. In contrast, the thresholds were not affected by the different carrier type when higher modulation rates (125, 160, and 320 Hz), were used. These results suggest that there might be a different temporal integration process for lower and higher modulation rates.

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