AMPLITUDE-MODULATION DETECTION OF NARROW-BAND NOISE - EFFECTS OF ABSOLUTE BANDWIDTH AND FREQUENCY REGION

Thresholds and psychometric functions for the detection of amplitude modulation were measured as a function of modulation frequency under several stimulus conditions. The first experiment investigated the relative importance of stimulus bandwidth and frequency region for amplitude-modulation detection. The stimulus bandwidth was either 200, 400, 800, or 1600 Hz. The frequency region was varied by adjusting the high-frequency cutoff of the noise to be either 600, 2200, or 4400 Hz. Temporal modulation transfer functions demonstrated the typical low-pass filter characteristic, with sensitivity to modulation decreasing with increasing modulation frequency. Time constants associated with the transfer functions were derived from low-pass filter functions fitted to the data. The time constants varied inversely with noise bandwidth (less-than-or-equal-to 1600 Hz) and were independent of frequency region. These results are consistent with estimates of temporal acuity based on previous studies of gap detection for narrow-band noise as well as estimates of temporal acuity using deterministic stimuli. In a second experiment, psychometric functions, plotted with modulation depth in dB, demonstrated somewhat steeper slopes as modulation frequency increased. The estimated slope values did not vary greatly with frequency region or noise bandwidth.