DETECTION OF CHANGES IN MODULATION DEPTH OF A TARGET SOUND IN THE PRESENCE OF OTHER MODULATED SOUNDS

These experiments are concerned with the comodulation masking release (CMR) and modulation discrimination interference (MDI) that occur when modulated carriers are added to a target modulated sound at frequencies remote from the target frequency. Thresholds were measured for detecting increments or decrements in the modulation index (m) of a 1000-Hz carrier (the target) sinusoidally amplitude modulated (SAM) at a 10-Hz rate, with that carrier presented either alone, or with a variety of added sounds. In experiments 1-3, the target had a standard value of m of 0.5. The added sounds had carrier frequencies of either 550 and 1550 Hz or 230 and 3300 Hz, and they were either unmodulated (m = 0) or SAM at a 10-Hz rate (in phase with the target), with m = 0.25, 0.5, or 0.75. In experiment 1, the target and added sounds were gated on synchronously for a duration of 1000 ms. The added sounds produced MDI, and the amount of MDI increased progressively with increases in the modulation index of the added sounds. In experiment 2, the added sounds were gated on 500 ms before and gated off 50 ms after the 100-ms target sound. This reduced the MDI, but substantial amounts of MDI remained, especially for the 550- and 1550-Hz added sounds. In experiment 3, the target and added sounds were gated on synchronously for 1550 ms, and the increment or decrement in m was gated on for 1000 ms after a delay of 500 ms. This generally gave rise to more MDI than was found in experiment 2. In experiment 4, the values of m for the target and added sounds were 0.75/0.75, 0.75/1.0, and 1.0/1.0, respectively. The added sound consisted of carriers at 230, 550, 1550, and 3300 Hz. All of the temporal conditions used in experiments 1-3 were examined. The added sounds produced MDI in some conditions and CMR in others. The largest CMR occurred in conditions where the modulation index for the added sounds was 1.0, and the whole target sound was delayed relative to the onset of the added sound. The results are discussed in terms of the factors influencing MDI and CMR, and in terms of the possible role of perceptual grouping mechanisms.