Modulation detection interference using random and sinusoidal amplitude modulation.

The detection of amplitude modulation (AM) of a target sound is made more difficult by the presence of a modulated sound some spectral distance from the target. This effect (modulation detection interference, or MDI) was examined for stimuli with random amplitude modulations (RAM) and for sounds with sinusoidal amplitude modulations (SAM) as a function of average modulation depth (m) of the interferer. In an experiment comparing comodulated and independent RAM targets and interferers, the amount of interference was not related to the modulation coherence of the target and interferer. Elevations in AM threshold increased as a function of m in a similar way for both conditions. The MDI for RAM and SAM targets and interferers was also compared. While no difference was found for AM detection of RAM and SAM, MDI was found to be greater for the RAM stimuli than for the SAM stimuli. A subsidiary experiment comparing RAM and SAM modulation depth discrimination indicated that RAM discrimination is more difficult than SAM discrimination. Taken together, these results are quantitatively consistent with a mechanism resembling AM discrimination as the underpinning of MDI in conditions where the target and interferer are synchronously gated.