Modulation detection interference: Effects of concurrent and sequential streaming

The presence of amplitude fluctuations in one frequency region can interfer e with our ability to detect similar fluctuations in another (remote) frequ ency region. This effect is known as modulation detection interference (MDI ). Gating the interfering and target sounds asynchronously is known to lead to a reduction in MDI, presumably because the two sounds become perceptual ly segregated. The first experiment examined the relative effects of carrie r and modulator gating asynchrony in producing a release from MDI. The targ et carrier was a 900-ms, 4.3-kHz sinusoid, modulated in amplitude by a 500- ms, 16-Hz sinusoid, with 200-ms unmodulated fringes preceding and following the modulation. The interferer (masker) was a 1-kHz sinusoid, modulated by a narrowband noise with a 16-Hz bandwidth, centered around 16 Hz. Extendin g the masker carrier for 200 ms before and after the signal carrier reduced MDI, regardless of whether the target and masker modulators were gated syn chronously or were gated with onset and offset asynchronies of 200 ms. Simi larly, when the carriers were gated synchronously, asynchronous gating of t he modulators did not produce a release from MDI. The second experiment mea sured MDI with a synchronous target and masker and investigated the effect of adding a series of precursor tones, which were designed to promote the f orming of a perceptual stream with the masker, thereby leaving the target p erceptually isolated. Four modulated or unmodulated precursor tones present ed at the masker frequency were sufficient to completely eliminate MDI. The results support the idea that MDI is due to a perceptual grouping of the m asker and target, and show that conditions promoting sufficient perceptual segregation of the masker and target can lead to a total elimination of MDI . (C) 200/ Acoustical Society of America.