Auditory stream segregation for alternating and synchronous tones.

Sound sequences, such as music, are usually organized perceptually into concurrent "streams." The mechanisms underlying this "auditory streaming" phenomenon are not completely known. The present study sought to test the hypothesis that synchrony limits listeners' ability to separate sound streams. To test this hypothesis, both perceptual-organization judgments and performance measures were used. In Experiment 1, listeners indicated whether they perceived sequences of alternating or synchronous tones as a single stream or as two streams. In Experiments 2 and 3, listeners detected rare changes in the intensity of "target" tones at one frequency in the presence of synchronous or asynchronous random-intensity "distractor" tones at another frequency. The results of these experiments showed that, for large frequency separations between the tones, the probability of perceiving two streams was lower on average for synchronous than for alternating tones, and that sensitivity to intensity changes in the target sequence was greater for asynchronous than for synchronous distractors. Overall, these results are consistent with the hypothesis that synchrony limits listeners' ability to form separate streams and/or to attend selectively to certain sounds in the presence of other sounds, even when the target and distractor sounds are well separated from each other in frequency.