Auditory stream segregation on the basis of amplitude-modulation rate

In this study, auditory stream segregation based on differences in the rate of envelope fluctuations--in the absence of spectral and temporal fine structure cues--was tested. The temporal sequences to segregate were composed of fully amplitude-modulated (AM) bursts of broadband noises A and B. All sequences were built by the reiteration of a ABA triplet where A modulation rate was fixed at 100 Hz and B modulation rate was variable. The first experiment was devoted to measuring the threshold difference in AM rate leading subjects to perceive the sequence as two streams as opposed to just one. The results of this first experiment revealed that subjects generally perceived the sequences as a single perceptual stream when the difference in AM rate between the A and B noises was smaller than 0.75 oct, and as two streams when the difference was larger than about 1.00 oct. These streaming thresholds were found to be substantially larger than, and not related to, the subjects' modulation-rate discrimination thresholds. The results of a second experiment demonstrated that AM-rate-based streaming was adversely affected by decreases in AM depth, but that segregation remained possible as long as the AM of either the A or B noises was above the subject's AM-detection threshold. The results of a third experiment indicated that AM-rate-based streaming effects were still observed when the modulations applied to the A and B noises were set individually, either at a constant level in dB above AM-detection threshold, or at levels at which they were of the same perceived strength. This finding suggests that AM-rate-based streaming is not necessarily mediated by perceived differences in AM depth. Altogether, the results of this study indicate that sequential sounds can be segregated on the sole basis of differences in the rate of their temporal fluctuations in the absence of other temporal or spectral cues.