Streaming and Jitter

The following demo explores the effect of temporal regularity, or rhythmicity, on stream segregation. It uses the stimuli used in the study by Rajendran and colleagues (2013 JASA-EL).

This demo works well with recent versions of Google Chrome, Firefox and Safari.

Rapidly alternating (ABAB...) tones are usually perceived, at least initially, as a single "trill"-like sound, but after a while the single auditory stream may appear to break into two, with either the A-A- or the -B-B sequence dominating the percept, and the other tone sequence becoming a "background" sound. The wider the frequency separation, the quicker generally the break-up into two streams. Here are two example ABAB sequences with frequency separations of either 1 semitone (heard by most as one stream throughout) or 10 semitones (heard by most as two streams after only a second or so).

Wider frequency separation is not the only factor that increases the likelihood that two streams are perceived instead of one. Another factor which seems to play a role is temporal irregularity. Here you can try 3 second long sequences of varying frequency separation, and you can also introduce varying degrees of temporal irregularity ("jitter") into the higher frequency tone sequence.

The five jitter values on offer replicate the values used in the experiment by Rajendran et al., in which temporal jitter was chosen from a uniform distribution around the "expected" timing of each B tone once every 200 ms.

What to try: Start the sequence with a 4 semitone separation and zero jitter. If your hearing is like mine, most of the time you will hear that as a single stream, of alternating high and low notes. Now switch the RMS jitter from 0 to the maximum, 18.48 ms. If your hearing is anything like mine, your percept should relatively quickly flip to have two different streams, one stream of regularly spaced low pips and one stream of irregularly spaced high pips. And when you switch back to zero jitter, the two streams may fuse again into a single low-high-low-high pattern. You can experiment with larger or smaller frequency steps. Larger frequency steps should make two stream percepts more likely even if there is no jitter. Larger jitter values should make two streams more likely even if the frequency separation is relatively large.

delta F (semitones) RMS Jitter (ms)
1 0
4 2.31
7 4.62
10 9.24