Streaming with Alternating Tones

When two tones are played alternately at a fixed, slow, rate, the result is a simple melody consisting of the two alternating tones. However, if the rate of presentation is fast enough and the frequency separation between the two tones is large enough, the melody breaks down into two streams, each consisting of tones of one frequency.

The following demonstration illustrates streaming. The horizontal slider changes the rate of presentation, while the vertical slider changes the frequency difference between the two tones. Start at the slowest presentation rate (leftmost position of the horizontal slider) and the smallest frequency difference (lowest position of the vertical slider). Now move to the fastest presentation rate and the largest frequency difference. What happens? Try to map the range of parameters in which the splitting of the sequence into two streams occurs.

Alternating Tones
Slide to change the tempo (rate = 6 tones/s)
Slide to change the frequency difference (df = 1 semitone)