Loss of rhythmic information as a result of stream segregation.
When a repeating cycle breaks into two streams, the rhythm of the full sequence is lost and replaced by those of the component streams (Panel 1). This change can be heard clearly if the rhythm of the whole sequence is quite different from those of the component streams. In the present example, we use triplets of tones separated by silences, HLH-HLH-HLH-... (where H represents a high tone, L a low one, and the hyphen corresponds to a silence equal in duration to a single tone). We perceive this pattern as having a galloping rhythm.
An interesting fact about this pattern is that when it breaks up into high and low streams, neither the high nor the low one has a galloping rhythm. We hear two concurrent streams of sound in each of which the tones are isochronous (equally spaced in time). One of these streams includes only the high tones (i.e., H-H-H-H-H-...), joined by dotted lines in Panel 1.
The apparent silences between H tones arise from two sources: Half of them are supplied by the actual silence, labeled S in the figure, that follows the second H tone in the HLH- sequence. The other apparent silences derive from the fact that perceptual organization has removed the low tones from the high-tone stream leaving behind gaps that are experienced as silences. These are labeled by the letter "G" in the figure.
Similarly, the low stream (Panel 1, bottom) is heard as containing only repetitions of the low tone, with three-unit silences between them (i.e., ---L---L---L--- L---...). Again, one of these silences (labeled S) is supplied by the inter-triplet silence of the HLH-HLH- sequence, but the other two (labeled G) are created in our perception to account for the two missing H tones, which have disappeared into their own stream. So stream segregation causes the original triplet rhythm, illustrated in Panel 2, to disappear and be replaced by the two isochronous rhythms of Panel 1, a more rapid high-frequency one and a slower low-frequency one. The change also affects the perceived melody. When we hear the sequence as a single stream, its melodic pattern is HLH-. This disappears when segregation occurs. The demon stration shows that both rhythms and melodies occur mainly within streams, and when the stream organization changes, so do the perceived melodies and rhythms.
It also shows the importance of speed and frequency separation of sounds in the formation of sub-streams. Segregation is favored both by faster sequences and by larger separations between the frequencies of high and low tones. The role of speed is seen as the sequence gradually speeds up. At slow speeds there is no segregation, but at high speeds there may be, depending on the frequency separation. In the first example, the H and L tones are far apart in frequency (about 18 semitones), as in Panel 1. At the slowest speed, people hear the one-stream percept, but as the sequence accelerates, a point is reached (which may vary from person to person) where stream segregation based on frequency differences inevitably occurs. In the second example (Panel 2), the H and L tones differ by only one semitone and the sequence usually fails to break into two separate streams even at the highest speed.
One can view speed as decreasing the temporal separation of the tones that lie in the same frequency range. Tones are believed to group according to their separations on frequency-by-time coordinates. When the speed is slow and the frequencies are close together, each tone's nearest neighbor is the tone of the other frequency; so a single-stream percept, of the type shown in Panel 2, is favored, as indicated by the dotted lines that connect the tones. When a high speed is coupled with a large frequency separation, the combination of these factors places the tones of the same frequency nearer together than they are to the tones of the other frequency. This causes a grouping by frequency to occur, creating the two stream percept of Panel 1.
First, the tones are far apart in frequency.
Next, the frequency separation is small.