Figure 2.

The effects of pitch proximity and tempo on determining the number of auditory objects in sequential streams of sounds. (A) When a higher tone repeats at a regular interval and a lower tone repeats at half the tempo of the higher tone, and they are arranged as in (A), all of the tones are perceived to come from a single sound source (as depicted by the dotted lines) and a gallop rhythm is heard. (B) When the higher and lower tones are sufficiently separated in frequency, they can no longer be integrated into a single stream. Two auditory objects are heard, one a repeating high tone and one a repeating low tone, and no gallop rhythm is perceived. This demonstrates that the auditory system expects a single sound source to remain reasonably consistent in pitch. (C) When the tempo of the sequence in (B) is slowed down, again the two pitches can be integrated into a single auditory object, and the gallop rhythm is heard again, consistent with the idea that the auditory system expects an auditory object to change pitch slowly. (Adapted from [41].)