Audio Demonstration of the Phenomenon of Auditory Stream Segregation

A sequence of tones that alternate between two frequencies (ABAB…) can give rise to two completely different percepts depending on the frequency separation (delta-F) between the A and B tones:

• If delta-F is relatively small (e.g., 1 semitone, about 6%), the sequence is usually perceived as a single stream of tones alternating in pitch (listen)
  

• However, if delta-F is large (e.g., 9 semitones), the sensation of pitch alternation is lost and most listeners perceive  two streams tones, each at a constant pitch  (listen). This is "stream segregation".

• At intermediate delta-Fs, an exciting thing happens: the sequence is usually perceived as a single stream at first, but after a few seconds of uninterrupted listening, it splits into two streams (listen). Another way to say this is that the probability of experiencing segregation usually increases over time (see). This effect is known as the "build-up" of segregation. We (Micheyl et al., 2005; see also) identified multi-second adaptation as a possible neural origin for this perceptual phenomenon. 
• Following the initial "build-up" phase, it is not uncommon for the percept to spontaneously switch back and forth between "one stream" and "two streams". Such switches in auditory percept are reminsicent of bistable percepts induced by ambiguous visual figures such as the Necker cubes.
  

Why study this? Because it is an important facet of our ears and brain’s remarkable ability to parse complex acoustic scenes, which likely plays an important role in everyday life.