Factors Influencing Sequential Stream Segregation

This paper reviews the factors that influence streaming, i.e. whether a sequence of sounds is heard as emanating from a single source (called fusion or coherence) or from more than one source (called fission or stream segregation). A traditional view is that streaming depends on "peripheral channelling", the degree to which successive sounds excite different "channels" either across frequency or across ears. However, a body of evidence suggests that other factors can play a strong role; stream segregation can occur when successive sounds are presented to the same ear and have essentially identical excitation patterns. The other factors include differences in: temporal envelope, fundamental frequency, phase spectrum and lateralisation produced by interaural time or intensity differences. Furthermore, stream segregation produced by these other factors can be obligatory, i.e. it occurs even in tasks where better performance would be achieved if successive sounds could be fused. To account for these findings, we propose the following hypothesis: the extent to which sequential stream segregation occurs is directly related to the degree of perceptual difference between successive sounds. Any sufficiently salient perceptual difference may lead to stream segregation, regardless of whether or not it involves peripheral channelling.