Subject: Re: perceptual segregation of sound From: Mark Every <m.every@xxxxxxxx> Date: Thu, 4 May 2006 16:23:19 +0100Dear List, Many thanks to all contributors for their enlightening replies to my initial question, this has been a very interesting discussion. > are we really capable of perceptually segregating multiple sources concurrently, or are we just focusing our attention on one source, and then shifting it very quickly to another source? I would like to summarise and reply to some comments raised, though mainly by conjecture on my part. Firstly, examples have been given (e.g. listening to music) whereupon repeated exposure to a sound and use of top-down processes, additional information is extracted from perceptual streams that were not initially the focus of attention. To make a loose analogy, repeated listening must be like learning a foreign language; we start off learning the most useful words/sounds and, over time, as these become part of our vocabulary, we redirect our attention at more subtle structures and relationships between words/sounds. However, it is evident that we can form multiple perceptual streams even from completely unfamiliar sounds, so let's try to isolate perceptual stream segregation from any additional complications inherent in repeated listening. Brian puts it thus: "top down processes are useful for resolving ambiguous cases". John's remark that: "survival requires an animal's sensory organs to produce a timely response to environmental information" implies that we should maximise the "potential evolutionary benefit" of perceived information in the shortest possible time. I would imagine that to this end, using all processing resources is better than using only some of them, so it would make sense to use spare resources to analyse any content that is not the main focus of attention (the basis of the "perceptual load" theory of selective attention that Erick mentioned). Erick's comments about the sensory buffer are also interesting in light of the above mentioned topic of repeated listening, since we can repeatedly process a sound in short-term memory even if it was physically heard once. However, he mentions a limit of around 4s for the sensory buffer. So, how is the situation Kevin described possible: " In my classes, I have had students who can "go back" to a sound (or sounds) they heard and extract components that they did not 'hear' when the sound was presented. In one case a student re-listened to a piece he had heard a couple of weeks previously.) "? Is 4s a typical limit for people with average memory, excluding those with photographic/eidetic memory? This was relatively clear in my mind before, but now I'm confused: what is attention? If attention can be present even at a cochlear level, then would we define it by its functionality rather than its "level in a perceptual hierarchy of processes?" Finally, to add to the arguments for preattentive structuring of sensory evidence into multiple streams, some experiments are described in (Bregman A.S., Auditory Scene Analysis, 1990, Chapter 2-Relation to Other Psychological Mechanisms: Attention) where perception of nonfocused streams in speech mixtures can extend even to recognition of words or associated meanings of words. However, as Erick pointed out, according to the "perceptual load" theory, perception of nonfocused streams will not necessarily always reach such an advanced state given different sounds and perception tasks, due to limited cognitive resources. Once again, thanks to all contributors. Mark -- Mark Every <m.every@xxxxxxxx> CVSSP, SEPS, University of Surrey