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I would like to comment on some characterizations of stream
segregation offered recently by Bruno Repp and to present my own,
by way of contrast.
According to Bruno,
"I believe the difference is that groups are sequential whereas
streams are interleaved:
xxxxx ooooo (groups)
Bruno's definitions are different from the ones that I use. For
me, a stream is the brain's representation of the sound emitted
by a single sound source over time. It doesn't matter whether
the sound is intermittent or continuous, interleaved with another
sound or not. A very long steady unbroken tone against a
background of silence is a stream. There are, of course,
variations in the structures of streams (e.g., steady vs.
intermittent, temporally random vs. rhythmic, etc.) and in the
presence of other sounds.
I don't believe that the choice of the definition that I use is
arbitrary. If all these types of streams have common factors
that influence them and common consequences of their formation --
as I believe they do -- this justifies treating them as variants
of the same type of mental representation.
Bruno also wrote:
"Stream segregation, I believe, is a phenomenon revealed by
judgments of the auditory scene that are made either directly (by
stating how many streams are heard) or indirectly (by judging
timing, or the like). In other words, streams are conscious
of the auditory input, or parsings that have consequences in
There is an issue about whether streams are necessarily
conscious. Obviously when a listener is asked to made a
judgement about a stream or about a sound within a stream, the
brain has to create a conscious representation of it. However I
believe that the same grouping processes that form streams also
operate on sound that is not being attended to by the listener.
Studies using the mismatch negativity paradigm with evoked EEG
potentials, carried out by Elyse Sussman at the Albert Einstein
School of Medicine, seem to show that this is so. Attention
seems to strengthen the stream segregation but not to be
essential for obtaining it. An old study I did with Alex
Rudnicky appeared to show that the perceptual organization of
sounds that listeners were trying to exclude from attention made
it easier or harder for them to do so. Therefore they were
organized without being at the focus of attention.
Bob Carlyon has evidence that seems to show that attention is a
prerequisite for streaming; so the question is not resolved.
However there is a strong possibility that grouping is
Still, it is likely that we do not perceive any conscious streams
until this presumed preattentive grouping gets together with
top-down processes to produce the conscious representation. It
is possible -- even likely -- that in some cases, top-down
processes can over-ride the groupings that result from bottom-up
Albert S. Bregman, Emeritus Professor
Dept of Psychology, McGill University
1205 Docteur Penfield Avenue
Montreal, QC, Canada H3A 1B1
Phone: +1 (514) 398-6103
Fax: +1 (514) 398-4896
Phone & Fax: +1 (514) 484-2592