Re: streams and groups (Al Bregman )

Subject: Re: streams and groups
From:    Al Bregman  <BREGMAN(at)PSYCH.MCGILL.CA>
Date:    Sun, 13 May 2001 21:03:51 -0400

Dear Bruno, Chuck, and List:

I guess I don't know exactly how the word "group" is used or
whether different people use it in the same way.  Here is my own
take on the terminology of streams and units.

For me, a stream is a mental construct, portraying either single
or multiple sounds extended in time.  The critical factor is that
they are treated as an extended output of a single sound source.
Whether they truly come from single or from multiple sound
sources is a physical or environmental description, not a
psychological one.  (Of course, the psychophysical question is
how the physical descriptions map onto the psychological ones.)

Within streams there can be "units".  A unit is a perceptual
entity, perceived whenever the sound is partitioned by
discontinuities.  The unit may emerge because it is bracketed by
silences, by its own sudden rise in intensity, or the sudden rise
in intensity that defines the beginning of the next unit (see the
work of Yoshitaka Nakajima).

A musical "note" (in the auditory sense) is an unit within a
melody; a single footstep is a unit within a sequence of steps.
A unit is a perceptual entity that is intended by Mother Nature
to correspond with a single "event" in nature (a single step of
an animal; a single activation of a piano key, a chirp of a
bird).

Units--being psychological--are the outputs, not the inputs, of
the auditory system.  The temporal variation of acoustic energy
is the input.

Perhaps the word "group" should be used to mean a cluster of
units within a stream -- perceived as a cluster due to the fact
that the acoustic differences are greater between the groups than
within the groups, or to the fact that there exist
discontinuities or silences that are greater between successive
groups than between the successive members of the same group.

It appears from the complex rhythms in music that groups can be
hierarchically organized as well (see the work of Mari Jones and
others on the representation of rhythms).

Like the streams in which they reside, "groups",  as defined
above, are psychological entities, not physical ones.  So they
could, under unusual circumstances, actually include sounds
coming from physically different sources.  However, the purpose
(from an evolutionary perspective) of this grouping of units
would appear to be the representation of environmental events
that have a common physical cause (e.g., the grouping of a set of
running steps within a longer set of walking steps).

I welcome any comments,

Al

-------------------------------------------------
Albert S. Bregman, Emeritus Professor
Dept of Psychology, McGill University
1205 Docteur Penfield Avenue
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