Objects Compared to Streams
Itisalsoabouttheconceptof‘‘auditorystreams.’’Anauditorystreamisour
perceptual grouping of the parts of the neural spectrogram that go together.
To see the reasons for bringing in this concept, it is necessary to consider the
relations between the physical world and our mental representations of it. As
we saw before, the goal of scene analysis is the recovery of separate descrip-
tions of each separate thing in the environment. What are these things? In vi-
sion, we are focused on objects. Light is reflected off objects, bounces back and
forth between them, and eventually some of it reaches our eyes. Our visual sense
uses this light to form separate descriptions of the individual objects. These
descriptions include the object’s shape, size, distance, coloring, and so on.
Then what sort of information is conveyed by sound? Sound is created when
things of various types happen. The wind blows, an animal scurries through a
clearing,thefireburns,apersoncalls.Acousticinformation,therefore,tellsus
about physical ‘‘happenings.’’ Many happenings go on at the same time in the
world, each one a distinct event. If we are to react to them as distinct, there has
to be a level of mental description in which there are separate representations
of the individual ones.
I refer to the perceptual unit that represents a single happening as an audi-
torystream.Whynotjustcallitasound?Therearetworeasonswhytheword
stream is better. First of all a physical happening (and correspondingly its
mental representation) can incorporate more than one sound, just as a visual
object can have more than one region. A series of footsteps, for instance, can
form a single experienced event, despite the fact that each footstep is a separate
sound. A soprano singing with a piano accompaniment is also heard as a coher-
ent happening, despite being composed of distinct sounds (notes). Furthermore,
the singer and piano together form a perceptual entity—the ‘‘performance’’—
that is distinct from other sounds that are occurring. Therefore, our mental
representations of acoustic events can be multifold in a way that the mere word
‘‘sound’’ does not suggest. By coining a new word, ‘‘stream,’’ we are free to
load it up with whatever theoretical properties seem appropriate.
A second reason for preferring the word ‘‘stream’’ is that the word ‘‘sound’’
refers indifferently to the physical sound in the world and to our mental expe-
rience of it. It is useful to reserve the word ‘‘stream’’ for a perceptual represen-
tation, and the phrase ‘‘acoustic event’’ or the word ‘‘sound’’ for the physical
cause.
I view a stream as a computational stage on the way to the full description of
an auditory event. The stream serves the purpose of clustering related qualities.
By doing so, it acts as a center for our description of an acoustic event. By way
of analogy, consider how we talk about visible things. In our verbal descrip-
tionsofwhatwesee,wesaythatanobjectis red, or that it is moving fast, that it
isnear,orthatitisdangerous.Inotherwords,thenotionofanobject,under-
stood whenever the word ‘‘it’’ occurs in the previous sentence, serves as a cen-
ter around which our verbal descriptions are clustered. This is not just a
convenience of language. The perceptual representation of an object serves the
same purpose as the ‘‘it’’ in the sentence. We can observe this when we dream.
When, for some reason, the ideas of angry and dog and green are pulled out
220 Albert S. Bregman