Selectivity and Perceptual Constancy
Categorization is a key 'top-down' process which is involved in perception.
Categories simplify. Categorization has a number of functions:
- it makes complexity manageable;
- it speeds up recognition;
- it reduces effort and learning;
- it makes the most of past experience;
- it enables the inferences about further attributes (going beyond what is
'given');
- it makes events predictable;
- it supports systematization;
- it bonds social behaviour (providing shared frameworks);
- it tailors the world to our purposes;
- it makes the world seem more meaningful.
The cost of these advantages is a loss of particularity and uniqueness in
perception and recall. For Romantics, it is also regarded as inducing a
sense of distance from the world. The way we categorize phenomena seems to
be a 'natural' 'reflection of reality', leading us to forget the role of
categorization in constructing the world.
Probably the most well-known example of the cultural diversity of categories
is that Eskimos have dozens of words for 'snow' - an assertion which is
frequently attributed to Benjamin Lee Whorf. Actually, Whorf seems never to
have claimed that Eskimos had more than five words for snow (Whorf 1956,
216). However, a more recent study - not of the Inuit but of the Koyukon
Indians of the subarctic forest - does list 16 terms for snow, representing
these distinctions:
- snow
- deep snow
- falling snow
- blowing snow
- snow on the ground
- granular snow beneath the surface
- hard drifted snow
- snow thawed previously and then frozen
- earliest crusted snow in spring
- thinly crusted snow
- snow drifted over a steep bank, making it steeper
- snow cornice on a mountain
- heavy drifting snow
- slushy snow on the ground
- snow caught on tree branches
- fluffy or powder snow
(Nelson 1983, 262-3)
I do not intend to discuss the controversial issue of the extent to which
the way we perceive the world may be influenced by the categories which
are embedded in the language available to us. I have discussed the so-called
Sapir-Whorf hypothesis elsewhere. Suffice it to say that words can be
found in English, as above, to refer to distinctions which we may not
habitually make, but that this does not rule out the possibility that the
categories which we employ may not only reflect our view of the world but
may also sometimes exercise subtle influences upon it.
There is research evidence that verbal labels may influence the recall of
visual images. In a well-known experiment by Carmichael, Hogan and Walter
(1932), observers were shown simple line drawings each of which was
associated with either of two verbal captions - e.g. a drawing of two
circles linked by a straight line bore either the caption 'eye-glasses' or
the caption 'dumb-bells'. The observers were then asked to reproduce the
drawings. Their reproductions showed a strong tendency to distort the
original image to make it closer to the verbal label which had been attached
to it.
The Stroop Colour-Word Test can be used to illustrate the difficulties
which we can experience in separating labels from what they refer to
(see below). Try counting the number of green words, for instance.
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A well-known study demonstrating the influence of language on
recall is that of Elizabeth Loftus (1974 & 1979). She showed observers a
short film of a traffic accident and asked how fast the cars had been
going. However, the wording of the question differed between the two
groups asked. Those in one group were asked 'About how fast were the cars
going when they hit each other?' whilst those in the second group were
asked 'About how fast were the cars going when they smashed into each
other?' Those who had been asked the question with the term smashed
gave higher estimates of the cars' speeds than the others did. A week later,
the same observers were asked whether they had seen any broken glass (there
had been none). More than twice as many of those questioned with the word
'smashed' reported seeing the nonexistent glass as those questioned with
the word 'hit'.
It is not only with respect to categorization thay perception is described
as 'selective'. Recovering his sight after 30 years of blindness, one man
reported:
When I could see again, objects literally hurled themselves at me. One
of the things a normal person knows from long habit is what not
to look at. Things that don't matter, or that confuse, are simply shut
out of their seeing minds. I had forgotten this, and tried to see
everything at once; consequently I saw almost nothing. (Muenzinger 1942)
Perception is unavoidably selective: we can't see all there is to see.
There are of course physiological limits (both for the human species and
for individuals); some argue that there are limits to cognitive
capacity. And then there are the constraints of our locational viewpoint:
we can't see things from every angle at once.
But in addition to such physical limits we focus on salient features and
ignore details which are irrelevant
to our current purposes or interests. Selectivity thus involves
omission. Some commentators use the 'filter' metaphor - we 'filter
out' data, but this suggests a certain passivity: we may also 'seek out'
data of a certain kind.
Selective attention is assisted by redundancy: we don't always need much
data in order to recognize something. Often we can manage with minimal
visual data, making use of what is called 'redundancy'. You may know those
'blocky' pictures of famous people in which you can just about recognize
who it is. Our schemata allow us to 'fill in gaps' because we know what
should be there. So selectivity also involves addition.
Selectivity also involves organization: foregrounding,
backgrounding and rearranging features. Objects, events or situations are
'sized up' in relation to our frames of reference, and these
influence how perception is structured (Newcomb 1952, 88-96).
Gordon Allport and Leo Postman (1945; Newcomb 1952, 88-96) offered a classic account of the
selectivity of perception in their study of rumour.
Whenever an event is open to divergent interpretations, reporting it involves transforming it.
The selection, retention, reporting and retelling of events routinely involves several kinds of
transformations. All of these involve simplifying events to make them more meaningful in
terms of personal interests, needs and experience. The process is exaggerated where memory
and retelling are involved, but it is already at work in the selectivity involved in the initial
perception of an event.
Contrary to the popular idea that rumours ‘snowball’, becoming more elaborate in the telling,
psychological studies suggest that retelling tends to make accounts shorter, more concise,
more easily grasped and told. There is an increasing tendency to use fewer words. Levelling is
the selective process by which certain details are omitted. However, items of
particular interest to the reporters, which confirm their expectations or help to structure their
reports, do tend to persist.
Sharpening is the reciprocal selective process of levelling. Alongside the loss of some details,
there also tends to be a pointing-up of a limited number of details which caught the
individual’s attention, often including attention-grabbing words. Temporal sharpening
involves a tendency to describe events in the present tense. Movement is often
emphasized or introduced. Items prominent because of their relative size or quantity
tend to be retained. Labels tend to persist. Primacy effects may lead to the
retention of items coming first in a series. Familiar symbols are also likely to be retained.
Explanations may be introduced, especially to produce ‘closure’.
Underlying the selective processes of levelling and sharpening, and of transpositions,
importations and other transformations involved in retellings, Allport and Postman
(1945) argue, is the process of assimilation. This involves the influence of habits, interests and
sentiments on reporters and listeners. Aspects of a story are sharpened or levelled to make
them more consistent with what is seen as the principal theme of the story, thus making the
story more coherent and ‘well-rounded’. Items relevant to the theme may be imported and
those irrelevant to the theme may be omitted. Apparent ‘gaps’ may be filled. And some details
may be changed to make them more consistent.
- Assimilation by condensation involves fusing several details into one.
- Assimilation to expectation involves transforming details into what one’s
habits of thought suggest they usually are.
- Assimilation to linguistic habits involves fitting phenomena into the familiar
frameworks of conventional verbal categories.
- Assimilation to interest involves retellings from the perspective of the
particular occupational interests or roles of the teller (especially where these interests are
shared with listeners), giving primary attention to details which reflect such interests.
- Assimilation to prejudice may simply involve assimilation to expectation or to
linguistic categories, but it may also involve deep emotional assimilation to hostility based
on racial, class or personal prejudices.
Selective perception is based on what seems to 'stand out'.
Much of this 'standing out' is related to our
purposes, interests, expectations, past experiences and the current
demands of the situation. However, some seems more widely-shared -
throughout a culture or even across the human species.
For instance, we seem to have a general preference for
features which are large and/or bright and/or moving, for the novel, the
surprising and the incongruous, and for what is meaningfully complex
('looking like things'), and our fixation tends to be on discontinuities,
corners and contours. We will turn to such apparently universal
features of human perception in discussing
Gestalt theories.
Some aspects of perception can be usefully discussed in terms of
'selectivity', but to see perception purely in terms of selectivity would be
reductive. It would court the danger of implying that perception is
relatively passive and would downplay the active construction of
reality.
This is related to what psychologists refer to as perceptual
constancy. Our perception of objects is far more constant or stable
than our retinal images. Retinal images change with the movement of the eyes,
the head and our position, together with changing light. If we relied only
on retinal images for visual perception we would always be conscious of
people growing physically bigger when they came closer, objects changing
their shapes whenever we moved, and colours changing with every shift in
lighting conditions. Counteracting the chaos of constant change in retinal
images, the visual properties of objects tend to remain constant in
consciousness. We are not usually conscious of people appearing to get
bigger as they approach us or of things appearing to change shape according
to the angles from which we view them. In relation to visual perception,
key 'constancies' are: size, shape, lightness and colour.
The following illustration demonstrates how a door appears to change shape
as it is opened. Shape constancy ensures that we are not typically conscious
of this.
With regard to shape constancy, R H Thouless published a paper entitled
'Phenomenal regression to the "real" object'
in the British Journal of Psychology in 1931. He reported an
experiment in which he exposed a circular disc at various angles and asked
observers to judge its shape each time. The observers did so by selecting a
matching disc from a series of circular and elliptical ones which they had
been given. When the disc was directly in front of them and in a vertical
plane the judgement was easy, of course. But when Thouless rotated the
disc away from the observer so that it appeared elliptical, the task
was more difficult. Judgements of shape reflected a compromise between the
shape as displayed at an angle (an ellipse) and the actual shape of the
object (the circle). Observers did not see the shape as it would be
on the retina but instead exhibited a 'phenomenal regression' - the
phenomenal or apparent shape was inbetween the tilted shape and the
vertical shape. This has been called a 'perceptual compromise'.
Familiarity of shape is also an explanation of the illusion generated by
a special 'room' called 'the Ames Room' (students in Wales: note that there is
an Ames Room at
Techniquest in Cardiff). Observers peer through a single
hole in a wall of this structure (thus having only monocular cues to depth,
as in looking at a photograph or painting, rather than a space around which
one could move). Two people of similar size within this special room would
look very different in size to observers as indicated in the following
illustration.
The reason for this strange illusion is to do with the extraordinary
construction of the room, the lefthand wall of which actually goes far
further back than the righthand wall, for instance. We are so used to
rooms being rectangular that we intepret everything within it on this
assumption. The Ames Room represents the mind making a habitual bet,
and only getting it wrong because of a careful conspiracy.