2. Introduction to Perception:
Perceptual Constancy
Perceptual constancy denotes the tendency of animals and humans to see
familiar objects as having standard shape, size, colour, or location regardless
of changes in the angle of perspective, distance, or lighting. The impression
tends to conform to the object as it is or is assumed to be, rather than
to the actual stimulus. Perceptual constancy is responsible for the ability
to identify objects under various conditions, which seem to be "taken into
account" during a process of mental reconstitution of the known image.
Even though the retinal image of a receding automobile shrinks in size, the
normal, experienced person perceives the size of the object to remain constant.
Indeed, one of the most impressive features of perceiving is the tendency of
objects to appear stable in the face of their continually changing stimulus
features. Though a dinner plate itself does not change, its image on the retina
undergoes considerable changes in shape and size as the perceiver and plate
move. What is noteworthy is stability in perception despite gross instability
in stimulation. Such matches between the object as it is perceived and the
object as it is understood to actually exist (regardless of transformations
in the energy of stimulation) are called perceptual constancies.
Dimensions of visual experience that exhibit constancy include size, shape,
brightness, and colour. Perceptual constancy tends to prevail for these dimensions
as long as the observer has appropriate contextual cues; for example, perception
of size constancy depends on cues that allow one a valid assessment of
his distance from the object. With distance accurately perceived, the apparent
size of an object tends to remain remarkably stable, especially for highly
familiar objects that have a standard size.
Psychologists
have proposed several explanations for the phenomenon of size constancy. First,
people learn the general size of objects through experience and use this knowledge
to help judge size. For example, we know that insects are smaller than people
and that people are smaller than elephants. In addition, people take distance
into consideration when judging the size of an object. Thus, if two objects
have the same retinal image size, the object that seems farther away will be
judged as larger. Even infants seem to possess size constancy.
Another explanation for size constancy involves the relative sizes of objects.
According to this explanation, we see objects as the same size at different
distances because they stay the same size relative to surrounding objects.
For example, as we drive toward a stop sign, the retinal image sizes of the
stop sign relative to a nearby tree remain constant - both images grow larger
at the same rate.
The experience of constancy may break down under extreme conditions. If distance
is sufficiently great, for example, the perceived size of objects will decrease;
thus, viewed from an airplane in flight, there seem to be "toy" houses, cars,
and people below.
Text taken from
Source of Figure
- Roger Shepard (1990). Mind Sights: Original Visual Illusions,
Ambiguities, and Other Anomalies. San Francisco: W. H. Freeman.
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