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. Thus, people’s heads all tend to look the same size regardless of distance; similarly, an object identified as a lump of coal tends to look black even when intensely illuminated.

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. To the extent that they prevail, the constancies lend the perceiver’s experience and behaviour relative stability. Imagine an alternative, kaleidoscopic perceptual world in which everything seems to change, solid objects apparently swelling, shrinking, and warping with every movement. Breakdown in perceptual constancy seems to complicate the course of some psychiatric disorders in which the perceptual boundary between the sufferer and the external world is weakened. Normal constancies also can be intentionally overcome, as in paintings of flabby watches and distorted people that apparently depict the unique perceptual world of the artist.

Age

That perceptual functioning should change with the perceiver’s age is expected on the grounds that psychological development stems from maturation and learning. Indeed, empirical evidence for age-related changes in perceiving is substantial. There are, for example, reliable data that perceptual constancies are enhanced with the person’s increasing age, improvement leveling off at about age ten. Similarly, there is a great deal of evidence for both decreased and increased susceptibility to various optical illusions with increasing age. Those illusions that become less pronounced with increasing age probably depend on the subject’s changes in scanning and on his increased ability to segregate parts of a pattern from one another; illusions that become more pronounced probably reflect the operation of expectancies that develop through experience. Anatomical and physiological changes in the eye itself also may account for some age-related perceptual changes.

Historically, the perceptual role of learning was a source of controversy. Vigorous denials that perceiving is influenced by learning are found in arguments of early Gestalt psychologists (e.g., Max Wertheimer, 1880–1943, a German). By contrast, heavy reliance is placed on learning processes in the writings of the German philosopher and scientist H.L.F. von Helmholtz (1821–94). Today, there is virtually full agreement that perceiving is modified by learning. Disputes now focus on the process of perceptual learning itself. Most theoretical alternatives reflect two underlying themes: discovery and enrichment. The discovery thesis is reflected in Eleanor J. Gibson’s view that perceptual learning is a process of discovering how to transform previously overlooked potentials of sensory stimulation into effective information. Enrichment theories depict perceptual learning as enriching sensory experience with specific associations and with rules for its interpretation that derive from past experience. Discovery theories propose that perceptual modification results from learning to respond to new aspects of sensory stimuli, while enrichment theories hold that such modification results from learning to respond differently to the same sensory stimuli.

Direct confrontations of these positions are rare, their advocates tending to differ in their selection of experimental procedures and learning situations. It may be that discovery and enrichment theories are compatible, simply accounting for different forms of perceptual learning.

General acceptance of the perceptual role of learning should not be taken to endorse the claim that perceiving originally depends on learning. Indeed, studies of human newborn and very young infants indicate highly organized and stable perceptual functions. Learning is to be regarded as supplementary to unlearned factors that mediate perceiving.