The stability in our perception of the environment in spite of wide variations in the conditions of observation is called perceptual constancy. But always there is a continuous change in respect of the objects and events around us. For example, the retinal image of our friend when he is at a distance is certainly smaller than the retinal image when he is near us.
Similarly, the retinal image of an elephant from a very distant place may be equal with the retinal image of an insect just near us. But we neither see our friend taller or shorter from different distances nor do we see the elephant as equal to the size of an insect. They maintain their size in our perception in spite of the fact that their retinal images change quite frequently. Imagine the confusion that would take place if we do not perceive our friend to be of same size when seen from different distances. On the other hand, consider the problems of the pet owner who would recognize his dog from the front but not from the side. But these problems do not actually take place with familiar objects due to perceptual constancies.
1. Size Constancy:
It is the tendency to perceive an object as being of the same size even when the size of its retinal image changes at different locations of distance (Rathus, 1990). Size constancy develops largely out of our constant experiences with the objects in the outside world. The role of experience in the development of size constancy can be illustrated by an incident concerning an African Pygmy named Kenge. Anthropologist Colin Turnbull (1961) reported that Kenge was taken for the first time from his home in the forest into the open country. When he spotted a herd of buffaloes grazing several miles away, Kenge asked what kind of ‘insects’ they were. He refused to accept the fact that they were buffaloes, and actually very large in size. During the drive, as the car gradually moved nearer to the buffaloes, Kenge observed the size of the buffalos changing quickly. Kenge muttered to himself and moved closer to Turnbull in fear. Even after Kenge saw that these animals were really buffaloes, he still wondered how they could grow large so quickly. He suspected that he was the victim of some sort of magic.
The above incident could be explained in the following way. Kenge lived in a thick forest where he had not seen animals as big as buffaloes. He had no experience of viewing large animals from a long distance. For that reason, he had not developed size constancy for distant objects. But he did not have any difficulty in displaying size constancy for familiar objects in his home.
2. Color Constancy:
It is the tendency to perceive an object as being of the same color even though lighting conditions change its appearance (Rathus, 1990). For example, a blue car may be perceived as the same blue car even if we look at it in bright sunlight, in dim illumination, or under a yellow streetlight in the evening. But if we do not know the actual color of the car, we may perceive the color of the car differently in different illuminations. In such situations, when clues to know the actual color of the car are unavailable, our oerception of color will depend upon wavelength of light reflected in our eyes.
3. Brightness Constancy:
It is the tendency to perceive an object as being just as bright even though lighting conditions change its intensity. (Rathus, 1990). It is same like color constancy.
4. Shape Constancy:
It is the tendency to perceive an object as being of the same shape although the retinal image varies in shape with change in the angle of vision. For example, we perceive a table as a rectangle from whatever angle we look at it. Actually the retinal image is a rectangle only when the table is seen from a straight position. From all other positions, the retinal image is a parallelogram. But we see the table as a rectangle from all locations of our vision. Similarly, in another example, a door is a rectangle only when viewed straight on. When we open the door, the retinal image becomes a trapezoid. But we perceive the door as the same rectangular door, and it does not matter from which angle we view it.
On the whole, it can be stated that we may have shape, size, color, and brightness constancies in our perceptions of the world as a result of our past experiences. The world therefore looks stable because of perceptual constancies. Had there been no perceptual constancies, our vision of the world would have been chaotic and unstable because of the frequent changes in our retinal image about the same object.