The most remarkable feature of the perceived world is that it is a stable world of objects even though the retinal images cast by them change rapidly as the observer moves from an object or looks at it from different angles.
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For instance, the image cast on retina by a window when we are standing in front of it would be square. But it may change rapidly if your step back or move to its left or right. But despite such changes in retinal images, we perceive the widow as rectangular or as having some constant shape or size.
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This stability of the environment as we experience it is termed as perceptual constancy. Our visual perceptions of size, shape, colour and brightness show this quality of stability. In other words, the tendency to see objects as relatively unchanged under widely altered conditions of illuminations, distance and position is called as perceptual constancy. We would now discuss:
(a) Lightness and colour constancy;
(b) Shape and size constancy; and
(c) Location constancy.
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We would discuss each of these briefly.
(a) Lightness and Colour Constancy:
Black velvet looks nearly as black to us in sunlight as it does in shadow, even though it reflects thousands of time more light toward our eyes when it is directly illuminated by the sun.
We refer to this phenomenon as lightness constancy. In other world, the tendency to see a familiar object as of the same brightness, regardless of light and shadow that change its stimulus properties are referred to as lightness constancy.
Although the effect holds under normal circumstances, a change in the surroundings can destroy it. Attach the black velvet to a white board and throw a bright light on both, and the velvet still looks black. But now place an opaque black screen with a small opening in it between you and the velvet so you can see only a small pitch of the velvet.
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This screen reduces what you see through the opening to the actual light reflected from the velvet, independent of its surrounding. Now the velvet looks white because the brightness that reaches your eye through the hole is more intense than that from the screen itself.
When we perceive objects in natural settings, several other objects are usually visible. Lightness constancy depends on the relations among the intensities of light reflected from the different objects.
Colour constancy shows a similar dependence on the presence of a heterogeneous field. For example, if you look at a ripe tomato through a tube that obscures the surroundings and the nature of the object, the tomato may appear any colour, i.e., blue, green, pink, etc. depending upon the wavelengths reflecting from it.
On the other hand, when we view an object in a normal scene, the illumination can change quite a bit while affecting very little the colours we perceive. However, colour constancy is far from perfect: Shoppers sometimes learn this the hard way by picking clothing in store lighting, only to discover that the colours look different at home. Psychologist does not completely understand colour constancy. Memory for colours may play a role, but the major influence is the relation between the light reflected from an object and the light reflected from surrounding surfaces.
(b) Shape and Size Constancy:
The image of an object on the retina may change as a result of the movement of the observer or the object but we may perceive the shape of the object in a constant manner. The fact that the perceived shape is constant while the retinal image changes is an example of shape constancy. In other words, shape constancy is a tendency to see a familiar object as of the same shape regardless of the viewing angle. Related to shape constancy in size constancy.
For example, if the object in our perceptual field moves back and forth its image on the retina changes but still we perceive the object to be of same size. This phenomenon is called as size constancy. In other words, the tendency to see a familiar object as of its actual size regardless of its distance is called as size constancy.
(c) Location Constancy:
Related to size constancy is location constancy. Despite the fact that a myriad of changing images strike the retina as we move, the position of fixed objects appear to remain constant. We tend to take this location constancy for granted, but it requires that the perceptual system take account of both our movements and the changing retinal images. The tendency to perceive the place at which a resting object is located as remaining the same even though the relationship to the observer has changed is called as location constancy.
Although all the examples of the constancy so far described are visual, and visual constancies are the most studied, constancies do occur in the other senses. For example, a person will hear the same tune if the frequencies of all of its notes double. All constancies depend on relations among features of the stimulus, the intensity of two adjacent regions, image size and distance, image movement and eye
movement, and so forth. Somehow, the perceptual system integrates these features to respond in a constant way, even though the individual features are changing.