The basis of perceptual constancy and perceptual illusion.

A wide range of perceptual illusions, including many of size, orientation, and movement, can be explained in terms of the mechanisms which normally initiate and maintain perceptual constancy. Perceptual constancy is the relative stability of the apparent value of object properties (size, shape, orientation, movement, etc.) when the representation at the eye (retinal image) is variant with change in observer position, posture, and movement. These constancies are consequent on stimuli for object distance and observer posture and motion. When the retinal image is invariant and these stimuli are manipulated, perceptual illusions occur. That is, the mechanisms which normally preserve constancy can be invoked to cause illusions.