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The MIT Encyclopedia of the Cognitive Sciences explains the illusion as an effect of "size and shape constancy [which] subjectively expand[s] the near-far dimension along the line of sight." [4] It classifies Shepard tables as an example of a geometrical illusion, in the category of an "illusion of size." [4]
Subjective constancy or perceptual constancy is the perception of an object or quality as constant even though our sensation of the object changes. [1] While the physical characteristics of an object may not change, in an attempt to deal with the external world, the human perceptual system has mechanisms that adjust to the stimulus.
Colour constancy is an example of subjective constancy and a feature of the human color perception system which ensures that the perceived color of objects remains relatively constant under varying illumination conditions. A green apple for instance looks green to us at midday, when the main illumination is white sunlight, and also at sunset ...
The moon illusion is a good example. Before invoking concepts of apparent distance and size constancy, it helps to be sure that the retinal image hasn't changed much when the moon looks larger as it descends to the horizon. Once the signals from the retina enter the visual cortex, a host of local interactions are known to take place.
Color constancy and brightness constancy are responsible for the fact that a familiar object will appear the same color regardless of the amount of light or color of light reflecting from it. An illusion of color difference or luminosity difference can be created when the luminosity or color of the area surrounding an unfamiliar object is changed.
Form perception is the recognition of visual elements of objects, specifically those to do with shapes, patterns and previously identified important characteristics. An object is perceived by the retina as a two-dimensional image, [1] but the image can vary for the same object in terms of the context with which it is viewed, the apparent size of the object, the angle from which it is viewed ...
In pictures B and D the eye recognizes disparate shapes as "belonging" to a single shape, in C a complete three-dimensional shape is seen, where in actuality no such thing is drawn. Reification can be explained by progress in the study of illusory contours , which are treated by the visual system as "real" contours.
It contradicts the Ebbinghaus illusion (1898) and the Obonai square illusion (1954). In fact, the central test shape (a cross) surrounded by large squares appears larger instead of smaller. Sarcone's Cross illusion consists of a cross (the test shape) surrounded by sets of squares of distinct size (the inducing shapes).