ABSTRACT
Numerous illusions have been dealt with in terms of contrast mechanisms. For instance, size contrast is usually invoked to explain the Ebbinghaus and Baldwin illusions. Directional contrast has been used to explain the Zoellner illusion, in which it can be reasoned that the direction of the test elements is contrasted with the direction of the other lines in the field. This chapter assesses some of the predictions that might be made if we assume that contrast is a strategy of difference extraction that results in the formation of some illusions. An interesting use of perceptual contrast as an explanatory device was offered by Kuennapas in an attempt to explain the horizontal-vertical illusion. His argument was based on a principle that figures and spaces may contrast against each other. In many respects both averaging and contrast theories of illusion formation are predominantly descriptive. For limited sets of stimulus configurations, each seems to have some usefulness.
