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   Also known as Cornsweet edge, Cornsweet-edge effect, Cornsweet stimulus, Cornsweet illusion, Craik-Cornsweet illusion, Craik-O'Brien-Cornsweet illusion, and Craik-O'Brien effect. The eponym Cornsweet effect refers to the American cognitive psychologist Tom Norman Cornsweet (b. 1929), who described the concomitant phenomenon in or shortly before 1970. The eponym Craik-O'Brien effect refers to two authors responsible for an earlier description of the same phenomenon. In 1940 the Scottish philosopher and psychologist Kenneth John William Craik (1914-1945) was the first to describe it in his doctoral thesis. However, Craik died in a car accident before his discovery was published. It was the American physicist Vivian O'Brien (b. 1924) who rediscovered the phenomenon and published it in 1958. All the above eponyms refer to a contrast illusion in which a difference in hue is observed between two adjacent fields of equal hue that are interconnected by a contrast edge with a colour gradient on one side. Only when the colour gradient is covered or removed, the perceptual system is able to recognize the two fields as being of the same hue. The mediation of the Cornsweet effect is commonly attributed to a cortical filling-in process, although it is generally acknowledged that its neurobiolog-ical correlates are not fully understood. Because it arises from the inherent properties of the perceptual system, the Cornsweet effect can be classified as a * physiological illusion. It should not be confused with a related contrast illusion called * Mach bands.

   References

   Cornsweet, T. (1970). Visual perception. New York, NY: Academic Press.

   Craik, K.J.W. (1940). Visual adaptation. Unpublished doctoral thesis, Cambridge University.

   Purves, D., Shimpi, A., Lotto, R.B. (1999). An empirical explanation of the Cornsweet effect. Neuroscience, 19, 8542-8551.