A powerful visual illusion produced by separating two identical grey areas with a blurred contour flanked by a narrow light strip merging into a narrow dark strip, resembling the appearance of Mach bands, causing the whole grey area adjacent to the light strip to appear much lighter than the grey area adjacent to the dark strip, the illusion being eliminated if the contour and flanking strips are covered with a narrow object such as a pencil or (if the contour is curved) a piece of string or cord (see illustration). Also called the Cornsweet illusion or the Craik-O'Brien-Cornsweet illusion. [Named after the Scottish philosopher and psychologist Kenneth John William Craik (1914–45) who was the first to describe it in his doctoral thesis in 1940, and who died following a car accident before publishing it, and the US physicist Vivian O'Brien (born 1924) who rediscovered and published it in 1958]
Craik-O'Brien effect. The left and right halves are identical, as can be confirmed by covering the join with a matchstick or similar object. In this simplified version of the effect, each half consists of a gradation from a lighter shade on the left to a darker shade on the right.