The Cornsweet illusion, also known as the Craik–O'Brien–Cornsweet illusion or the Craik–Cornsweet illusion, is an

optical illusion Within visual perception, an optical illusion (also called a visual illusion) is an illusion caused by the visual system and characterized by a visual perception, percept that arguably appears to differ from reality. Illusions come in a wide v ...

that was described in detail by Tom Cornsweet in the late 1960s.

Kenneth Craik Kenneth James William Craik (; 1914 – 1945) was a Scottish philosopher and psychologist. Life He was born in Edinburgh on 29 March 1914, the son of James Craik, a solicitor. The family lived at 13 Abercromby Place in Edinburgh's Second New ...

and

Vivian O'Brien Vivian "Vob" O'Brien (1924 – December 24, 2010) was an American applied mathematician and physicist whose research included fluid dynamics and visual perception. She worked for many years as a researcher at Johns Hopkins University, and is the ...

had made earlier observations in a similar vein. The original version of the illusion involved a rapidly spinning black-and-white disk, painted in a way that would create the appearance of a gradient effect when in motion. An equivalent static version of illusion is composed of a gray rectangle where the left half fades to a lighter shade as it approaches a vertical center line, and the right half fades to a darker gray approaching the same line. As a result, the whole left half of the rectangle appears lighter than the right half, but in fact the brightness of both areas is exactly the same. This can be seen by blacking out the region containing the edge. This phenomenon is similar to the phenomenon of

simultaneous contrast A contrast effect is the enhancement or diminishment, relative to normal, of perception, cognition or related performance as a result of successive (immediately previous) or simultaneous exposure to a stimulus of lesser or greater value in the s ...

and

Mach bands Mach bands is an optical illusion named after the physicist Ernst Mach. It exaggerates the contrast between edges of the slightly differing shades of gray, as soon as they contact one another, by triggering edge-detection in the human visual s ...

, but differs from it in two important respects. *In Mach bands, the effect is seen only on areas that are close to the intensity gradient. In the Craik–O'Brien–Cornsweet illusion, a very small area (the central "edge") affects the perception of entire large areas, portions of which are distant from the edge. *In the Cornsweet illusion, the region adjacent to the light part of the edge appears lighter, and the region adjacent to the dark part of the edge appears darker, just the opposite of the usual contrast effects. Cornsweet luminance explanation

A far more convincing and dramatic version of the effect can be seen in the article by Purves, Lotto, and Nundy, where it is presented within a quasi-realistic image of solid, illuminated objects.[Archived copy https://web.archive.org/web/20150228222043/https://www.americanscientist.org/include/popup_fullImage.aspx?key=a%2F9TzeCnMGhUDKw5aZ77Tg4fFGhe5fVLOriginal was at http://www.americanscientist.org/include/popup_fullImage.aspx?key=a/9TzeCnMGhUDKw5aZ77Tg4fFGhe5fVL Quasi-realistic image] from Purves, Lotto, and Nundy (2002) showing a much more convincing presentation of the Cornsweet illusion. These writers give an explanation of this and other illusions, in which the visual system and brain are posited to generate percepts on an empirical basis that is much like a reflex. In their words, "... erceptionaccords not with the features of the retinal stimulus or the properties of the underlying objects, but with what the same or similar stimuli have typically signified in the past."

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by Michael Bach. {{Optical illusions Optical illusions

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