Cornsweet Illusion

The Cornsweet illusion, also called the Craik-O'Brien-Cornsweet illusion or the Craik-Cornsweet illusion, is an optical illusion described in detail by Tom Cornsweet in the late 1960s, building on comparable earlier findings by Kenneth Craik and Vivian O'Brien. Its classic static form is a gray rectangle whose left half gradually lightens toward a vertical line in the center while its right half gradually darkens toward that same line; this creates the strong impression that the entire left half of the rectangle is brighter than the entire right half, even though both halves are actually the same overall brightness, and covering up the central edge makes the illusion disappear immediately. The Cornsweet illusion shares its basic mechanism with simultaneous contrast and with Mach bands but differs in two important ways: the small central edge shapes the perceived brightness of regions far away from it, unlike Mach bands, and the brightness relationship it produces runs opposite to ordinary contrast effects, since the areas next to the lighter edge appear lighter rather than darker. Researchers including Purves, Lotto and Nundy later built a far more dramatic version of the illusion using realistic images of illuminated objects, arguing that it demonstrates how the visual system generates what we perceive on an empirical, reflex-like basis rather than by directly registering the true physical brightness of a scene.