Optical illusion

Within visual perception, an optical illusion (also called a visual illusion[2]) is an illusion caused by the visual system and characterized by a visual percept that arguably appears to differ from reality. Illusions come in a wide variety; their categorization is difficult because the underlying cause is often not clear[3] but a classification[1][4] proposed by Richard Gregory is useful as an orientation. According to that, there are three main classes: physical, physiological, and cognitive illusions, and in each class there are four kinds: Ambiguities, distortions, paradoxes, and fictions.[4] A classical example for a physical distortion would be the apparent bending of a stick half immerged in water; an example for a physiological paradox is the motion aftereffect (where, despite movement, position remains unchanged).[4] An example for a physiological fiction is an afterimage.[4] Three typical cognitive distortions are the Ponzo, Poggendorff, and Müller-Lyer illusion.[4] Physical illusions are caused by the physical environment, e.g. by the optical properties of water.[4] Physiological illusions arise in the eye or the visual pathway, e.g. from the effects of excessive stimulation of a specific receptor type.[4] Cognitive visual illusions are the result of unconscious inferences and are perhaps those most widely known.[4]

The checker shadow illusion. Although square A appears a darker shade of gray than square B, in the image the two have exactly the same luminance.
Drawing a connecting bar between the two squares breaks the illusion and shows that they are the same shade.
Gregory's categorization of illusions [1]
In this animation, Mach bands exaggerate the contrast between edges of the slightly differing shades of gray as soon as they come in contact with one another.

Pathological visual illusions arise from pathological changes in the physiological visual perception mechanisms causing the aforementioned types of illusions; they are discussed e.g. under visual hallucinations.

Optical illusions, as well as multi-sensory illusions involving visual perception, can also be used in the monitoring and rehabilitation of some psychological disorders, including phantom limb syndrome[5] and schizophrenia.[6]

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