Summary
In visual perception, an optical illusion (also called a visual illusion) 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 but a classification 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. 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). An example for a physiological fiction is an . Three typical cognitive distortions are the Ponzo, Poggendorff, and Müller-Lyer illusion. Physical illusions are caused by the physical environment, e.g. by the optical properties of water. Physiological illusions arise in the eye or the visual pathway, e.g. from the effects of excessive stimulation of a specific receptor type. Cognitive visual illusions are the result of unconscious inferences and are perhaps those most widely known. 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 and schizophrenia. A familiar phenomenon and example for a physical visual illusion is when mountains appear to be much nearer in clear weather with low humidity (Foehn) than they are. This is because haze is a cue for depth perception, signalling the distance of far-away objects (Aerial perspective).
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Related concepts (26)
Optical illusion
In visual perception, an optical illusion (also called a visual illusion) 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 but a classification 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.
Illusion
An illusion is a distortion of the senses, which can reveal how the mind normally organizes and interprets sensory stimulation. Although illusions distort the human perception of reality, they are generally shared by most people. Illusions may occur with any of the human senses, but visual illusions (optical illusions) are the best-known and understood. The emphasis on visual illusions occurs because vision often dominates the other senses.
Hallucinogen persisting perception disorder
Hallucinogen persisting perception disorder (HPPD) is a non-psychotic disorder in which a person experiences apparent lasting or persistent visual hallucinations or perceptual distortions after a previous use of drugs, including but not limited to psychedelics, dissociatives, entactogens, tetrahydrocannabinol (THC), and SSRIs. Despite being designated as a hallucinogen-specific disorder, the specific contributory role of psychedelic drugs is unknown.
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