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Optical illusion

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This article is about visual perception. See Optical Illusion for information about the Time Requiem album.

An optical illusion characterized by visually perceived images that, at least in common sense terms, are deceptive or misleading. Therefore, the information gathered by the eye is processed by the brain to give, on the face of it, a percept that does not tally with a physical measurement of the stimulus source. A conventional assumption is that there are physiological illusions that occur naturally and cognitive illusions that can be demonstrated by specific visual tricks that say something more basic about how human perceptual systems work.

An optical illusion. Square A is exactly the same shade of grey as square B. See Same color illusion
The Scintillating grid illusion or Hermann grid illusion. Dark spots seem to appear and disappear very quickly at the intersections
File:Gradient.illusion.arp.jpg
Simultaneous Contrast Illusion. The grey bar is the same shade throughout
Penrose triangle, an impossible object created (independently) by Oscar Reutersvärd and Roger Penrose.
File:Illusion4.gif
An optical illusion. The two circles seem to move when the viewer's head is moving forwards and backwards while looking at the black dot.
Floor tiles at the Basilica of St. John Lateran in Rome. The pattern creates an illusion of three-dimensional boxes.
File:OpticalIllusion.JPG
1535 wood carving by Erhard Schön. Appears to be scribbled black ink, but features the condensed images (from left to right) of Charles V, Ferdinand I Pope Paul III, and Francis I. It can be expanded on a computer and rotated 90° counterclockwise

Physiological illusions

Physiological illusions, such as the afterimages following bright lights or adapting stimuli of excessively longer alternating patterns (contingent perceptual aftereffect), are presumed to be the effects on the eyes or brain of excessive stimulation of a specific type - brightness, tilt, color, movement, and so on. The theory is that stimuli have individual dedicated neural paths in the early stages of visual processing, and that repetitive stimulation of only one or a few channels causes a physiological imbalance that alters perception.

Example movie which produces distortion illusion after you watch it and look away.


Cognitive illusions

Cognitive illusions are assumed to arise by interaction with assumptions about the world, leading to "unconscious inferences", an idea first suggested in the 19th century by Hermann Helmholtz. Cognitive illusions are commonly divided into ambiguous illusions, distorting illusions, paradox illusions, or fiction illusions.

Ambiguous illusions are pictures or objects that elicit a perceptual 'switch' between the alternative interpretations. The Necker cube is a well known example; another instance is the Rubin vase.

Distorting illusions are characterized by distortions of size, length, or curvature. A striking example is the Café wall illusion. Another example is the famous Mueller-Lyer illusion.

Paradox illusions are generated by objects that are paradoxical or impossible, such as the Penrose triangle or impossible staircases seen, for example, in the work of M. C. Escher. The triangle is an illusion dependent on a cognitive misunderstanding that adjacent edges must join.

Fictional illusions are defined as the perception of objects that are genuinely not there to all but a single observer, such as those induced by schizophrenia or hallucinogenic drugs. These are more properly called hallucinations.

Well-known illusions include:

Many famous artists have worked extensively with optical illusions, including M.C. Escher, Salvador Dalí, Giuseppe Arcimboldo, Marcel Duchamp, Oscar Reutersvärd, and Charles Allan Gilbert.Optical illusion is also used in film by the technique of forced perspective.

A possible explanation of many visual illusions

The explanation of illusions is widely debated. Recent evidence implies that visual illusions are simply the signature of the empirical statistical way all visual percepts are generated. In this interpretation, these phenomena are simply a consequence of the way vision has evolved to solve the inverse problem (see references and links below).

References

Eagleman, D.M. (2001) Visual Illusions and Neurobiology. Nature Reviews Neuroscience. 2(12): 920-6. (pdf)

Gregory Richard (1997) Knowledge in perception and illusion. Phil. Trans. R. Soc. Lond. B 352:1121-1128 (pdf)

Purves D, Lotto B (2002) Why We See What We Do: An Empirical Theory of Vision. Sunderland, MA: Sinauer Associates.

Purves D, Lotto RB, Nundy S (2002) Why We See What We Do. American Scientist 90 (3): 236-242.

Purves D, Williams MS, Nundy S, Lotto RB (2004) Perceiving the intensity of light. Psychological Rev. Vol. 111: 142-158.

Yang Z, Purves D (2003) A statistical explanation of visual space.Nature Neurosci 6: 632-640.

See also

Magic tap. (There is a metal pipe running up through the column of water, supporting the tap and bringing new water to the top.)
Wikimedia Commons has media related to Optical illusion.
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