Optical illusion 53497 226057371 2008-07-16T17:12:34Z Pleasantville 3058640 rvv [[WP:UNDO|Undid]] revision 226028639 by [[Special:Contributions/192.148.249.103|192.148.249.103]] ([[User talk:192.148.249.103|talk]]) [[Image:Grey square optical illusion.PNG|thumb|right|An optical illusion. Square A is exactly the same shade of grey as square B. See [[Same color illusion]]]] :''This article is about visual perception. See [[Optical Illusion (Album)]] for information about the Time Requiem album.'' An '''optical illusion''' (also called a '''visual illusion''') is characterized by [[visual perception|visually perceived]] images that differ from objective reality. The information gathered by the eye is processed by the brain to give a [[percept]] that does not tally with a physical measurement of the stimulus source. There are three main types of illusion - literal optical illusions that create images that are different from the objects that make them, physiological illusions that are the effects on the eyes and brain of excessive stimulation of a specific type - brightness, tilt, color, movement, and [[cognitive]] illusions where the eye and brain make unconscious inferences. ==Physiological illusions== [[Image:Grid illusion.svg|thumb|right|A scintillating [[grid illusion]]. Shape position and colour and 3d [[contrast (vision)|contrast]] converge to produce the illusion of grey dots at the intersections.]] '''Physiological illusions''', such as the [[afterimage]]s 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, etc. 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. The Hermann [[grid illusion]] and [[Mach bands]] are two illusions that are best explained using a biological approach. [[Lateral inhibition]], where in the [[receptive field]] of the retina light and dark receptors compete with one another to become active, has been used to explain why we see bands of increased brightness at the edge of a color difference when viewing Mach bands. Once a receptor is active it inhibits adjacent receptors. This inhibition creates contrast, highlighting edges. In the Hermann grid illusion the grey spots appear at the intersection because of the inhibitory response which occurs as a result of the increased dark surround.<ref>Pinel, J. (2005) Biopsychology (6th ed.). Boston: Allyn & Bacon. ISBN 0-205-42651-4 </ref> [[Lateral inhibition]] has also been used to explain the Hermann [[grid illusion]], but [[grid illusion#The cause of both Scintillating and Hermann grid illusions|this has been disproved]]. ==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 von Helmholtz|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 [[Müller-Lyer illusion]]. # '''Paradox illusions''' are generated by objects that are paradoxical or impossible, such as the [[Penrose triangle]] or [[Penrose staircase|impossible staircases]] seen, for example, in [[M. C. Escher]]'s ''[[Ascending and Descending]]'' and ''[[Waterfall (M. C. Escher)|Waterfall]]''. 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 [[observation|observer]], such as those induced by [[schizophrenia]] or a [[hallucinogen]]. These are more properly called [[hallucinations]]. ==Explanation of cognitive illusions== ===Perceptual organization=== [[Image:Cup or faces paradox.svg|left|thumb|Reversible figure and vase|200px]] <!-- Unsourced image removed: [[Image:Illusion-9.gif|thumb|Left - Right Conflict]] --> [[Image:Duck-Rabbit illusion.jpg|thumb|Duck-Rabbit illusion]] [[Image:My Wife and My Mother-In-Law (Hill).png|thumb|My Wife and My Mother-In-Law|100px]] To make sense of the world it is necessary to organize incoming sensations into information which is meaningful. [[Gestalt psychology|Gestalt psychologists]] believe one way this is done is by perceiving individual sensory stimuli as a meaningful whole.<ref name="Myers">Myers, D. (2003). Psychology in Modules, (7th ed.) New York: Worth. ISBN 0-7167-5850-4 </ref> Gestalt organization can be used to explain many illusions including the Duck-Rabbit illusion where the image as a whole switches back and forth from being a duck then being a rabbit and why in the [[Figure-ground (perception)|figure-ground]] illusion the figure and ground are reversible. [[Image:Kanizsa triangle.svg|thumb|Kanizsa triangle]] In addition, Gestalt theory can be used to explain the [[Illusory Contours|illusory contours]] in the [[Kanizsa triangle|Kanizsa Triangle]]. A floating white triangle, which does not exist, is seen. The brain has a need to see familiar simple objects and has a tendency to create a "whole" image from individual elements.<ref name="Myers">Myers, D. (2003). Psychology in Modules, (7th ed.) New York: Worth. ISBN 0-7167-5850-4 </ref> Gestalt means "whole" in German. However, another explanation of the Kanizsa Triangle is based in [[evolutionary psychology]] and the fact that in order to survive it was important to see form and edges. The use of perceptual organization to create meaning out of stimuli is the principle behind other well-known illusions including [[impossible objects]]. Our brain makes sense of shapes and symbols putting them together like a jigsaw puzzle, formulating that which isn't there to that which is believable. ===Depth and motion perception=== Illusions can be based on an individual's ability to see in three dimensions even though the image hitting the retina is only two dimensional. The [[Ponzo illusion]] is an example of an illusion which uses monocular cues of depth perception to fool the eye.[[Image:Ponzo illusion.gif|left|thumb|Ponzo Illusion]] In the Ponzo illusion the converging [[Parallel (geometry)|parallel lines]] tell the brain that the image higher in the [[visual field]] is further away therefore the brain perceives the image to be larger, although the two images hitting the retina are the same size. The [[Optical illusion]] seen in a [[diorama]]/[[false perspective]] also exploits assumptions based on monocular cues of [[depth perception]]. The [[M. C. Escher]] painting ''[[Waterfall (M. C. Escher)|Waterfall]]'' exploits rules of depth and proximity and our understand of the physical world to create an illusion. Like [[depth perception]], [[motion perception]] is responsible for a number of sensory illusions. Film [[animation]] is based on the illusion that the brain perceives a series of slightly varied images produced in rapid succession as a moving picture. Likewise, when we are moving, as we would be while riding in a vehicle, stable surrounding objects may appear to move. We may also perceive a large object, like an airplane, to move more slowly, than smaller objects, like a car, although the larger object is actually moving faster. The [[Phi phenomenon]] is yet another example of how the brain perceives motion, which is most often created by blinking lights in close succession. ===Color and brightness constancies=== [[Image:Gradient-optical-illusion.svg|thumb|left|Simultaneous Contrast Illusion. The horizontal grey bar is the same shade throughout]] [[Image:ColourIllusion2.jpg|thumb|In this illusion, the second card from the left seems to be a stronger shade of pink in the top picture. In fact they are the same colour, but the brain changes its assumption about colour due to the colour cast of the surrounding photo.]] Perceptual constancies are sources of illusions. [[Color constancy]] and brightness constancy are responsible for the fact that a familiar object will appear the same color regardless of the amount of or colour of light reflecting from it. An illusion of color or contrast difference can be created when the luminosity or colour of the area surrounding an unfamiliar object is changed. The contrast of the object will appear darker against a black field which reflects less light compared to a white field even though the object itself did not change in color. Similarly, the eye will compensate for colour contrast depending on the colour cast of the surrounding area. ===Object consistencies=== Like color, the brain has the ability to understand familiar objects as having a consistent shape or size. For example a door is perceived as rectangle regardless as to how the image may change on the retina as the door is opened and closed. Unfamiliar objects, however, do not always follow the rules of shape constancy and may change when the perspective is changed. The Shepard illusion of the changing table is an example of an illusion based on distortions in shape constancy. ===Future perception=== Researcher Mark Changizi of [[Rensselaer Polytechnic Institute]] in New York says optical illusions are due to a neural lag which most humans experience while awake. When light hits the retina, about one-tenth of a second goes by before the brain translates the signal into a visual perception of the world. Scientists have known of the lag, yet they have debated over how humans compensate, with some proposing that our motor system somehow modifies our movements to offset the delay. Changizi asserts that the human visual system has evolved to compensate for neural delays, generating images of what will occur one-tenth of a second into the future. This foresight enables human to react to events in present. This allows humans to perform reflexive acts like catching a fly ball and to maneuver smoothly through a crowd. <ref name="Bryner"> [http://news.yahoo.com/s/livescience/20080602/sc_livescience/keytoallopticalillusionsdiscovered;_ylt=AiMAgfs0zWgjCtEjx7DQDDtxieAA Key to All Optical Illusions Discovered], Jeanna Bryner, Senior Writer, LiveScience.com 6/2/08. His research on this topic is detailed in the May/June issue of the journal Cognitive Science. </ref> Illusions occur when our brains attempt to perceive the future, and those perceptions don't match reality. For example, one illusion called the [[Hering illusion]], looks like bike spokes around a central point, with vertical lines on either side of this central, so-called vanishing point. The illusion tricks us into thinking we are moving forward, and thus, switches on our future-seeing abilities. Since we aren't actually moving and the figure is static, we misperceive the straight lines as curved ones. Chnagizi said: <blockquote> "Evolution has seen to it that geometric drawings like this elicit in us premonitions of the near future. The converging lines toward a vanishing point (the spokes) are cues that trick our brains into thinking we are moving forward - as we would in the real world, where the door frame (a pair of vertical lines) seems to bow out as we move through it - and we try to perceive what that world will look like in the next instant." <ref name="Bryner"> </ref> </blockquote> ==Illusions== [[Image:Revolving circles.svg|thumb|right|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.]] [[Image:OpticalIllusionStJohnLateran.jpg|thumb|right|Floor tiles at the [[Basilica of St. John Lateran]] in [[Rome]]. The pattern creates an illusion of three-dimensional boxes.]] [[Image:Spinning Dancer.gif|thumb|[[The Spinning Dancer]] appears to move both clockwise and counter-clockwise]] <!-- Unsourced image removed: [[Image:Moving circles.jpg|thumb|500px|Are these circles moving ? The answer is no!]] --> *[[Ames room]] illusion *[[Ames Trapezoid Window]] illusion *[[Autostereogram]] *[[Barberpole illusion]] *[[Benham's top]] *[[Bezold Effect]] *[[Blivet]] (also known as the [[Impossible trident illusion]]) *[[Café wall illusion]] *[[Chubb illusion]] *[[Cornsweet illusion]] *[[Ebbinghaus illusion]] *[[Ehrenstein illusion]] *[[Flash lag illusion]] *[[Fraser spiral illusion]] *[[Grid illusion]] *[[Hering illusion]] *[[Hollow-Face illusion]] *[[Isometric illusion]] *[[Jastrow illusion]] *[[Kanizsa triangle]] *[[Leaning tower illusion]] *[[Lilac chaser]] *[[Mach bands]] *[[Magnetic Hill]] *[[McCollough effect]] *[[Missing square puzzle]] *[[Moon illusion]] *[[Motion illusion]] *[[Müller-Lyer illusion]] *[[Necker cube illusion]] *[[Orbison illusion]] *[[Penrose triangle]] also known as Impossible triangle illusion *[[Peripheral drift illusion]] *[[Phi phenomenon]] *[[Poggendorff illusion]] *[[Ponzo illusion]] *[[Pulfrich effect]] or ''Pulfrich pendulum illusion'' *[[Rubin vase]] *[[Same color]] *[[Sander illusion]] *[[Size-weight illusion]] *[[The Spinning Dancer]] *[[Wagon-wheel effect]] *[[White's illusion]] *[[Wundt illusion]] *[[Zöllner illusion]] Artists have worked with optical illusions, including [[M. C. Escher]], [[Bridget Riley]], [[Salvador Dalí]], [[Giuseppe Arcimboldo]], [[Marcel Duchamp]], [[Oscar Reutersvärd]], and [[Charles Allan Gilbert]]. Also some contemporary artists are experimenting with illusions, including: [[Octavio Ocampo]], [[Dick Termes]], [[Shigeo Fukuda]], [[Patrick Hughes (artist)]], [[István Orosz]], [[Rob Gonsalves]] and [[Akiyoshi Kitaoka]]. Optical illusion is also used in film by the technique of [[forced perspective]]. ===Cognitive processes hypothesis=== The hypothesis claims that visual illusions are because the neural circuitry in our visual system evolves, by neural learning, to a system that makes very efficient interpretations of usual 3D scenes based in the emergence of simplified models in our brain that speed up the interpretation process but give rise to optical illusions in unusual situations. In this sense, the cognitive processes hypothesis can be considered a framework for an understanding of optical illusions as the signature of the empirical statistical way vision has evolved to solve the inverse problem [http://www.richardgregory.org/papers/knowl_illusion/knowledge-in-perception.pdf]. Research indicates that 3D vision capabilities emerge and are learned jointly with the planning of movements. After a long process of learning, an internal representation of the world emerges that is well adjusted to the perceived data coming from closer objects. The representation of distant objects near the horizon is less "adequate". In fact, it is not only the Moon that seems larger when we perceive it near the horizon. In a photo of a distant scene, all distant objects are perceived as smaller than when we observe them directly using our vision. The retinal image is the main source driving vision but what we see is a "virtual" 3D representation of the scene in front of us. We don't see a physical image of the world. We see objects; and the physical world is not itself separated into objects. We see it according to the way our brain organizes it. The names, colors, usual shapes and other information about the things we see pop up instantaneously from our neural circuitry and influence the representation of the scene. We "see" the most relevant information about the elements of the best 3D image that our neural networks can produce. <!-- I'm commenting this next sentence out because I believe that brightness, colour and shape are real physical properties ......The perceived properties of the objects, such as brightness, angular size, and color, are unconsciously "determined" and are not real physical properties. --> The illusions arise when the "judgments" implied in the unconscious analysis of the scene are in conflict with reasoned considerations about bite. ==Gallery== <gallery> Image:Illusion_movie.ogg|<p align=center>Stare at the center an example movie which produces distortion illusion after watching the movie then looking away. Image:Glass_tesseract_animation.gif|<p align=center>[[Tesseract]] Image:Café wall.svg|<p align=center>[[Café wall illusion]] Image:Lilac-Chaser.gif|<p align=center>[[Lilac chaser]] <a href="http://opticalillusion4u.webs.com/">[[Image:http://opticalillusion4u.webs.com/squares.gif]]</a> </gallery> == References == {{reflist}} *[[David Eagleman|Eagleman, D.M.]] (2001) Visual Illusions and Neurobiology. Nature Reviews Neuroscience. 2(12): 920-6. [http://nba.uth.tmc.edu/homepage/eagleman/papers/Eagleman.NatureRevNeuro.Illusions.pdf (pdf)] *Gregory Richard (1997) ''Knowledge in perception and illusion''. Phil. Trans. R. Soc. Lond. B 352:1121-1128. [http://www.richardgregory.org/papers/knowl_illusion/knowledge-in-perception.pdf (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. *Renier, L., Laloyaux, C., Collignon, O., Tranduy, D., Vanlierde, A., Bruyer, R., De Volder, A.G. (2005). The Ponzo illusion using auditory substitution of vision in sighted and early blind subjects. Perception, 34, 857–867. *Renier, L., Bruyer, R., & De Volder, A. G. (2006). Vertical-horizontal illusion present for sighted but not early blind humans using auditory substitution of vision. Perception & Psychophysics, 68, 535–542. *Yang Z, Purves D (2003) ''A statistical explanation of visual space.'' Nature Neurosci 6: 632-640. ==See also== * [[Adaptation (eye)]] * [[Alice in Wonderland syndrome]] * [[Auditory illusion]] * [[Camouflage]] * [[Contingent perceptual aftereffect]] * [[Contour rivalry]] * [[Depth perception]] * [[Infinity pool]] * [[Kinetic depth effect]] * [[Emmert's law]] * [[Entoptic phenomenon]] * [[Mirage]] * [[Forced perspective]] - application used in film and architecture to create the illusion of larger, more distant objects. * [[Gestalt psychology]] * [[Gravity hill]] * [[Multistable Perception]] * [[Op Art]] * [[Trompe l'oeil]] ==External links== {{Commons|Optical illusion}} <!-- --> <!-- --> <!-- Please do not add advertising links to this article. --> <!-- --> <!-- --> * [http://opticalillusion4u.webs.com/rotating.htm Physiological Illusions] * [http://opticalillusion4u.webs.com/ Optical Illusions] * [http://optillusions.com Opt Illusions] * [http://www.metacafe.com/watch/951520/impossible_water/ Impossible Water Illusion] * [http://www.visualfunhouse.com Daily Optical Illusions] by VisualFunHouse * [http://www.octavioocampo.com.mx/ Octavio Ocampo´s Optical Illusions] Octavio Ocampo's Optical Illusions * [http://www.michaelbach.de/ot/ Optical Illusions & Visual Phenomena] by Michael Bach. * [http://www.moillusions.com/ Optical Illusions Database] Daily illusions * [http://conflusions.com/ Optical Illusions Resource] Specialising in new illusions * [http://www.cfar.umd.edu/~fer/optical/ Theory of Optical Illusions] * [http://www.cs.brown.edu/stc/outrea/greenhouse/nursery/optical_illusions/home.html Optical Illusions: Computer Art in the Classroom] * [http://ghshauseniw.de/projekte/optillu/optfolge1.htm Optical-Illusions-Project GH-School Hausen/Germany] * [http://www.archimedes-lab.org/index_optical.html Visual and Perception Illusions, Archimedes Laboratory] * [http://lite.bu.edu/vision/applets/lite/lite/lite.html Project LITE Atlas of Visual Phenomena] * [http://www.123opticalillusions.com/ Optical Illusions Archive] Directory of illusion images. *"[http://demonstrations.wolfram.com/SpiralOrNot/ Spiral Or Not?]" by Enrique Zeleny, [[The Wolfram Demonstrations Project]]. * [http://www.johnsadowski.com/big_spanish_castle.php See black and white in colour] (May need to be moved in terms of section, but the most amazing example of this phenomenon I've seen). <!-- --> <!-- --> <!-- Please do not add advertising links to this article. --> <!-- --> <!-- --> <!-- --> [[Category:Optical illusions| ]] [[Category:Optical phenomena]] [[ar:خداع بصري]] [[ca:Il·lusió òptica]] [[cs:Optický klam]] [[da:Synsbedrag]] [[de:Optische Täuschung]] [[es:Ilusión óptica]] [[eo:Optika iluzio]] [[fa:خطای دید]] [[fr:Illusion d'optique]] [[gl:Ilusión óptica]] [[ko:착시]] [[hr:Perceptivne varke]] [[id:Ilusi optis]] [[is:Sjónvilla]] [[it:Illusione ottica]] [[he:אשליה אופטית]] [[ka:ოპტიკური ილუზია]] [[nl:Gezichtsbedrog]] [[ja:錯視]] [[no:Optisk illusjon]] [[pl:Złudzenie optyczne]] [[pt:Ilusão de óptica]] [[ro:Iluzie optică]] [[ru:Оптические иллюзии]] [[sk:Optický klam]] [[fi:Optinen harha]] [[sv:Optisk illusion]] [[th:ภาพลวงตา]] [[vi:Ảo ảnh (quang học)]] [[zh:错视]]