Difference between revisions of "Illusion" - New World Encyclopedia

From New World Encyclopedia
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:''This article is about the phenomenon known as an illusion. For the novel by Richard Bach, please see [[Illusions (novel)]]. Musical groups: For the English band see [[Illusion (UK band)]], for the group from Poland see [[Illusion (band)]]. For the Japanese software company, see [[Illusion Soft]].
 
  
 
An '''illusion''' is a distortion of a sensory [[perception]], revealing how the brain normally organizes and interprets sensory stimulation. While illusions distort reality, they are generally shared by most people <ref> Solso, R. L. (2001). Cognitive psychology (6th ed.). Boston: Allyn and Bacon.ISBN 0-205-30937-2</ref>.
 
An '''illusion''' is a distortion of a sensory [[perception]], revealing how the brain normally organizes and interprets sensory stimulation. While illusions distort reality, they are generally shared by most people <ref> Solso, R. L. (2001). Cognitive psychology (6th ed.). Boston: Allyn and Bacon.ISBN 0-205-30937-2</ref>.
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Illusions can occur with each of the human [[sense]]s, but visual illusions are the most well known and understood. The emphasis on visual illusions occurs because [[Visual perception|vision]] often dominates the other senses. For example, individuals watching a ventriloquist will perceive the voice is coming from the dummy since they are able to see the dummy mouth the words<ref>McGurk,H. & MacDonald, J.(1976). "Hearing lips and seeing voices", Nature 264, 746-748.</ref>.
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Illusions can occur with each of the human [[sense]]s, but visual illusions are the most well known and understood. The emphasis on visual illusions occurs because [[Visual perception|vision]] often dominates the other senses. For example, individuals watching a ventriloquist will perceive the voice is coming from the dummy since they are able to see the dummy mouth the words<ref>McGurk,H. & MacDonald, J.(1976). "Hearing lips and seeing voices," Nature 264, 746-748.</ref>.
 
Some illusions are based on general assumptions the brain makes during [[perception]]. These assumptions are made using organizational principles, like [[Gestalt]], an individual's ability of [[depth perception]] and motion perception, and perceptual constancy. Other illusions occur because of biological sensory structures within the human body or conditions outside of the body within one’s physical environment.  
 
Some illusions are based on general assumptions the brain makes during [[perception]]. These assumptions are made using organizational principles, like [[Gestalt]], an individual's ability of [[depth perception]] and motion perception, and perceptual constancy. Other illusions occur because of biological sensory structures within the human body or conditions outside of the body within one’s physical environment.  
  
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[[Image:Illusion-9.gif|thumb|Left - Right Conflict]]
 
[[Image:Illusion-9.gif|thumb|Left - Right Conflict]]
 
[[Image:Duck-Rabbit_illusion.jpg|thumb|Duck-Rabbit illusion]]
 
[[Image:Duck-Rabbit_illusion.jpg|thumb|Duck-Rabbit illusion]]
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In order to make sense of the world it is necessary to organize incoming sensations into information which is meaningful. 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>.[[Image:Facevase.JPG|left|thumb|Reversible figure and ground]]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]] illusion the figure and ground are reversible. [[Image:Kanizsatriangle.png|thumb|Kanizsa triangle]] In addition, Gestalt theory can be used to explain the illusory contours in the Kanizsa Triangle. Here 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 created 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]] ‎and sounds like the [[auditory illusion]], the [[Shepard tone]].
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In order to make sense of the world it is necessary to organize incoming sensations into information which is meaningful. 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>.[[Image:Facevase.JPG|left|thumb|Reversible figure and ground]]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]] illusion the figure and ground are reversible. [[Image:Kanizsatriangle.png|thumb|Kanizsa triangle]] In addition, Gestalt theory can be used to explain the illusory contours in the Kanizsa Triangle. Here 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 created 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]] ‎and sounds like the [[auditory illusion]], the [[Shepard tone]].
  
 
===Depth and motion perception===
 
===Depth and motion perception===
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===Perceptual constancies===
 
===Perceptual constancies===
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[[Image:Chubbillusion.gif|thumb|]]<!-- Image with unknown copyright status removed: [[Image:Trippy2.jpg|thumb|]] —>
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Perceptual constancies are sources of many 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 light reflecting from it. An illusion of color difference can be created, however, when the luminosity of the area surrounding an unfamiliar object is changed. The color 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. 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.
 
Perceptual constancies are sources of many 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 light reflecting from it. An illusion of color difference can be created, however, when the luminosity of the area surrounding an unfamiliar object is changed. The color 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. 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.
  
 
==Biological approach==
 
==Biological approach==
 
===Vision===
 
===Vision===
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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
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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>.
 
</ref>.
  
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===Other senses===
 
===Other senses===
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Illusions can occur with the other senses including that of taste, smell and touch. It was discovered that even if some portion of the taste receptor on the tongue became damaged that illusory taste could be produced by tactile stimulation. <small> Todrank, J & Bartoshuk, L.M., 1991</small>. Evidence of [[Olfactory]] illusions occurred when positive or negative verbal labels were given prior to olfactory stimulation  
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Illusions can occur with the other senses including that of taste, smell and touch. It was discovered that even if some portion of the taste receptor on the tongue became damaged that illusory taste could be produced by tactile stimulation. <small> Todrank, J & Bartoshuk, L.M., 1991</small>. Evidence of [[Olfactory]] illusions occurred when positive or negative verbal labels were given prior to olfactory stimulation  
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<small> Herz R. S. & Von Clef J., 2001</small>. Examples of [[Tactile illusion|Touch illusion]]s include [[Phantom limb]], the [[Thermal grill illusion]], and the tactile illusion which occurs when the middle finger is crossed over the pointer finger and the fingers are ran along the bridge of the nose to the tip with one finger on each side of the nose . In this illusion two “noses” are felt at the tip. Interestingly, with [[Tactile illusion|Touch illusion]]s similar brain sights are activated during illusory stimulation as actual stimulation <small> Gross, L 2006 </small>.
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<small> Herz R. S. & Von Clef J., 2001</small>. Examples of [[Tactile illusion|Touch illusion]]s include [[Phantom limb]], the [[Thermal grill illusion]], and the tactile illusion which occurs when the middle finger is crossed over the pointer finger and the fingers are ran along the bridge of the nose to the tip with one finger on each side of the nose . In this illusion two “noses” are felt at the tip. Interestingly, with Touch illusions similar brain sights are activated during illusory stimulation as actual stimulation <small> Gross, L 2006 </small>.
 
===Disorders===
 
===Disorders===
 
Some illusions occur as result of an illness or a disorder. While these types of illusions are not shared with everyone they are typical of each condition. For example migraine suffers often report [[Fortification illusions]]….
 
Some illusions occur as result of an illness or a disorder. While these types of illusions are not shared with everyone they are typical of each condition. For example migraine suffers often report [[Fortification illusions]]….
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*Stage magic is a popular form of entertainment based on illusion. Magicians use tricks to give their audiences the impression that seemingly impossible events have occurred. See [[magic (illusion)]].
 
*Stage magic is a popular form of entertainment based on illusion. Magicians use tricks to give their audiences the impression that seemingly impossible events have occurred. See [[magic (illusion)]].
 
*In [[fantasy]] works, actual [[magic (fantasy)|magic]] may work by affecting the senses or producing an image, rather than producing a real change; this magic is frequently called illusion to distinguish it from more substantive forms of magic.
 
*In [[fantasy]] works, actual [[magic (fantasy)|magic]] may work by affecting the senses or producing an image, rather than producing a real change; this magic is frequently called illusion to distinguish it from more substantive forms of magic.
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*Mimes are known for a repertoire of illusions that are created by physical means. The [[mime artist]] creates an illusion of acting upon or being acted upon an unseen object. These illusions exploit the audience's assumptions about the physical world. Well known examples include "walls, "climbing stairs", "leaning", "descending ladders", "pulling and pushing", [[et cetera]]. Amongst mimes, these illusions are sometimes referred to as pantomime
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*Mimes are known for a repertoire of illusions that are created by physical means. The [[mime artist]] creates an illusion of acting upon or being acted upon an unseen object. These illusions exploit the audience's assumptions about the physical world. Well known examples include "walls, "climbing stairs," "leaning," "descending ladders," "pulling and pushing," [[et cetera]]. Amongst mimes, these illusions are sometimes referred to as pantomime
  
 
In [[psychiatry]] and [[philosophy]] the term ''illusion'' refers to a specific form of sensory distortion. Unlike a [[hallucination]], which is a sensory experience in the absence of a stimulus, an illusion describes a misinterpretation of a true sensation so it is perceived in a distorted manner. For example, hearing voices regardless of the environment would be a hallucination, whereas hearing voices in the sound of running water (or other auditory source) would be an illusion.
 
In [[psychiatry]] and [[philosophy]] the term ''illusion'' refers to a specific form of sensory distortion. Unlike a [[hallucination]], which is a sensory experience in the absence of a stimulus, an illusion describes a misinterpretation of a true sensation so it is perceived in a distorted manner. For example, hearing voices regardless of the environment would be a hallucination, whereas hearing voices in the sound of running water (or other auditory source) would be an illusion.
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Perhaps less common than visual illusions (or maybe more subtle) [http://www.roblesdelatorre.com/gabriel/haptics.htm touch illusions] also exist ([http://www.roblesdelatorre.com/gabriel/GR-VH-Nature2001.pdf Robles-De-La-Torre & Hayward 2001]). These "illusory" tactile objects can be used to create "virtual objects" (see the MIT Technology Review article [http://www.technologyreview.com/read_article.aspx?id=17363&ch=biotech&sc=&pg=1 The Cutting Edge of Haptics]).
 
Perhaps less common than visual illusions (or maybe more subtle) [http://www.roblesdelatorre.com/gabriel/haptics.htm touch illusions] also exist ([http://www.roblesdelatorre.com/gabriel/GR-VH-Nature2001.pdf Robles-De-La-Torre & Hayward 2001]). These "illusory" tactile objects can be used to create "virtual objects" (see the MIT Technology Review article [http://www.technologyreview.com/read_article.aspx?id=17363&ch=biotech&sc=&pg=1 The Cutting Edge of Haptics]).
  
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==References ==
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==Optical illusions==
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An '''optical illusion''' is always characterized by [[visual perception|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.
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[[image:Optical.greysquares.arp.jpg|250px|thumb|right|An optical illusion. Square A is exactly the same shade of grey as square B. See [[Same color illusion]]]]
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[[Image:320px-Grid_illusion.svg.png|thumb|256px|The [[Grid illusion|Scintillating grid illusion]] or Hermann grid illusion. Dark spots seem to appear and disappear very quickly at the intersections]]
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[[image:Gradient-optical-illusion.svg|thumb|right|250px|Simultaneous Contrast Illusion. The grey bar is the same shade throughout]]
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[[image:350px-Revolving_circles.svg.png|thumb|right|250px|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.]]
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[[Image:OpticalIllusionStJohnLateran.jpg|thumb|right|250px|Floor tiles at the [[Basilica of St. John Lateran]] in [[Rome]]. The pattern creates an illusion of three-dimensional boxes.]]
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[[Image:Grifo mágico.JPG|thumb|right|250px|Magic tap, which appears to float in the sky with an endless supply of water. In actuality, there is a pipe hidden in the stream of water.]]
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[[Image:Spheres.JPG|thumb|right|250px|An example of the [[Ebbinghaus illusion|Ebbinghaus]] or "same size" illusion. Although, in perspective the sphere to the right seems larger, in reality both spheres are exactly the same size.]]
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==Physiological illusions==
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'''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, 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.
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[[Image:Illusion_movie.ogg]] Example movie which produces distortion illusion after you watch it and look away.
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==Cognitive illusions==
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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.
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(a). '''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]].
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(b). '''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]].
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(c). '''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.
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(d). '''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 a [[hallucinogen]]. These are more properly called [[hallucinations]].
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==Well-known illusions==
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*[[Ames room]] illusion
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*[[Ames Trapezoid Window]] illusion
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*[[Barberpole illusion]]
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*[[Benham's top]]
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*[[Bezold Effect]]
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*[[Blivet]] (also known as the [[Impossible trident illusion]])
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*[[Cafe wall illusion]]
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*[[Chubb illusion]]
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*[[Cornsweet illusion]]
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*[[Ebbinghaus illusion]]
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*[[Ehrenstein illusion]]
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*[[Flash lag illusion]]
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*[[Fraser spiral illusion]]
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*[[Grid illusion]]
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*[[Hering illusion]]
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*[[Hermann grid illusion]]
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*[[Hollow-Face illusion]]
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*[[Impossible cube]]
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*[[Isometric illusion]]
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*[[Jastrow illusion]]
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*[[Kanizsa triangle]]
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*[[Lilac chaser]]
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*[[Mach bands]]
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*[[Missing square puzzle]]
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*[[Moon illusion]]
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*[[Motion illusion]]
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** [[anomalous motion illusion]]
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** [[Relative motion illusion]]
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*[[Muller-Lyer distortion illusion]]
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*[[Necker cube illusion]]
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*[[Orbison illusion]]
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*[[Penrose triangle]] aka Impossible triangle illusion
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*[[Peripheral drift illusion]]
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*[[Phi phenomenon]]
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*[[Poggendorff illusion]]
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*[[Ponzo illusion]]
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*[[Pulfrich effect]] or ''Pulfrich pendulum illusion''
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*[[Rubin vase]]
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*[[Same color]]
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*[[Sander illusion]]
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*[[Size-weight illusion]]
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*[[White's illusion]]
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*[[Wundt illusion]]
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*[[Zollner illusion]]
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Many 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 optical illusion, including: [[Dick Termes]], [[Shigeo Fukuda]], [[Patrick Hughes]], [[István Orosz]], [[Rob Gonsalves]] and [[Akiyoshi Kitaoka]]. Optical illusion is also used in film by the technique of [[forced perspective]].
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Some visual illusions such as the Ponzo illusion and the Vertical-horizontal illusions can also occur when using an auditory-to-vision sensory substitution device.
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==Auditory illusions==
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An '''auditory illusion''' is an illusion of [[hearing (sense)|hearing]], the [[sound]] equivalent of an optical illusion: the listener hears either sounds which are not present in the stimulus, or "impossible" sounds. In short, audio illusions highlight areas where the human ear and brain, as organic, makeshift tools, differ from perfect audio receptors (for better or for worse).
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Examples of auditory illusions:
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* the [[Shepard tone]] or scale, and the [[Deutsch tritone paradox]]
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* hearing a [[missing fundamental]] frequency, given other parts of the [[Harmonic series (music)|harmonic series]]
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* Various psychoacoustic tricks of [[lossy]] [[Audio compression]]
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* [[Octave illusion]]/Deutsch's High-Low Illusion
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* [[Deutsch's scale illusion]]
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* [[Glissando illusion]]
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* [[Illusory continuity of tones]]
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* [[McGurk Effect]]
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==Notes ==
 
<references/>
 
<references/>
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==References==
 
*{{cite journal|author=Flanagan, J.R., Lederman, S.J.|url=http://brain.phgy.queensu.ca/flanagan/papers/FlaLed_NAT_01.pdf|title=Neurobiology: Feeling bumps and holes, News and Views|journal=Nature|volume=412|issue=6845|pages=389–91|year=2001}}
 
*{{cite journal|author=Flanagan, J.R., Lederman, S.J.|url=http://brain.phgy.queensu.ca/flanagan/papers/FlaLed_NAT_01.pdf|title=Neurobiology: Feeling bumps and holes, News and Views|journal=Nature|volume=412|issue=6845|pages=389–91|year=2001}}
 
* {{cite journal|author=Hayward V, Astley OR, Cruz-Hernandez M, Grant D, Robles-De-La-Torre G|url=http://www.roblesdelatorre.com/gabriel/VH-OA-MC-DG-GR-04.pdf|title=Haptic interfaces and devices|journal=Sensor Review |volume=24|issue=1|pages=16–29|year=2004}}
 
* {{cite journal|author=Hayward V, Astley OR, Cruz-Hernandez M, Grant D, Robles-De-La-Torre G|url=http://www.roblesdelatorre.com/gabriel/VH-OA-MC-DG-GR-04.pdf|title=Haptic interfaces and devices|journal=Sensor Review |volume=24|issue=1|pages=16–29|year=2004}}
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Robles-De-La-Torre G.|url=http://www.roblesdelatorre.com/gabriel/GR-IEEE-MM-2006.pdf|title=The Importance of the Sense of Touch in Virtual and Real Environments|journal=IEEE Multimedia|volume=13|issue=3, Special issue on Haptic User Interfaces for Multimedia Systems|pages=24–30|year=2006}}
 
Robles-De-La-Torre G.|url=http://www.roblesdelatorre.com/gabriel/GR-IEEE-MM-2006.pdf|title=The Importance of the Sense of Touch in Virtual and Real Environments|journal=IEEE Multimedia|volume=13|issue=3, Special issue on Haptic User Interfaces for Multimedia Systems|pages=24–30|year=2006}}
  
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== See also ==
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* [[Trompe l'oeil]]
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[[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)]
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* [[Auditory illusions]]
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* [[Delusion]]
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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)]
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* [[Hallucination]]
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* [[Simulated Reality]]
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Purves D, Lotto B (2002) ''Why We See What We Do: An Empirical Theory of Vision''. Sunderland, MA: Sinauer Associates.
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* [[Hermann von Helmholtz|Helmholtz, Hermann von]]
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* [[Augmented reality]] for a more radical approach to the possibility of illusion
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Purves D, Lotto RB, Nundy S (2002) ''Why We See What We Do''. American Scientist 90 (3): 236-242.
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* [[Maya (Hinduism)]]
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Purves D, Williams MS, Nundy S, Lotto RB (2004) ''Perceiving the intensity of light''. Psychological Rev. Vol. 111: 142-158.
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Renier, L., Laloyaux, C., Collignon, O., Tranduy, D., Vanlierde, A., Bruyer, R.,
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De Volder, A.G. (2005). The Ponzo illusion using auditory substitution of vision in sighted and early blind subjects. Perception, 34, 857–867.
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Renier, L., Bruyer, R., & De Volder, A. G. (2006). Vertical-horizontal illusion
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present for sighted but not early blind humans using auditory substitution of
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vision. Perception & Psychophysics, 68, 535–542.
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Yang Z, Purves D (2003) ''A statistical explanation of visual space.''Nature Neurosci 6: 632-640.
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== External links ==
 
== External links ==
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*[http://www.technologyreview.com/read_article.aspx?id=17363&ch=biotech&sc=&pg=1 The Cutting Edge of Haptics] Using touch illusions to create virtual objects with sharp borders. An article in MIT's Technology review by Duncan Graham-Rowe.  
 
*[http://www.technologyreview.com/read_article.aspx?id=17363&ch=biotech&sc=&pg=1 The Cutting Edge of Haptics] Using touch illusions to create virtual objects with sharp borders. An article in MIT's Technology review by Duncan Graham-Rowe.  
 
* [http://www.magiciansofillusion.com Magicians of Illusion]
 
* [http://www.magiciansofillusion.com Magicians of Illusion]
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* [http://www.moillusions.com/ Daily updated Optical Illusions Blog]
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* [http://www.ophtasurf.com/en/illusion.htm selection of the best optical illusions]
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* [http://www.cfar.umd.edu/~fer/optical/ Theory of Optical Illusions]
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* [http://www.cs.brown.edu/stc/outrea/greenhouse/nursery/optical_illusions/home.html Optical Illusions: Computer Art in the Classroom]
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* [http://ghshauseniw.de/projekte/optillu/optfolge1.htm Optical-Illusions-Project GH-School Hausen/Germany]
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* [http://www.archimedes-lab.org/index_optical.html Explore, learn and have Fun with Optical Illusions]
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* [http://www.michaelbach.de/ot/ Optical Illusions & Visual Phenomena]
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* [http://www.ritsumei.ac.jp/~akitaoka/index-e.html Akiyoshi's illusion pages]
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* [http://lite.bu.edu/vision/applets/lite/lite/lite.html Project LITE Atlas of Visual Phenomena]
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* [http://hlavolamy.szm.sk/brainteasers/optical-illusions.htm Collection of Optical Illusions]
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*[http://www.kyushu-id.ac.jp/~ynhome/ENG/Demo/illusions.html Demonstrations of various auditory illusions at] [[Kyushu Institute of Design]]
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*[http://psy.ucsd.edu/~ddeutsch/ Diana Deutsch's Web Page]
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*[http://www.radio.cbc.ca/programs/quirks/test/audill.htm "You must be hearing things," CBC Radio's Quirks & Quarks for Kids]
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Revision as of 23:45, 16 March 2007


An illusion is a distortion of a sensory perception, revealing how the brain normally organizes and interprets sensory stimulation. While illusions distort reality, they are generally shared by most people [1]. Illusions can occur with each of the human senses, but visual illusions are the most well known and understood. The emphasis on visual illusions occurs because vision often dominates the other senses. For example, individuals watching a ventriloquist will perceive the voice is coming from the dummy since they are able to see the dummy mouth the words[2]. Some illusions are based on general assumptions the brain makes during perception. These assumptions are made using organizational principles, like Gestalt, an individual's ability of depth perception and motion perception, and perceptual constancy. Other illusions occur because of biological sensory structures within the human body or conditions outside of the body within one’s physical environment.

Cognitive approach

Perceptual organization

File:Illusion-9.gif
Left - Right Conflict
Duck-Rabbit illusion

In order to make sense of the world it is necessary to organize incoming sensations into information which is meaningful. Gestalt psychologists believe one way this is done is by perceiving individual sensory stimuli as a meaningful whole[3].

File:Facevase.JPG
Reversible figure and ground

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 illusion the figure and ground are reversible.

File:Kanizsatriangle.png
Kanizsa triangle

In addition, Gestalt theory can be used to explain the illusory contours in the Kanizsa Triangle. Here 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 created a "whole" image from individual elements [3]. 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 ‎and sounds like the auditory illusion, the Shepard tone.

Depth and motion perception

Illusions can be based on an individual's ability to see in three dimensions even through 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.

Ponzo Illusion

In the Ponzo illusion the converging parallel lines tells the brain 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 exploits rules of depth and proximity and our understand of the physical world to create an impossible 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 at a faster rate. The Phi phenomenon is yet another example of how the brain perceives motion. The Phi phenomenon is an illusion created when adjacent lights are blinked on and off to create a sense of motion as in Christmas lighting or a neon sign. ????

Perceptual constancies

Perceptual constancies are sources of many 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 light reflecting from it. An illusion of color difference can be created, however, when the luminosity of the area surrounding an unfamiliar object is changed. The color 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. 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.

Biological approach

Vision

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 [4].

Lateral inhibition has also been used to explain the Hermann grid illusion, but this has recently been disproved

Other senses

Illusions can occur with the other senses including that of taste, smell and touch. It was discovered that even if some portion of the taste receptor on the tongue became damaged that illusory taste could be produced by tactile stimulation. Todrank, J & Bartoshuk, L.M., 1991. Evidence of Olfactory illusions occurred when positive or negative verbal labels were given prior to olfactory stimulation Herz R. S. & Von Clef J., 2001. Examples of Touch illusions include Phantom limb, the Thermal grill illusion, and the tactile illusion which occurs when the middle finger is crossed over the pointer finger and the fingers are ran along the bridge of the nose to the tip with one finger on each side of the nose . In this illusion two “noses” are felt at the tip. Interestingly, with Touch illusions similar brain sights are activated during illusory stimulation as actual stimulation Gross, L 2006 .

Disorders

Some illusions occur as result of an illness or a disorder. While these types of illusions are not shared with everyone they are typical of each condition. For example migraine suffers often report Fortification illusions….

Physical approach

  • Mirages are optical distortions through the atmosphere that may be photographed. While the perceived reality (such as water in the desert) is illusory, the visual image (of a reflective surface) is real.
  • Rainbows
  • Antisolar rays
  • Reflection
  • Refraction

Paranormal activity and illusion

Illusion in art and magic

  • Stage magic is a popular form of entertainment based on illusion. Magicians use tricks to give their audiences the impression that seemingly impossible events have occurred. See magic (illusion).
  • In fantasy works, actual magic may work by affecting the senses or producing an image, rather than producing a real change; this magic is frequently called illusion to distinguish it from more substantive forms of magic.
  • Mimes are known for a repertoire of illusions that are created by physical means. The mime artist creates an illusion of acting upon or being acted upon an unseen object. These illusions exploit the audience's assumptions about the physical world. Well known examples include "walls, "climbing stairs," "leaning," "descending ladders," "pulling and pushing," et cetera. Amongst mimes, these illusions are sometimes referred to as pantomime

In psychiatry and philosophy the term illusion refers to a specific form of sensory distortion. Unlike a hallucination, which is a sensory experience in the absence of a stimulus, an illusion describes a misinterpretation of a true sensation so it is perceived in a distorted manner. For example, hearing voices regardless of the environment would be a hallucination, whereas hearing voices in the sound of running water (or other auditory source) would be an illusion.

Perhaps less common than visual illusions (or maybe more subtle) touch illusions also exist (Robles-De-La-Torre & Hayward 2001). These "illusory" tactile objects can be used to create "virtual objects" (see the MIT Technology Review article The Cutting Edge of Haptics).

Optical illusions

An optical illusion is always 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.

File:Optical.greysquares.arp.jpg
An optical illusion. Square A is exactly the same shade of grey as square B. See Same color illusion
File:320px-Grid illusion.svg.png
The Scintillating grid illusion or Hermann grid illusion. Dark spots seem to appear and disappear very quickly at the intersections
File:Gradient-optical-illusion.svg
Simultaneous Contrast Illusion. The grey bar is the same shade throughout
File:350px-Revolving circles.svg.png
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:Grifo mágico.JPG
Magic tap, which appears to float in the sky with an endless supply of water. In actuality, there is a pipe hidden in the stream of water.
File:Spheres.JPG
An example of the Ebbinghaus or "same size" illusion. Although, in perspective the sphere to the right seems larger, in reality both spheres are exactly the same size.

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.

File:Illusion movie.ogg 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.

(a). 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.

(b). 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.

(c). Paradox illusions are generated by objects that are paradoxical or impossible, such as the Penrose triangle or impossible staircases seen, for example, in M. C. Escher's Ascending and Descending and Waterfall. The triangle is an illusion dependent on a cognitive misunderstanding that adjacent edges must join.

(d). 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 a hallucinogen. These are more properly called hallucinations.

Well-known illusions

Many 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 optical illusion, including: Dick Termes, Shigeo Fukuda, Patrick Hughes, István Orosz, Rob Gonsalves and Akiyoshi Kitaoka. Optical illusion is also used in film by the technique of forced perspective.

Some visual illusions such as the Ponzo illusion and the Vertical-horizontal illusions can also occur when using an auditory-to-vision sensory substitution device.

Auditory illusions

An auditory illusion is an illusion of hearing, the sound equivalent of an optical illusion: the listener hears either sounds which are not present in the stimulus, or "impossible" sounds. In short, audio illusions highlight areas where the human ear and brain, as organic, makeshift tools, differ from perfect audio receptors (for better or for worse).

Examples of auditory illusions:

  • the Shepard tone or scale, and the Deutsch tritone paradox
  • hearing a missing fundamental frequency, given other parts of the harmonic series
  • Various psychoacoustic tricks of lossy Audio compression
  • Octave illusion/Deutsch's High-Low Illusion
  • Deutsch's scale illusion
  • Glissando illusion
  • Illusory continuity of tones
  • McGurk Effect


Notes

  1. ↑ Solso, R. L. (2001). Cognitive psychology (6th ed.). Boston: Allyn and Bacon.ISBN 0-205-30937-2
  2. ↑ McGurk,H. & MacDonald, J.(1976). "Hearing lips and seeing voices," Nature 264, 746-748.
  3. ↑ 3.0 3.1 Myers, D. (2003). Psychology in Modules, (7th ed.) New York: Worth. ISBN 0-7167-5850-4
  4. ↑ Pinel, J. (2005) Biopsychology (6th ed.). Boston: Allyn & Bacon. ISBN 0-205-42651-4

References
ISBN links support NWE through referral fees


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.

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.


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