Thechecker 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 thecontrast between edges of the slightly differing shades of gray as soon as they come in contact with one another.
Invisual perception, anoptical illusion (also called avisual illusion[2]) is anillusion caused by thevisual system and characterized by a visualpercept that arguably appears to differ fromreality. 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 byRichard 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 immersed in water; an example for a physiological paradox is themotion aftereffect (where, despite movement, position remains unchanged).[4] An example for a physiological fiction is anafterimage.[4] Three typical cognitive distortions are thePonzo,Poggendorff, andMüller-Lyer illusion.[4]Cognitive visual illusions are the result ofunconscious inferences and are perhaps those most widely known.[4]
Optical illusions, as well as multi-sensory illusions involving visual perception, can also be used in the monitoring and rehabilitation of somepsychological disorders, includingphantom limb syndrome[5] andschizophrenia.[6]
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 fordepth perception,[7] signalling the distance of far-away objects (Aerial perspective).
The classical example of a physical illusion is when a stick that is half immersed in water appears bent. This phenomenon was discussed byPtolemy (c. 150)[8] and was often a prototypical example for an illusion.
Physiological illusions, such as theafterimages[9] 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 or interaction with contextual or competing stimuli of a specific type—brightness, color, position, tile, size, movement, etc. The theory is that a stimulus follows its individual dedicated neural path in the early stages of visual processing and that intense or repetitive activity in that or interaction with active adjoining channels causes aphysiologicalimbalance that alters perception.
The Hermanngrid illusion andMach bands are twoillusions that are often explained using a biological approach.Lateral inhibition, where inreceptive fields of the retina receptor signals from light and dark areas compete with one another, 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 gray spots that appear at the intersections at peripheral locations are often explained to occur because oflateral inhibition by the surround in larger receptive fields.[10] However,lateral inhibition as an explanation of the Hermanngrid illusionhas been disproved.[11][12][13][14][15]More recent empirical approaches to optical illusions have had some success in explaining optical phenomena with which theories based on lateral inhibition have struggled.[16]
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 theGerman physicist and physicianHermann Helmholtz.[17] Cognitive illusions are commonly divided intoambiguous illusions, distorting illusions, paradox illusions, or fiction illusions.
Ambiguous illusions are pictures or objects that elicit a perceptual "switch" between the alternative interpretations. TheNecker cube is a well-known example; other instances are theRubin vase and the "squircle", based onKokichi Sugihara's ambiguous cylinder illusion.[18]
Fictions are when a figure is perceived even though it is not in the stimulus, like with theKanizsa triangle, usingillusory contours.[19][20]
Specific examples of typical cognitive illusions include:
The Ponzo illusion, where two parallel lines of the same length appear to be different sizes due to their placement within converging lines that create a false sense of depth.
The Poggendorff illusion, where two straight lines partially obsecured by an intervening shape, typically a rectangle, appears misaligned when, in fact, they are collinear.
The Müller-Lyer illusion, where two lines of the same length appear to be different lengths due to the outward or inward pointing arrow-like fins attached to their ends.[4]
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.[21] Gestalt organization can be used to explain many illusions including therabbit–duck illusion where the image as a whole switches back and forth from being a duck then being a rabbit and why in thefigure–ground illusion the figure and ground are reversible.[citation needed]
In addition, gestalt theory can be used to explain theillusory contours in theKanizsa's 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.[21]Gestalt means "form" or "shape" in German. However, another explanation of the Kanizsa's triangle is based inevolutionary 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 includingimpossible objects. The brain makes sense of shapes and symbols putting them together like a jigsaw puzzle, formulating that which is not there to that which is believable.[citation needed]
Thegestalt principles of perception govern the way different objects are grouped. Good form is where the perceptual system tries to fill in the blanks in order to see simple objects rather than complex objects. Continuity is where the perceptual system tries to disambiguate which segments fit together into continuous lines. Proximity is where objects that are close together are associated. Similarity is where objects that are similar are seen as associated. Some of these elements have been successfully incorporated into quantitative models involving optimal estimation or Bayesian inference.[22][23]
The double-anchoring theory, a popular but recent theory of lightness illusions, states that any region belongs to one or more frameworks, created by gestalt grouping principles, and within each frame is independently anchored to both the highest luminance and the surround luminance. A spot's lightness is determined by the average of the values computed in each framework.[24]
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. ThePonzo illusion is an example of an illusion which uses monocular cues of depth perception to fool the eye. But even with two-dimensional images, the brain exaggerates vertical distances when compared with horizontal distances, as in thevertical–horizontal illusion where the two lines are exactly the same length.
In the Ponzo illusion the convergingparallel lines tell the brain that the image higher in thevisual field is farther away, therefore, the brain perceives the image to be larger, although the two images hitting theretina are the same size. The optical illusion seen in adiorama/false perspective also exploits assumptions based on monocular cues ofdepth perception. TheM.C. Escher paintingWaterfall exploits rules of depth and proximity and our understanding of the physical world to create an illusion. Likedepth perception,motion perception is responsible for a number of sensory illusions. Filmanimation 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. Thephi phenomenon is yet another example of how the brain perceives motion, which is most often created by blinking lights in close succession.
The ambiguity of direction of motion due to lack of visual references for depth is shown inthe spinning dancer illusion. The spinning dancer appears to be moving clockwise or counterclockwise depending on spontaneous activity in the brain where perception is subjective. Recent studies show on the fMRI that there are spontaneous fluctuations in cortical activity while watching this illusion, particularly the parietal lobe because it is involved in perceiving movement.[25]
Two identical objects behind each other have the same retinal images as two similar objects next to each other. At a small distance between A and B the brain chooses to see option C,D. This results in an illusion if the real objects are present at positions A,B and not at C,D (double-nail illusion).
Midsagittal-strip illusion. In 3D plane CD is seen, AB is not.
When a thin object like a razor blade is held in the midsagittal plane, then it is seen at a right angle to the viewing direction (Midsagittal-strip illusion).
This illusion suggests that the visual system detects the disparity of edges (rims) with equal contrast sign only.
When a black disc is present hovering in front of a white disc, then this can be perceived as it physically is, or as a truncated white cone. If a physical white cone with a black top is presented, then this can be perceived as it physically is, or as a black disc hovering above a white disc. In other words, the observer cannot distinguish between seeing a disc on a pin above a white background, and a white truncated cone with a black top-plane (Ambiguous 3D-surfaces).
This illusion suggests that the visual system detects the disparity (depth) of equal-sign edges and fills in the orientation of surfaces in between.
A denotes the actual position of the pendule, A′ its previous position (as perceived by the darkened eye) and A* its apparent position; similarly for B and C.
When viewing the swinging movement of the rain wiper of a car, and holding a grey filter or dark sunglass in front of one of the eyes, the pendulum appears to make an elliptical movement in depth. It even appears to move through the glass. (Pulfrich illusion).
This is suggests that the signals of the covered eye are processed with a delay.
When stereoimages are swapped (pseudoscopy) binocular depth is inversed and conflicts with monocular depth cues. Perceived depth appears to correspond with the inversed disparity, but the apparent size of objects looks different. Nearby objects appear bigger and far objects appear smaller than normal.
Simultaneous contrast illusion. The background is acolor gradient and progresses from dark gray to light gray. The horizontal bar appears to progress from light grey to dark grey, but is in fact just one color.
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 light or color of light reflecting from it. An illusion of color difference or luminosity difference can be created when the luminosity or color of the area surrounding an unfamiliar object is changed. The luminosity of the object will appear brighter against a black field (that reflects less light) than against a white field, even though the object itself did not change in luminosity. Similarly, the eye will compensate for color contrast depending on the color cast of the surrounding area.
In addition to the gestalt principles of perception, water-color illusions contribute to the formation of optical illusions. Water-color illusions consist of object-hole effects and coloration. Object-hole effects occur when boundaries are prominent where there is a figure and background with a hole that is 3D volumetric in appearance. Coloration consists of an assimilation of color radiating from a thin-colored edge lining a darker chromatic contour. The water-color illusion describes how the human mind perceives the wholeness of an object such as top-down processing. Thus, contextual factors play into perceiving the brightness of an object.[26]
"Shepard tables" deconstructed. The two tabletops appear to be different, but they are the same size and shape.
Just as it perceives color and brightness constancies, the brain has the ability to understand familiar objects as having a consistent shape or size. For example, a door is perceived as a rectangle regardless of 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. TheShepard tables illusion[27] is an example of an illusion based on distortions in shape constancy.
ResearcherMark Changizi ofRensselaer Polytechnic Institute in New York has a more imaginative take on optical illusions, saying that they 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 how humans compensate, with some proposing that our motor system somehow modifies our movements to offset the delay.[28]
Changizi asserts that the human visual system has evolved to compensate for neural delays by generating images of what will occur one-tenth of a second into the future. This foresight enables humans to react to events in the present, enabling humans to perform reflexive acts like catching a fly ball and to maneuver smoothly through a crowd.[29] In an interview with ABC Changizi said, "Illusions occur when our brains attempt to perceive the future, and those perceptions don't match reality."[30] For example, an illusion called theHering illusion looks like bicycle spokes around a central point, with vertical lines on either side of this central, so-called vanishing point.[31]The illusion tricks us into thinking we are looking at a perspective picture, and thus according to Changizi, switches on our future-seeing abilities. Since we are not actually moving and the figure is static, we misperceive the straight lines as curved ones.Changizi said:
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.[29]
Apathological visual illusion is a distortion of a real external stimulus[32] and is often diffuse and persistent. Pathological visual illusions usually occur throughout the visual field, suggesting global excitability or sensitivity alterations.[33] Alternatively visual hallucination is the perception of an external visual stimulus where none exists.[32] Visual hallucinations are often from focal dysfunction and are usually transient.
These symptoms may indicate an underlying disease state and necessitate seeing a medical practitioner. Etiologies associated with pathological visual illusions include multiple types ofocular disease,migraines,hallucinogen persisting perception disorder,head trauma, andprescription drugs. If a medical work-up does not reveal a cause of the pathological visual illusions, the idiopathic visual disturbances could be analogous to the altered excitability state seen in visual aura with no migraine headache. If the visual illusions are diffuse and persistent, they often affect the patient's quality of life. These symptoms are often refractory to treatment and may be caused by any of the aforementioned etiologies, but are often idiopathic. There is no standard treatment for these visual disturbances.
Therubber hand illusion (RHI), amulti-sensory illusion involving bothvisual perception andtouch, has been used to study howphantom limb syndrome affects amputees over time.[5]Amputees with the syndrome actually responded to RHI more strongly than controls, an effect that was often consistent for both the sides of the intact and the amputated arm.[5] However, in some studies, amputees actually had stronger responses to RHI on their intact arm, and more recent amputees responded to the illusion better than amputees who had been missing an arm for years or more.[5] Researchers believe this is a sign that thebody schema, or an individual's sense of their own body and its parts, progressively adapts to the post-amputation state.[5] Essentially, the amputees were learning to no longer respond to sensations near what had once been their arm.[5] As a result, many have suggested the use of RHI as a tool for monitoring an amputee's progress in reducing their phantom limb sensations and adjusting to the new state of their body.[5]
Other research used RHI in the rehabilitation of amputees withprosthetic limbs.[34] After prolonged exposure to RHI, the amputees gradually stopped feeling a dissociation between the prosthetic (which resembled the rubber hand) and the rest of their body.[34] This was thought to be because they adjusted to responding to and moving a limb that did not feel as connected to the rest of their body or senses.[34]
RHI may also be used to diagnose certain disorders related to impairedproprioception or impaired sense oftouch in non-amputees.[34]
Top-down processing involves using action plans to make perceptual interpretations and vice versa. (This is impaired in schizophrenia.)
Schizophrenia, a mental disorder often marked byhallucinations, also decreases a person's ability to perceive high-order optical illusions.[6] This is because schizophrenia impairs one's capacity to performtop-down processing and a higher-level integration of visual information beyond the primary visual cortex,V1.[6] Understanding how this specifically occurs in the brain may help in understanding how visualdistortions, beyond imaginaryhallucinations, affect schizophrenic patients.[6] Additionally, evaluating the differences between how schizophrenic patients and unaffected individuals see illusions may enable researchers to better identify where specific illusions are processed in thevisual streams.[6]
An example of theperipheral drift illusion: alternating lines appear to be moving horizontally left or right.An example of thehollow face illusion which makes concave masks appear to be jutting out (or convex)An example ofmotion induced blindness: while fixating on the flashing dot, the stationary dots may disappear due to the brain prioritizing motion information. This is called theTroxler Effect.
One study on schizophrenic patients found that they were extremely unlikely to be fooled by a three dimensional optical illusion, thehollow face illusion, unlike non-affected volunteers.[35] Based onfMRI data, researchers concluded that this resulted from a disconnection between their systems forbottom-up processing of visual cues and top-down interpretations of those cues in theparietal cortex.[35] In another study on themotion-induced blindness (MIB) illusion (pictured right), schizophrenic patients continued to perceive stationary visual targets even when observing distracting motion stimuli, unlike non schizophreniccontrols, who experienced motion induced blindness.[36] The schizophrenic test subjects demonstrated impaired cognitive organization, meaning they were less able to coordinate their processing ofmotion cues and stationary image cues.[36]
Op art is a style of art that uses optical illusions to create an impression of movement, or hidden images and patterns.Trompe-l'œiluses realistic imagery to create the optical illusion that depicted objects exist in three dimensions.
Tourists attractions employing large-scale illusory art allowing visitors to photograph themselves in fantastic scenes have opened in several Asian countries, such as theTrickeye Museum andHong Kong 3D Museum.[38][39]
The hypothesis claims that visual illusions occur 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.[40]
Research indicates that 3D vision capabilities emerge and are learned jointly with the planning of movements.[41] That is, as depth cues are better perceived, individuals can develop more efficient patterns of movement and interaction within the 3D environment around them.[41] 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".[further explanation needed] In fact, it is not only theMoon 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.
Some images need to be viewed in full resolution to see their effect.
Motion aftereffect: this video produces a distortion illusion when the viewer looks away after watching it.
Ebbinghaus illusion: the orange circle on the left appears smaller than that on the right, but they are in fact the same size.
Café wall illusion: the parallel horizontal lines in this image appear sloped.
Checker version: the diagonal checker squares at the larger grid points make the grid appear distorted.
Checker version with horizontal and vertical central symmetry
Lilac chaser: if the viewer focuses on the black cross in the center, the location of the disappearing dot appears green.
Motion illusion: contrasting colors create the illusion of motion.
Watercolor illusion: this shape's yellow and blue border create the illusion of the object being pale yellow rather than white[42]
Subjective cyan filter, left: subjectively constructed cyan square filter above blue circles, right: small cyan circles inhibit filter construction[43][44]
Pinna's illusory intertwining effect[45] and Pinna illusion (scholarpedia).[46] The picture shows squares spiralling in, although they are arranged in concentric circles.
Phenakistoscope which is spun displaying the illusion of motion of a man bowing and a woman curtsying to each other in a circle at the outer edge of the disc, 1833
Ahybrid image constructed from low-frequency components of a photograph ofMarilyn Monroe (left inset) and high-frequency components of a photograph ofAlbert Einstein (right inset). The Einstein image is clearer inthe full image.
An ancient Roman geometric mosaic. The cubic texture induces aNecker-cube-like optical illusion.
A set of colorful spinning disks that create illusion. The disks appear to move backwards and forwards in different regions.
Pinna-Brelstaff illusion: the two circles seem to move when the viewer's head is moving forwards and backwards while looking at the black dot.[47]
TheSpinning Dancer appears to move both clockwise and counter-clockwise.
^In the scientific literature the term "visual illusion" is preferred because the older term gives rise to the assumption that the optics of the eye were the general cause for illusions (which is only the case for so-calledphysical illusions). "Optical" in the term derives from the Greekoptein = "seeing", so the term refers to an "illusion of seeing", not tooptics as a branch of modern physics. A regular scientific source for illusions are the journalsPerception andi-Perception
^Bach, Michael; Poloschek, C. M. (2006)."Optical Illusions"(PDF).Adv. Clin. Neurosci. Rehabil.6 (2):20–21. Archived fromthe original(PDF) on January 20, 2021. RetrievedDecember 29, 2017.
^abcdefgDeCastro, Thiago Gomes; Gomes, William Barbosa (2017-05-25). "Rubber Hand Illusion: Evidence for a multisensory integration of proprioception". Avances en Psicología Latinoamericana. 35 (2): 219.doi:10.12804/revistas.urosario.edu.co/apl/a.3430.ISSN 2145-4515.
^abcdeKing, Daniel J.; Hodgekins, Joanne; Chouinard, Philippe A.; Chouinard, Virginie-Anne; Sperandio, Irene (2017-06-01). "A review of abnormalities in the perception of visual illusions in schizophrenia". Psychonomic Bulletin & Review. 24 (3): 734–751.doi:10.3758/s13423-016-1168-5.ISSN 1531-5320.
^Goldstein, E. Bruce (2002).Sensation and Perception. Pacific Grove, CA: Wadsworth.ISBN0-534-53964-5., Chpt. 7
^Wade, Nicholas J. (1998).A natural history of vision. Cambridge, MA: MIT Press.
^Pinel, J. (2005) Biopsychology (6th ed.). Boston: Allyn & Bacon.ISBN0-205-42651-4
^Lingelbach B, Block B, Hatzky B, Reisinger E (1985). "The Hermann grid illusion -- retinal or cortical?".Perception.14 (1): A7.
^Geier J, Bernáth L (2004). "Stopping the Hermann grid illusion by simple sine distortion".Perception. Malden Ma: Blackwell. pp. 33–53.ISBN978-0631224211.{{cite book}}: CS1 maint: publisher location (link)
^Geier J, Bernáth L, Hudák M, Séra L (2008). "Straightness as the main factor of the Hermann grid illusion".Perception.37 (5):651–665.doi:10.1068/p5622.PMID18605141.S2CID21028439.
^Bach, Michael (2008). "Die Hermann-Gitter-Täuschung: Lehrbucherklärung widerlegt (The Hermann grid illusion: the classic textbook interpretation is obsolete)".Ophthalmologe.106 (10):913–917.doi:10.1007/s00347-008-1845-5.PMID18830602.S2CID1573891.
^Bernal, B., Guillen, M., & Marquez, J. (2014). The spinning dancer illusion and spontaneous brain fluctuations: An fMRI study. Neurocase (Psychology Press), 20(6), 627-639.
^Bach, Michael (January 4, 2010) [16 August 2004]."Shepard's "Turning the Tables"".michaelbach.de. Michael Bach. Archived fromthe original on December 27, 2009. RetrievedJanuary 27, 2010.
^abcdChrist, Oliver; Reiner, Miriam (2014-07-01). "Perspectives and possible applications of the rubber hand and virtual hand illusion in non-invasive rehabilitation: Technological improvements and their consequences". Neuroscience & Biobehavioral Reviews. Applied Neuroscience: Models, methods, theories, reviews. A Society of Applied Neuroscience (SAN) special issue. 44: 33–44.doi:10.1016/j.neubiorev.2014.02.013.ISSN 0149-7634
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 (7):857–867.doi:10.1068/p5219.PMID16124271.S2CID17265107.
![Watercolor illusion: this shape's yellow and blue border create the illusion of the object being pale yellow rather than white[42]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aSubjectively_constructed_water-color.svg)
![Subjective cyan filter, left: subjectively constructed cyan square filter above blue circles, right: small cyan circles inhibit filter construction[43][44]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aOptical_illusion_-_subjectively_constructed_cyan_sqare_filter_above_blue_cirles.svg)
![Pinna's illusory intertwining effect[45] and Pinna illusion (scholarpedia).[46] The picture shows squares spiralling in, although they are arranged in concentric circles.](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aPinna%2527s_illusory_intertwining_effect.gif)
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![Pinna-Brelstaff illusion: the two circles seem to move when the viewer's head is moving forwards and backwards while looking at the black dot.[47]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aRevolving_circles.svg)
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