Optical illusion 1
An optical illusion. The square A is exactly the
same shade of grey as square B. See Same color
An optical illusion (also called a visual illusion) is characterized by
visually perceived images that differ from objective reality. The
information gathered by the eye is processed in the brain to give a
percept that does not tally with a physical measurement of the stimulus
source. There are three main types: literal optical illusions that create
images that are different from the objects that make them,
physiological ones 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. They can also be known as "mind games".
A scintillating grid illusion. Shape, position,
colour, and 3D contrast converge to produce the
illusion of black dots at the intersections.
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, 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 gray spots appear at the intersection because of the inhibitory response which occurs as a
result of the increased dark surround.
Lateral inhibition has also been used to explain the Hermann grid illusion,
but this has been disproved. More recent "empirical" approaches to optical illusions have had some success in
explaining optical phenomena with which theories based on lateral inhibition have struggled (e.g. Howe et al. 2005
Optical illusion 2
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.
1. 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.
2. 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.
3. 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.
4. 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
Explanation of cognitive illusions
Reversible figure and vase
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.
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.
Optical illusion 3
In addition, Gestalt theory can be used to explain the illusory contours
in the 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.
Gestalt means "form" or "shape" 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.
In the Ponzo illusion the converging parallel lines tell the brain that the
image higher in the visual field is farther 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 understanding 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.
Optical illusion 4
Color and brightness constancies
Simultaneous Contrast Illusion. The background
is a colour gradient and progresses from dark
grey to light grey. The horizontal bar appears to
progress from light grey to dark grey, but is in
fact just one colour.
In this illusion, the colored regions appear rather
different, roughly orange and brown. In fact they
are the same colour, and in identical immediate
surrounds, but the brain changes its assumption
about colour due to the global interpretation of
the surrounding image. Also, the white tiles that
are shadowed are the same colour as the grey tiles
outside the shadow.
Perceptual constancies are sources of illusions. Colour constancy and
brightness constancy are responsible for the fact that a familiar object
will appear the same colour regardless of the amount of or colour of
light reflecting from it. An illusion of colour 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 that reflects less light compared to a white
field even though the object itself did not change in colour. Similarly,
the eye will compensate for colour contrast depending on the colour
cast of the surrounding area.
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
an example of an illusion based on distortions in shape constancy.
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
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 the present.
This allows humans to perform reflexive acts like catching a fly ball and to maneuver smoothly through a crowd.
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.
"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
Optical illusion 5
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
Artists have worked with optical illusions, including M. C. Escher,
Bridget Riley, Salvador Dalí, Giuseppe Arcimboldo, Marcel Duchamp,
Oscar Reutersvärd, Victor Vasarely 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.
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
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. The illusions arise when the "judgments" implied in the unconscious analysis of the
scene are in conflict with reasoned considerations about it.
Optical illusion 6
The Spinning Dancer appears to move both
clockwise and counter-clockwise
Ebbinghaus illusion Café wall illusion Lilac chaser
• Adaptation (eye)
• Alice in Wonderland syndrome
• Auditory illusion
• Barber's pole
• Contingent perceptual aftereffect
• Contour rivalry
• Depth perception
• Emmert's law
• Entoptic phenomenon
• Forced perspective - application used in film and architecture to create the illusion of larger, more distant objects.
• Gestalt psychology
• Gravity hill
• Infinity edge pool
• Kinetic depth effect
Optical illusion 7
• Multistable Perception
• Op Art
• Trompe l'oeil
• Visual reorientation illusions
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Writer, LiveScience.com 6/2/08. His research on this topic is detailed in the May/June issue of the journal Cognitive Science.
 Knowledge in perception and illusion by Richard L. Gregory (http://www.richardgregory.org/papers/knowl_illusion/
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• 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)
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Ponzo illusion using auditory substitution of vision in sighted and early blind subjects. Perception, 34, 857–867.
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blind humans using auditory substitution of vision. Perception & Psychophysics, 68, 535–542.
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• Optical Illusions (http://opticalillusion4u.webs.com/) Detailed explanation & relevant images
• Optical Illusions Database (http://www.moillusions.com/) by Mighty Optical Illusions
• Optical illusions and perception paradoxes (http://www.archimedes-lab.org/index_optical.html) by
• http://ilusaodeotica.com (http://ilusaodeotica.com) hundreds of optical illusions
• Project LITE Atlas of Visual Phenomena (http://lite.bu.edu/vision/applets/lite/lite/lite.html)
• Akiyoshi's illusion pages (http://www.ritsumei.ac.jp/~akitaoka/index-e.html) Professor Akiyoshi
KITAOKA's anomalous motion illusions
• Spiral Or Not? (http://demonstrations.wolfram.com/SpiralOrNot/) by Enrique Zeleny, Wolfram
Optical illusion 8
• Magical Optical Illusions (http://rangki.com/optical/) by Rangki
• Hunch Optical Illusions (http://hunch.com/optical-illusions/) by Hunch
Article Sources and Contributors 9
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Image Sources, Licenses and Contributors
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