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Illusion

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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 [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.



Contents

Cognitive approach

Perceptual organization

Illusion:Duck-Rabbit illusion
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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].
Illusion:Reversible figure and ground
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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.
Illusion:Kanizsa triangle
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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.
Illusion:Ponzo Illusion
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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

Illusion:Chubbillusion
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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.

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, explains the Hermann grid illusion and 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].

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

Paranormal activity and illusion

Illusion in art and magic

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).

References

  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. ^ a b Myers, D. (2003). Psychology in Modules, (7th ed.) New York: Worth. ISBN 0716758504
  4. ^ Pinel, J. (2005) Biopsychology (6th ed.). Boston: Allyn & Bacon. ISBN 0205426514

See also

Categories


Cleanup from October 2006 | All pages needing cleanup | Illusions | Reality | Psychology

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