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illusion
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{{about|perception|figures of speech|allusion|other uses}}File:Montre mysterieuse-IMG 4639.jpg|thumb|The transparent dial in this "mystery watchmystery watchAn illusion is a distortion of the senses, which can reveal how the human brain normally organizes and interprets sensory stimulation. Though illusions distort our perception of reality, they are generally shared by most people.Solso, R. L. (2001). Cognitive psychology (6th ed.). Boston: Allyn and Bacon. {{ISBN|0-205-30937-2}}Illusions may occur with any of the human senses, but visual illusions (optical illusions) are the best-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.JOURNAL, McGurk, Hj., MacDonald, J., 1976, Hearing lips and seeing voices, Nature, 264, 746–748, 10.1038/264746a0, 1012311, Some illusions are based on general assumptions the brain makes during perception. These assumptions are made using organizational principles (e.g., Gestalt theory), an individual's capacity for depth perception and motion perception, and perceptual constancy. Other illusions occur because of biological sensory structures within the human body or conditions outside the body within one's physical environment.The term illusion refers to a specific form of sensory distortion. Unlike a hallucination, which is a distortion in the absence of a stimulus, an illusion describes a misinterpretation of a true sensation. 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.

Optical

File:Grey square optical illusion.svg|250px|thumb|right|An optical illusion. Square A is exactly the same shade of grey as Square B. (See Checker shadow illusionChecker shadow illusionAn optical illusion is characterised by visually perceived images that 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.The human brain constructs a world inside our head based on what it samples from the surrounding environment. However, sometimes it tries to organise this information it thinks best while other times it fills in the gaps.Yoon Mo Jung and Jackie (Jianhong) Shen (2008), J. Visual Comm. Image Representation, 19(1):42-55, First-order modeling and stability analysis of illusory contours.Yoon Mo Jung and Jackie (Jianhong) Shen (2014), arXiv:1406.1265, Illusory shapes via phase transition.
This way in which our brain works is the basis of an illusion.

Auditory

An auditory illusion is an illusion of hearing, the auditory 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). One example of an auditory illusion is a shepard tone.

Tactile

Examples of tactile illusions include phantom limb, the thermal grill illusion, the cutaneous rabbit illusion and a curious illusion that occurs when the crossed index and middle fingers are run along the bridge of the nose with one finger on each side, resulting in the perception of two separate noses. The brain areas activated during illusory tactile perception are similar to those activated during actual tactile stimulation.Gross, L 2006 THIS REFERENCE IS INCOMPLETE Tactile illusions can also be elicited through haptic technology.Robles-De-La-Torre & Hayward 2001 These "illusory" tactile objects can be used to create "virtual objects".The Cutting Edge of Haptics (MIT Technology Review article)

Temporal

A temporal illusion is a distortion in the perception of time, which occurs when the time interval between two or more events is very narrow (typically less than a second). In such cases, a person may momentarily perceive time as slowing down, stopping, speeding up, or running backwards.

Other senses

Illusions can occur with the other senses including those involved in food perception. Both soundZampini M & Spence C (2004) "The role of auditory cues in modulating the perceived crispness and staleness of potato chips". Journal of Sensory Studies 19, 347-363. and touchBarnett-Cowan M (2010) "An illusion you can sink your teeth into: Haptic cues modulate the perceived freshness and crispness of pretzels". Perception 39, 1684-1686. have been shown to modulate the perceived staleness and crispness of food products. It was also 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 (smell) illusions occurred when positive or negative verbal labels were given prior to olfactory stimulation.Herz R. S. & Von Clef J., 2001

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 sufferers often report fortification illusions.

Neuroscience

Perception is linked to specific brain activity and so can be elicited by brain stimulation. The (illusory) percepts that can be evoked range from simple phosphenes (detections of lights in the visual field) to high-level percepts. In a single-case study on a patient undergoing presurgical evaluation for epilepsy treatment, electrical stimulation at the left temporo-parietal junction evoked the percept of a nearby (illusory) person who "closely 'shadowed' changes in the patient's body position and posture".JOURNAL, Arzy, S, Seeck, M, Ortigue, S, Spinelli, L, Blanke, O, 2006, Induction of an illusory shadow person,weblink Nature, 443, 287, 10.1038/443287a, 16988702, {{citation|last=Hopkin|first=Michael|title=Brain Electrodes Conjure up Ghostly Visions|journal=Nature|doi=10.1038/news060918-4|date=20 September 2006|url=http://www.nature.com/news/2006/060918/full/news060918-4.html}}

See also

References

{{reflist}}

External links

{{commons category|Illusions}} {{Sensory system}}{{Mental processes}}{{Authority control}}

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