Editing Optical illusion (section)

{{short description|Visually perceived images that differ from objective reality}}
{{about|visual perception|the album by Time Requiem|Optical Illusion (album)|the film|Optical Illusions (film)}}
{{multiple image
| image1            = Checker shadow illusion.svg
| image2            = Grey square optical illusion proof2.svg
| caption1          = The [[checker shadow illusion]]. Although square A appears a darker shade of gray than square B, in the image the two have exactly the same luminance.
| caption2          = Drawing a connecting bar between the two squares breaks the illusion and shows that they are the same shade.
| width2            = 196
| direction         = vertical
}}
[[File:Gregory categorization of illusions 1991.png|thumb|right|upright=1.5|Gregory's categorization of illusions<ref name=" Gregory" />]]
[[File:Mach bands - animation.gif|thumb|In this animation, [[Mach bands]] exaggerate the [[contrast (vision)|contrast]] between edges of the slightly differing shades of gray as soon as they come in contact with one another.]]

In [[visual perception]], an '''optical illusion''' (also called a '''visual illusion'''<ref>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-called ''physical illusions''). "Optical" in the term derives from the Greek ''optein'' = "seeing", so the term refers to an "illusion of seeing", not to [[optics]] as a branch of modern physics. A regular scientific source for illusions are the journals [https://uk.sagepub.com/en-gb/eur/perception/journal202440 ''Perception''] and [http://journals.sagepub.com/home/ipe ''i-Perception'']</ref>) is an [[illusion]] caused by the [[visual system]] and characterized by a visual [[perception|percept]] that arguably appears to differ from [[reality]]. Illusions come in a wide variety; their categorization is difficult because the underlying cause is often not clear<ref name="Bach">{{cite journal | last1 = Bach | first1 = Michael | last2 = Poloschek | first2 = C. M. | title = Optical Illusions | journal = Adv. Clin. Neurosci. Rehabil. | volume = 6 | issue = 2 | pages = 20–21 | year = 2006 | url = http://www.dfisica.ubi.pt/~hgil/p.v.2/Ilusoes-Visuais/Visual-Illusions.2.pdf | access-date = 2017-12-29 | archive-date = 2021-01-20 | archive-url = https://web.archive.org/web/20210120054520/http://www.dfisica.ubi.pt/~hgil/p.v.2/Ilusoes-Visuais/Visual-Illusions.2.pdf | url-status = dead }}</ref> but a classification<ref name=" Gregory">{{cite journal | last1 = Gregory | first1 = Richard | title = Putting illusions in their place | journal = Perception| volume = 20 | issue = 1 | pages = 1–4 | year = 1991 | doi=10.1068/p200001| pmid = 1945728 | s2cid = 5521054 }}</ref><ref name=" Gregory_97">{{cite journal | last1 = Gregory | first1 = Richard L.| title = Visual illusions classified| journal = Trends in Cognitive Sciences| volume = 1 | issue=5 | pages = 190–194 | year = 1997 | url = http://invibe.net/biblio_database_dyva/woda/data/att/c9cf.file.pdf | doi=10.1016/s1364-6613(97)01060-7| pmid = 21223901| s2cid = 42228451}}</ref> proposed by [[Richard 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.<ref name="Gregory_97" /> 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 the [[motion aftereffect]] (where, despite movement, position remains unchanged).<ref name="Gregory_97" /> An example for a physiological fiction is an [[afterimage]].<ref name="Gregory_97" /> Three typical cognitive distortions are the [[Ponzo illusion|Ponzo]], [[Poggendorff illusion|Poggendorff]], and [[Müller-Lyer illusion|Müller-Lyer]] illusion.<ref name="Gregory_97" /> Physical illusions are caused by the physical environment, e.g. by the optical properties of water.<ref name="Gregory_97" /> Physiological illusions arise in the eye or the visual pathway, e.g. from the effects of excessive stimulation of a specific receptor type.<ref name="Gregory_97" /> [[Cognitive]] visual illusions are the result of [[unconscious inference]]s and are perhaps those most widely known.<ref name="Gregory_97" />

[[#Pathological visual illusions (distortions)|Pathological visual illusions]] arise from pathological changes in the physiological [[visual perception]] mechanisms causing the aforementioned types of illusions; they are discussed e.g. under [[hallucination#Visual|visual hallucinations]].

Optical illusions, as well as multi-sensory illusions involving visual perception, can also be used in the monitoring and rehabilitation of some [[Psychological disorder|psychological]] disorders, including [[Phantom limb|phantom limb syndrome]]<ref name=":02">DeCastro, 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 (identifier)|doi]]:10.12804/revistas.urosario.edu.co/apl/a.3430. [[ISSN (identifier)|ISSN]] 2145-4515.</ref> and [[schizophrenia]].<ref name=":22">King, 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 (identifier)|doi]]:10.3758/s13423-016-1168-5. [[ISSN (identifier)|ISSN]] 1531-5320.</ref>