Kanizsa's Triangle
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file:Kanizsa triangle.svg, ''Kanizsa's triangle'': These spatially separate fragments give the impression of a bright white triangle, defined by a sharp illusory contour, occluding three black circles and a black-outlined triangle. An illusory contour or subjective contour is a visual illusion that evokes the perception of an edge without a luminance or color change across that edge. Illusory brightness and depth ordering often accompany illusory contours. Friedrich Schumann is often credited with the discovery of illusory contours around the beginning of the 20th century, but they are present in art dating to the Middle Ages. Gaetano Kanizsa’s 1976 ''Scientific American'' paper marked the resurgence of interest in illusory contours for vision scientists.


Common types

The Ehrenstein illusion of a bright disc Perhaps the most famous example of an illusory contour is the triangle configuration popularized by Gaetano Kanizsa. Kanizsa figures trigger the percept of an illusory contour by aligning circles with wedge-shaped portions removed in the visual field such that the edges form a shape. Although not explicitly part of the image, Kanizsa figures evoke the
perception Perception () is the organization, identification, and interpretation of sensory information in order to represent and understand the presented information or environment. All perception involves signals that go through the nervous syste ...
of a shape, defined by a sharp illusory contour. Typically, the shape seems brighter than the background, even though the luminance is in reality homogeneous. Additionally, the illusory shape seem to be closer to the viewer than the inducers. Kanizsa figures involve modal completion of the illusory shape and amodal completion of the inducers. Closely related to Kanizsa figures is the
Ehrenstein illusion The Ehrenstein illusion is an optical illusion of brightness or color perception. The visual phenomena was studied by the German psychologist Walter H. Ehrenstein (1899–1961) who originally wanted to modify the theory behind the Hermann grid ...
. Instead of employing circles with missing wedges, the Ehrenstein illusion triggers an illusory contour percept via radial line segments. Ehrenstein's discovery was originally contextualized as a modification of the Hermann grid. In abutting line gratings, illusory contours are created at the boundary between two misaligned gratings. In these so-called abutting line gratings, the illusory contour is perpendicular to the inducing elements.


Cortical responses

It is thought that early visual cortical regions such as V1 V2 in the
visual system The visual system is the physiological basis of visual perception (the ability to perception, detect and process light). The system detects, phototransduction, transduces and interprets information concerning light within the visible range to ...
are responsible for forming illusory contours. Studies using human neuroimaging techniques have found that illusory contours are associated with activity in the deep layers of primary visual cortex.


Uses

Olympic Games The modern Olympic Games (Olympics; ) are the world's preeminent international Olympic sports, sporting events. They feature summer and winter sports competitions in which thousands of athletes from around the world participate in a Multi-s ...
logos from 1972, 1984, 1988, and 1994 all feature illusory contours, as does
Ellsworth Kelly Ellsworth Kelly (May 31, 1923 – December 27, 2015) was an American painter, sculptor, and printmaker associated with hard-edge painting, Color field painting and minimalism. His works demonstrate unassuming techniques emphasizing line, col ...
's 1950s series. Jacob Gestman Geradts often used the Kanizsa illusion in his silkscreen prints, for instance in his work ''Formula 1'' (1991).


Related phenomena

Neon color spreading: the cyan circle's contours are illusory Visual illusions are useful stimuli for studying the neural basis of
perception Perception () is the organization, identification, and interpretation of sensory information in order to represent and understand the presented information or environment. All perception involves signals that go through the nervous syste ...
because they hijack the
visual system The visual system is the physiological basis of visual perception (the ability to perception, detect and process light). The system detects, phototransduction, transduces and interprets information concerning light within the visible range to ...
's innate mechanisms for interpreting the visual world under normal conditions. For example, objects in the natural world are often only partially visible. Illusory contours provide clues for how the visual system constructs surfaces when portions of the surface's
edge Edge or EDGE may refer to: Technology Computing * Edge computing, a network load-balancing system * Edge device, an entry point to a computer network * Adobe Edge, a graphical development application * Microsoft Edge, a web browser developed by ...
are not visible. The encoding of surfaces is thought to be an indispensable part of visual perception, forming a critical intermediate stage of visual processing between the initial analysis of visual features and the ability to recognize complex stimuli like
faces The face is the front of the head that features the eyes, nose and mouth, and through which animals express many of their emotions. The face is crucial for human identity, and damage such as scarring or developmental deformities may affect the ...
and scenes.


See also

* * * * * *


References


Further reading

* * {{ cite journal, journal=Trends Neurosci, year=1991 , volume=14, issue=3, pages=112–119, title=Subjective contours--bridging the gap between psychophysics and physiology., author1=Peterhans, E. , author2=von der Heydt, R., pmid=1709535, doi=10.1016/0166-2236(91)90072-3, s2cid=11553954 , quote=Phenomena of contour, color and movement perception have been used to identify functions of neurons and to reveal functional differences between cortical areas that application of classical receptive-field concepts has not suggested.


External links


Illusory contours figures
Many unpublished drawings (fr) Optical illusions Triangles