Illusory contours or subjective contours are

visual illusions Within visual perception, an optical illusion (also called a visual illusion) is an illusion caused by the visual system and characterized by a visual percept that arguably appears to differ from reality. Illusions come in a wide variety; the ...

that evoke 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 Gaetano Kanizsa ( he, גאטאנו קאניזסא; 18 August 1913 – 13 March 1993) was an Italian psychologist and artist of Jewish and Slovenian Catholic descent who last served as a founder of the Institute of Psychology of Trieste. Biography ...

’s 1976 ''Scientific American'' paper marked the resurgence of interest in illusory contours for vision scientists.

Common types of illusory contours

Kanizsa figures

Perhaps the most famous example of an illusory contour is the

Pac-Man originally called ''Puck Man'' in Japan, is a 1980 maze action video game developed and released by Namco for arcades. In North America, the game was released by Midway Manufacturing as part of its licensing agreement with Namco America. Th ...

configuration popularized by Gaetano Kanizsa. Kanizsa figures trigger the percept of an illusory contour by aligning Pac-Man-shaped inducers in the visual field such that the edges form a shape. Although not explicitly part of the image, Kanizsa figures evoke the

percept 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 system ...

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.

Ehrenstein illusion

Closely related to Kanizsa figures is the

Ehrenstein illusion The Ehrenstein illusion is an optical illusion studied by the German psychologist (1899 – 1961) in which the sides of a square placed inside a pattern of concentric circles take an apparent curved shape. File:Ehrenstein.svg, Original Ehren ...

. Instead of employing Pac-Man inducers, 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.

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.

In art and graphic design

Olympic Olympic or Olympics may refer to Sports Competitions * Olympic Games, international multi-sport event held since 1896 ** Summer Olympic Games ** Winter Olympic Games * Ancient Olympic Games, ancient multi-sport event held in Olympia, Greece b ...

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, c ...

's 1950s series.

Jacob Gestman Geradts Jacob (or Jaap) Gestman Geradts (born 9 December 1951, The Hague) is a Dutch pin up artist painter. Biography Geradts studied electronics at the Delft Technical University where he received his master's degree in 1977. He then became a pr ...

often used the Kanizsa illusion in his silkscreen prints, for instance in his work ''Formula 1'' (1991).

Cortical responses

It is thought that early visual cortical regions such as V1 V2 in the

visual system The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photoreceptor cells, the optic nerve, the optic tract and the visual cortex) which gives organisms the sense of sight (the a ...

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.

Related visual phenomena

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 system ...

because they hijack the

'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 an animal's 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 affe ...

and scenes. *

Amodal perception Amodal perception is the perception of the whole of a physical structure when only parts of it affect the sensory receptors. For example, a table will be perceived as a complete volumetric structure even if only part of it—the facing surface—pro ...

Autostereogram An autostereogram is a two-dimensional (2D) image that can create the optical illusion of a three-dimensional (3D) scene. Autostereograms use only one image to accomplish the effect while normal stereograms require two. The 3D scene in an a ...

Filling-in In vision, filling-in phenomena are those responsible for the completion of missing information across the physiological blind spot, and across natural and artificial scotomata. There is also evidence for similar mechanisms of completion in normal ...

Gestalt psychology Gestalt-psychology, gestaltism, or configurationism is a school of psychology that emerged in the early twentieth century in Austria and Germany as a theory of perception that was a rejection of basic principles of Wilhelm Wundt's and Edward T ...

Negative space Negative space, in art, is the empty space around and between the subject(s) of an image. Negative space may be most evident when the space around a subject, not the subject itself, forms an interesting or artistically relevant shape, and s ...

Phantom contour A phantom contour is a type of illusory contour. Most illusory contours are seen in still images, such as the Kanizsa triangle and the Ehrenstein illusion. A phantom contour, however, is perceived in the presence of moving or flickering images wit ...

* Reification

References

External links

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