Illusory contours

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.

Illusory contours or subjective contours are visual illusions that evoke the perception of an edge without a luminance or color change across that edge. Illusory brightness and depth ordering frequently accompany illusory contours. Friedrich Schumann is often credited with the discovery of illusory contours around the beginning of the twentieth century,^[1] however illusory contours are present in art dating to the Middle Ages. Gaetano Kanizsa’s 1976 Scientific American paper marks the resurgence of interest in illusory contours for vision scientists.

Common types of illusory contours

Kanizsa Figures (a.k.a. Pac-Man Configurations): Perhaps the most famous example of an illusory contour is the Pac-Man configuration popularized by Gaetano Kanizsa.^[2] Kanizsa figurers trigger the percept of an illusory contour by aligning Pac-Man-shaped inducers in the visual field such that the edges form a shape. Though not explicitly part of the image, Kanizsa figures evoke the percept of a shape, defined by a sharp illusory contour. Typically, the shape seems brighter than the background though 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.

The Ehrenstein illusion is of a bright disk.

Closely related to Kanizsa figures is the Ehrenstein illusion. 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.^[3]

Abutting Line Gratings: Illusory contours are also created at the boundary between two misaligned gratings.^[4] In these so-called abutting line gratings, the illusory contour is perpendicular to the inducing elements.

Explanation

It is thought that early visual cortical regions such as V1 V2 are responsible for forming illusory contours.^[5][6]

Illusory contours in art and graphic design

Olympic logos from ’72, ’88, ’84, and ’94 all feature illusory contours as does Ellsworth Kelly's 1950s series

Related visual phenomena

Neon color spreading: the cyan circle's contours are illusory

Visual illusions are useful stimuli for studying the neural basis of perception because they hijack the visual system’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 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 and scenes.^[7]

• Amodal perception
• Autostereogram
• Filling-in
• Gestalt psychology
• Negative space
• Reification

References

Further reading

• Coren, S (1972), "Subjective contour and apparent depth", Psychological Review, 79 (4): 359–367, doi:10.1037/h0032940, PMID 5038153 
• Peterhans, E.; von der Heydt, R. (1991). "Subjective contours--bridging the gap between psychophysics and physiology.". Trends Neurosci. 14 (3): 112–119. doi:10.1016/0166-2236(91)90072-3. PMID 1709535. 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)