Illusory contours

Friedrich Schumann is often credited with the discovery of illusory contours around the beginning of the 20th century,[1] 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.

[2] 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.

Instead of employing circles with missing wedges, the Ehrenstein illusion triggers an illusory contour percept via radial line segments.

[5] Studies using human neuroimaging techniques have found that illusory contours are associated with activity in the deep layers of primary visual cortex.

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.
The Ehrenstein illusion is of a bright disc
Neon color spreading : the cyan circle's contours are illusory