Illusory contours

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, 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. (Wikipedia)


Common Types of Illusory Contours

Kanizsa triangle

Kanizsa triangle is a classic example of illusory contours. This figure comprises three black circles with equal wedges cut out of them facing the center point and three black angles on a white background. But many observers see a white triangle on top of three black disks and an outline triangle. Also, the nonexistent white triangle appears to be brighter than the surrounding area, but in fact it has the same brightness as the background. (Wikipedia)

Kanizsa triangle

These spatially separate fragments give the impression a bright white triangle, defined by a sharp illusory contour, occluding three black circles and a black-outlined triangle.

It is due to human visual system’s ability to assemble a coherent picture from ambiguous fragments in an image.

Kanizsa Figures (a.k.a. Pac-Man Configurations): Note that illusory contour popularized by Gaetano Kanizsa has the Pac-Man figures.

Kanizsa square

This is a square version of Kanizsa Figure with the Pac-Man Configurations

Kanizsa aquare

This problem-solving aspect of perception is strikingly illustrated in a by the famous illusory rectangle of Italian psychologist Gaetano Kanizsa and neuropsychologist Richard L. Gregory of the University of Bristol in England. Your brain regards it as highly unlikely that some malicious scientist has deliberately aligned four Pac-men in this manner and instead interprets it parsimoniously as a white opaque rectangle partially covering four black disks in the background. Remarkably, you even fill in, or “hallucinate,” the edges of the phantom rectangle. The main goal of vision, it would seem, is to segment the scene to discover object boundaries so that you can identify and respond to them.

Ehrenstein illusion

Closely related to Kanizsa figures is the Ehrenstein illusion, an optical illusion in which a circle appears at the end points of a series of lines.
The Ehrenstein illusion is one of the most popular subjective contour illusions—illusions that create the impression of a shape even though a large portion of the contour is nonexistent. The original Ehrenstein illusion triggers an illusory contour percept via radial line segments

Ehrenstein illusion
adding a circle (bottom)
destroys the illusion of
a bright central disk.
Ehrenstein illusion

The ends of the dark segments produce the illusion of circles. The apparent figures have the same color as the background, but appear brighter.

In 1941 German psychologist Walter Ehrenstein demonstrated that a bright circular patch conspicuously fills the central gap between a series of radial lines. The patch and the circular border delineating it have no correlate in the physical stimulus; they are illusory. The bright illusory surface seems to lie slightly in front of the radial lines.

The length, width, number and contrast of the radial lines determine the strength of this phenomenon. The spatial configuration of the lines necessary for the illusion to take effect implies the existence of neurons that respond to the termination of a line. Such cells, called end stopped neurons, have been identified in the visual cortex, and they may explain this effect. These local signals combine and become inputs to another (second-order) neuron, which fills in the central area with enhanced brightness.

Ehrenstein illusion   Ehrenstein illusion with a black annulus   Ehrenstein illusion with a colored annulus

We experimented with variations in the chromatic properties of the central gap. First we added a black annulus, or ring, to the Ehrenstein figure, and the brightness of the central gap disappeared entirely—the illusion was destroyed, as Ehrenstein had already noticed. We suspect that this effect arises because the ring silences the cells that signal line terminations.

If the annulus is colored, however, other cells may be excited by this change. When we added a colored annulus, the white disk not only appeared much brighter (self-luminous) than it did in the Ehrenstein figure, it also had a dense appearance, as if a white paste had been applied to the surface of the paper (b). This phenomenon surprised us; self-luminosity and surface qualities do not ordinarily appear together and have even been considered to be opposing, or mutually exclusive, modes of appearance. We call this phenomenon anomalous brightness induction. As occurs with the watercolor effect, cells in early cortical areas are candidates for causing this illusion.

Note: The name "Ehrenstein illusion" is also associated with a type of Perspective illusion


Grid version of Ehrenstein illusion

Grid version of Ehrenstein illusion

Grid version of Ehrenstein illusion

Grid version of Ehrenstein illusion


Illusory 3-Dimensional contours

Illusory Pyramid

Rotating Reversals

Illusory 3-Dimensional Pyramid
by vision scientists Pietro Guardini and Luciano Gamberini, both then at the University of Padua in Italy, won second prize in the 2007 Best Illusion of theYear Contest.
The illusory pyramid is a novel variant of the classic Kanizsa triangle, in which the phantom shape of a triangle arises from the placement of three Pac-Man shapes at an imagined triangle’s corners. Guardini and Gamberini’s illusion adds a background, formed by three patches with different levels of gray, to the three Pac-Men. As the angle formed by the intersection of the three gray segments varies, the illusory triangle becomes a pyramid and then reverts.

Watch at Original animated version of "The Illusory Contoured Tilting Pyramid"  

Gestalt Spikes

Rotating Reversals

Gestalt Spikes
Gestalt principle of closure... in this principle, though the edges of the circle are not defined entirely, our minds continue the edges and it appears to be a white circle surrounded by spikes of various sizes.
There is no real circle in the picture, but we can see a circle.
Gestalt principle of closure... mind's eye fills in the sphere.