Kanizsa Triangle

A triangle that isn't drawn

Three black discs, each with a wedge cut out, are arranged with their missing wedges pointing towards the same centre. No triangle is drawn, yet you see one: a white triangle with straight, sharp edges, lying on top of the three discs and partly covering them. It even looks brighter than the white page around it.

Look closely at the triangle's edges between the discs and there's nothing there: no line, no change of shade. The edges and the brightness exist only in your perception. The demo lets you rotate the discs so the cut-outs no longer line up, and the triangle disappears.

Edges the brain constructs

The triangle's edges are called illusory contours or subjective contours. The visual system builds them and then treats them like real edges, right down to making the enclosed area look brighter.

The reason is occlusion. In the real world, when something has a neat triangular bite out of it, the likeliest explanation is that something triangular is in front of it. When three discs all have bites that line up, the simplest explanation for all three at once is a single triangle covering them. The brain doesn't just conclude that a triangle is probably there; it draws one, with visible edges and a brightness difference, because a solid object in front of a background usually does differ from it a little.

This is related to the way you see a cat as one animal when a fence post hides its middle, filling in the part you can't see. The difference is that here the filled-in part is something you do see: visible edges in front of the discs, not hidden parts behind something. Knowing this makes no difference; the triangle stays put even when you know it isn't drawn.

Not the same as figure and ground

The Kanizsa triangle is often grouped with Rubin's vase, but the two work differently. Rubin's vase makes you choose which of two regions is the object, flipping between two readings of an edge that is actually drawn. The Kanizsa triangle involves no choice at all: the brain has already made an object that isn't on the page.

Kanizsa's figure

The Italian psychologist Gaetano Kanizsa introduced the figure in 1955 as part of his research into how much of what we see is constructed by the visual system (Kanizsa, 1955; Kanizsa, 1976). It has since been one of the most studied figures in perception research. A key finding came from Rüdiger von der Heydt and colleagues, who recorded from single neurons in monkeys' visual cortex and found cells in the area called V2 that responded to illusory contours much as they did to real ones (von der Heydt, Peterhans & Baumgartner, 1984). The edge you see isn't just a manner of speaking: the brain produces a real edge signal where the image has no edge. The Penrose triangle is a very different triangle that also exists only in perception.