The Triangle That Isn’t

Edges that are not there

There is no triangle and no brightness difference: the middle is the same white as the rest of the page. The three cut-outs line up so exactly that the simplest explanation is a white triangle lying on top of three whole discs, and the visual system adopts that explanation — drawing in edges, and even a slight glow, to go with it.

Gaetano Kanizsa described it in 1955. Cells in the visual cortex respond to these illusory edges much as they do to real ones, which is part of how we see the outline of something partly hidden behind something else.

• Illusory contours — Wikipedia