Café wall illusion

The café wall illusion, also known as the Münsterberg illusion, is a geometrical-optical illusion in which the parallel straight dividing lines between staggered rows with alternating dark and light "bricks" appear to be sloped, not parallel as they really are.

A version of the illusion was first described by Hugo Münsterberg in 1894. It has been rediscovered several times, including under the name kindergarten illusion in 1898 by A. H. Pierce, and under its current name in 1973 by Richard Gregory. According to Gregory, this effect was observed by a member of his laboratory, Steve Simpson, in the tiles of the wall of a café at the bottom of St Michael's Hill, Bristol.

In the construction of the illusion often each "brick" is surrounded by a layer of "mortar" intermediate between the dark and light colours of the "bricks".

In attempts at its deconstruction, the illusion was ascribed largely to the irradiation illusion (apparent greater size of a white area than of a black one), and the image disappears when black and white are replaced by different colours of the same brightness. But a component of the illusion remains even when all optical and retinal components are factored out. Contrast polarities seem to be the determining factor in the tilt's direction.