The café wall illusion, also known as the Münsterberg illusion,[1] 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.[2] [3] [4] It has been rediscovered several times, including under the name kindergarten illusion in 1898 by A. H. Pierce,[5] and under its current name in 1973 by Richard Gregory.[6] 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".[7]
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),[8] and the image disappears when black and white are replaced by different colours of the same brightness.[9] 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.[10]