The café wall optical illusion was first described by Richard Gregory, professor of neuropsychology at the University of Bristol, in 1979.

When alternating columns of dark and light tiles are vertically misaligned, they can give the impression that the horizontal rows of tiles narrow at one end.

The effect depends on the presence of a visible line of gray mortar between the tiles. 

This effect occurs when there is a detectable line of gray mortar separating the tiles, causing the rows of horizontal lines to appear tapered at one end.

This visual illusion was first noted by a member of Professor Gregory’s lab, who spotted the unique effect created by a tiling pattern on a café wall located at the base of St Michael’s Hill in Bristol.

The café, close to the university, was tiled with alternate rows of offset black and white tiles, with visible mortar lines in between. 

Diagonal lines are perceived because of the way neurons in the brain interact. 

Different types of neurons react to the perception of dark and light colours, and because of the placement of the dark and light tiles, different parts of the grout lines are dimmed or brightened in the retina. 

Where there is a brightness contrast across the grout line, a small scale asymmetry occurs whereby half the dark and light tiles move toward each other forming small wedges. 

The café wall optical illusion was first described by Richard Gregory, professor of neuropsychology at the University of Bristol, in 1979. The unusual visual effect was noticed in the tiling pattern on the wall of a nearby café. Both are shown in this image

These little wedges are then integrated into long wedges with the brain interpreting the grout line as a sloping line.

Professor Gregory’s findings surrounding the café wall illusion were first published in a 1979 edition of the journal Perception.

The café wall illusion has helped neuropsychologists study the way in which visual information is processed by the brain. 

The illusion has also been used in graphic design and art applications, as well as architectural applications.

The effect is also known as the Munsterberg illusion, as it was previously reported in 1897 by Hugo Munsterberg who referred to it as the ‘shifted chequerboard figure.’

It has also been called the ‘illusion of kindergarten patterns’, because it was often seen in the weaving of kindergarten students. 

The illusion has been used in graphic design and art applications, as well as architectural applications, like the Port 1010 building in the Docklands region of Melbourne, Australia