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

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

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

This visual trick occurs especially when a gray line of mortar is visible between the tiles, making the rows of horizontal lines appear tapered.

The illusion was first spotted by a member of Professor Gregory’s lab, who observed a peculiar visual effect from the tile 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