This
geometrical illusion was named by
British psychologist Richard Gregory after a café wall in Bristol in the UK, with offset alternating dark and light tiles, and thin lines of mortar of grey shade in between the rows. It looks like the rows are tilted, with alternating lines tilted in opposite directions, while they're actually parallel.
It was first described under the name Kindergarten illusion at the end of the 19th century, and it is a variant of the shifted-chessboard “Münsterberg” illusion. It was proposed then that
irradiation illusion alone seems adequate in explaining the phenomenon.
A key aspect of the illusion seems to be the local contrast between the mortar lines and the tiles that changes depending on whether it is between two dark tiles, two light tiles, or a dark tile and a light tile. Contrast polarities seem to be the determining factor in the tilt's direction. While the precise cause of the illusion is still not well understood, it appears to involve interactions between the neurons in the visual cortex which code for contrast and orientation.
With this interactive widget, you can explore the effect of several parameters on this illusion: for example if you set the mortar thickness to 0 or to a high value, the tilted appearance will completely disappear, while even the thinest mortar line will make it reappear.
References:
Pierce (1898)
The illusions of the kindergarten patterns. Psychological Review 5(3):233–253
Day (1978)
A Note on the Münsterberg or Café Wall Illusion. Perception 7(1):123-124
Gregory & Heard (1979)
Border locking and the Café Wall illusion. Perception 8(4):365–380
Kitaoka et al. (2004)
Contrast polarities determine the direction of Cafe Wall tilts. Perception 33 (1):11–20
Takeuchi (2005)
The effect of eccentricity and the adapting level on the café wall illusion. Perception & Psychophysics 67:1113–1127
Westheimer (2007)
Irradiation, Border Location, and the Shifted-Chessboard Pattern. Perception 36(4):483–494
