What to observe. Above, the tiles are moving left and right in alternating rows. In the ‘half-shifted condition’, the ‘mortar lines’ (the horizontal lines between the tiles) appear to slope alternately upward and downward. This gives the impression that the tiles are wedge-shaped. As seen when the tiles align or make up a chequerboard, the lines are actually parallel, and all tiles are perfectly square and of the same size. So during the movement, the illusion “comes and goes”.
Origin.
The Café Wall Illusion was first reported by Richard
L. Gregory and Priscilla Heard in 1979. A member of Gregory’s
lab had noticed that the front of a café (St Michael's Hill, Bristol, England)
had been adorned with black and white ceramic tiles. The mortar between
adjacent rows of tiles was visually apparent, and the black/white pattern
was offset by half a tile width in alternating rows.
Comment. This illusion demonstrates the effect of some simple image processing occurring at the retina combined with some complex processing in the cortical cells of the striate cortex. The incoming image is first filtered by the centre-surround operator of the retina. The apparent tilt of the mortar lines is caused by orientation-sensitive simple cells in the striate cortex. The cells interact with one another to interpret the diagonal bands produced by the retina as a single continuous line, tilted in the direction of the diagonal bands.
Sources
Gregory RL & Heard P (1979) Border locking and the Café Wall illusion. Perception 8:365–380 on-line, reprint (PDF)
Lulich DP & Stevens KA (1989) Differential contributions of circular and elongated spatial filters to the Café Wall illusion. Biol Cybernetics 61:427–435
Nice interactive Java Applet at <www.cs.ubc.ca/nest/imager/contributions/flinn/Illusions/CW/cw.html>
For an explanatory text see Brain Connection
