Café Wall Illusion

Straight lines that refuse to look straight

The demo shows rows of black and white tiles, each row shifted sideways from the one above, with thin grey "mortar" lines in between. Every mortar line is perfectly straight and horizontal; hold a ruler to the screen if you like. Yet they look tilted and wedge-shaped, alternately converging and spreading apart along their length.

It needs exactly the right ingredients

The effect is surprisingly fussy. The tiles need strong contrast, ideally black and white. The rows need to be offset by part of a tile rather than lined up. And the mortar has to be a mid grey: make it black or white and the tilt almost disappears. The demo's controls let you change these and watch the illusion switch on and off.

Small shifts that add up

The tilt starts at the edges. Where a grey mortar line meets a black tile, the boundary is registered slightly differently from where the same line meets a white tile, and early visual processing effectively nudges the perceived position of the line by a tiny amount, in opposite directions at dark and light tiles. Richard Gregory and Priscilla Heard called this "border locking" (Gregory & Heard, 1979). Because the tiles alternate and the rows are offset, these tiny nudges line up into a consistent slant along the whole line, much like the "twisted cord" illusion, where a cord of black and white strands laid in a straight line looks tilted. Akiyoshi Kitaoka and colleagues later showed that the direction of the tilt is set by the arrangement of light and dark contrasts at the tile corners (Kitaoka, Pinna & Brelstaff, 2004).

Because the distortion is produced so early in visual processing, knowing how it works doesn't help. The lines still look crooked even when you've just measured them.

Turning it up and down

A few changes control the strength of the tilt. A thicker mortar line weakens it, because the grey band no longer sits close enough to both kinds of tile edge. Lining the rows up removes it, and so does shifting alternate rows by a whole tile, which turns the wall into a checkerboard. Reducing the contrast of the tiles, from black and white towards two similar greys, softens it, even though the geometry hasn't changed.

A real café in Bristol

The illusion is named after a real building, a café at the bottom of St Michael's Hill in Bristol, tiled in alternating black and white with grey mortar. A member of Richard Gregory's laboratory noticed the effect there, and Gregory and Priscilla Heard described it in 1979. A very similar pattern had been drawn by the psychologist Hugo Münsterberg in the 1890s, but it was the Bristol wall that made it famous. Like the Hermann grid, it's a reminder that some illusions happen in the brain's earliest processing, before any interpretation comes in.