The Café Wall Illusion Will Make You Think You're Not Seeing Straight

Optical illusions have that unique ability to both delight and infuriate. The café wall illusion is no exception. How could a slightly misaligned checkerboard be such a source of frustration?! Our advice: Just have fun with it. No one can really explain what's happening here anyway.

Check Mate

The café wall illusion may be something you've seen before. It's not a new optical puzzle; it dates back to the turn of the 20th century. The image itself is a simple one: Imagine a checkerboard, but there is a thin separation between the rows. In addition, the tiles are just so slightly skewed so that their corners don't meet at the same points. The result of all these elements hitting your eyeballs at once? A wiggly design with slanted lines and wedge-shaped rows. The kicker is that all the lines you're seeing are parallel. Get out a ruler and test it yourself, if you feel so compelled. Check out the illusion below, or check out a super cool animated version of it from illusion master Michael Bach.

Lock Down

The modern version of this illusion was first reported by neuropsychology professor Richard L. Gregory and experimental cognitive psychologist Priscilla Heard, both of the University of Bristol, in 1979. The two described in their published research that the illusion was first spotted by a member of their lab, Steve Simpson, when he noticed some weirdness on the wall of a — wait for it — café nearby in St. Michael's Hill. Simpson noted that the parallel lines didn't seem parallel, though they definitely were. What caused the unexpected illusion were visible mortar lines separating the rows. After testing out different models to see when the illusion started to click, Gregory and Heard found that the illusion works only when the mortar lines are relatively narrow and when the mortar is not much darker than the dark tiles or lighter than the light tiles

So, what's happening here? According to the Illusion Index, the precise cause of the weirdness is not well understood. It's likely that the illusion involves interactions between the neurons in the visual cortex that deal with orientation. Gregory and Heard suggest a "border-locking" theory to describe why this illusion works. Their idea basically says that highly contrasting shades help your brain lock in the locations of different objects. So, if the rows of black and white tiles were right on top of each other with no border, our brains could more easily see where the corners line up or don't. With a narrow gray line between the rows, our brains can't quite "lock down" where things are happening in relation to each other.

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Want more illusions? Check out Al Seckel's "The Ultimate Book of Optical Illusions." We handpick reading recommendations we think you may like. If you choose to make a purchase through that link, Curiosity will get a share of the sale.

Written by Joanie Faletto May 4, 2018

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