Ready for a little vision test? Akiyoshi Kitaoka, a Kyoto-based psychologist who studies visual illusions, created this eye-bending image that appears to bulge and bend. In the image, shared on Syfy.com, the horizontal and vertical lines actually run straight across and down, but they look like they ripple, and the shapes (Kitaoka calls them turtles) look like they’re different shades of gray, even though they’re an identical color.

As Phil Plait explains for Syfy, the key is in the corners—the turtle “legs,” if you will. “At each vertex between turtles, they form a rotated square divided into four smaller squares," he writes. "Note how they're offset from one another, giving a twist to the vertices.” If you zoom in closely on the image, the lines begin to straighten out.

The difference in the colors, meanwhile, is a result of the contrast between the black and white pixels outlining the turtles. If the outlines of the turtles were entirely black or entirely white, instead of a combination, the grays would look identical. But the contrast between the two fools your eyes into thinking they're different.

To see more of Kitaoka’s illusion art, you can follow him on Twitter @AkiyoshiKitaoka. Then, go check out these other amazing optical illusions.

[h/t Syfy]