Perceptual Illusion of the Week!

This is called the "Craik-O'Brien-Cornsweet Illusion," and it is very similar to the more famous Adelson "Checkershadow Illusion." What we see here is, let us say, a plastic protective case which is propped up and open to display top and bottom, the bottom being white and the top grey. However, astonishingly, the top and bottom of the case, on your screen, are precisely the same shade of grey. [Illustration from Dale Purves and Beau Lotto.]

Don't believe it? Mask off all your screen except the top and bottom of the case, or just click here. You can find some related and similar illusions here.
Want a preview of next week's illusion? Click here.