This week’s visual illusion is related to Mach bands, and
similar in some ways to the watercolor effect. It’s called the
Craik-O’Brien-Cornsweet effect (or just the Cornsweet effect)1.
This is the best example I’ve ever seen (from here): What you
should see is a dark square over a light square (almost white).
Now take your finger and place it over the boundary between the
two squares. What do you see? Two squares of the same color!
What’s going on here? Well, at the boundary between the two
squares, the top one really is dark, and the bottom one really
is white. The visual system seems to be taking the dark gray and
white at the edges of the squares, and spreading it across them
(sort of like the visual system spreads the boundary colors
throughout the center of the watercolor effect figures), so that
otherwise identical squares look very different. Here is a more
traditional version2: How this happens, exactly, is still a
matter of contention, but most explanations do include our old
friend the center-surrou