I’ve long been puzzled by ambiguous figures, ever since I saw the famous Necker cube as a boy. What changes in the brain when the perception “flips”? (There had better be something.)
I often have trouble along these lines when I’m looking at photographs of the surface of the Moon or of Mars: depending on the direction of the shading, the craters and rilles often look like bumps rather than depressions. Here’s a good example:
With a little effort (especially if I tilt my head to the left, so the shaded of the crater rim at left is at the “top”), I can make this crater look like a shallow dome with a depressed rim. Sometimes I have to work hard to get these sorts of pictures to “pop” the right way, even when I know what they are supposed to look like.
Here’s another, somewhat different example. In this case, flashing pairs of dots are presented in diagonally opposite corners of a square or rectangle: first northeast/southwest, then northwest/southeast. We tend to see this as movement, in one of two ways: either we see the dots sliding from side to side across the top and bottom, or up and down along the sides. (I strongly favor the latter, it seems.) Try it for yourself here.