As you all know, I love visual illusions, and this may be one of my favorites. This picture is pretty small (go here for a bigger version), but you should be able to figure out what's going on by watching it for a moment.
Notice that as the face flips over, you briefly see the concave surface of the back or inside of the mask, but it quickly switches back to a convex, upside-down mask. That's not because the image changes, though. Instead, your brain decides that faces can't be hollow, so it changes it for you. This is called depth inversion, or the "hollow face illusion." Interestingly, depth inversion doesn't seem to occur for most objects (e.g., a "hollow potato")1. Thus it is often taken as evidence for the existence of a special system for processing faces.
Perhaps the most striking aspect of the illusion is how resistant it is to ordinary depth cues. As Richard Gregory wrote2:
This bias of seeing faces as convex is so strong it counters competing monocular depth cues, such as shading and shadows, and also very considerable unambiguous information from the two eyes signalling stereoscopically that the object is hollow... The effect is weaker when the mask is placed upside down, strongest for a typical face. If the mask is rotated [as in the above image -- Chris], or the observer moves, it appears to rotate in the opposite to normal direction, at twice the speed; because distances are reversed motion parallax becomes effectively reversed. (p. 1122)The overriding of normal depth cues indicates that the effect is the result of top-down influences. In other words, our knowledge of faces is influencing our visual perception of them.
It's also interesting to note that that the illusion is reduced in schizophrenics3, when you're really really drunk (or when alcoholics are going through withdrawal)4, or when you're high on marijuana5. Apparently each of these states involves a disconnect between knowledge and sensory systems. Some researchers have argued that this disconnect may explain visual hallucinations in schizophrenia or during marijuana highs6.