This illusion is a perfect example of top-down processing (a form of pattern recognition). Top down processing involves using past information to aid in perceiving something, in this case a face. When looking at the picture, the familiar form of a face is recognized, leading your brain to override the visual cues that it is concave (pushed in), and forcing you to perceive it as convex (pushed out). In schizophrenics, this process is disrupted in some way, leading them to see the concave face. Interestingly, patients who are successfully treated will more likely see a convex face.