Now You Know Schizophrenia sufferers aren’t fooled by an optical illusion known as the “hollow mask”. In the hollow mask illusion, viewers perceive a concave face (like the back side of a hollow mask) as a normal convex face. The illusion exploits our brain’s strategy for making sense of the visual world: uniting what it actually sees — known as bottom-up processing — with what it expects to see based on prior experience — known as top-down processing. ”All the models in our head have a face coming out, so whenever we see a face, of course if has to come out.” This powerful expectation overrides visual cues, like shadows and depth information, that indicate anything to the contrary. But patients with schizophrenia are undeterred by implausibility: They see the hollow face for what it is. (Source)
The mask is convex; the right side of the mask is light and if it was concave, that means that the light source is on the left side. This is impossible because the left side of the nose is in shadow. The light must be coming from the right and the mask must be convex. It can still be true in the way that it detects schizophrenia, but for different reasons.