Hollow-Face illusion

According to Richard Gregory, "The strong visual bias of favouring seeing a hollow mask as a normal convex face is evidence for the power of top-down knowledge for vision".

[1] 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.

This dragon's head seems to follow the viewer's eyes everywhere (even up or down), when lighting, perspective and/or stereoscopic cues are not strong enough to tell its face is actually hollow.

Keen observers will note that the head doesn't actually follow them, but appears to turn twice as fast around its center than they do themselves.

[4][5] The Hollow-Face illusion is weaker among people with schizophrenia and other populations with psychotic symptoms, perhaps as a result of reduced tendency to interpret any kind of ambiguous 3D object as convex.