Hollow-Face/Rotating Mask Illusions

The Hollow-Face illusion is an optical illusion in which a mask or mould appears as a normal convexed (pushed out) face when in actuality it is concaved (hollow). Head seems to turn or the eyes seem to follow you as you move around the mask. It is a powerful illusion in that even viewers that consciously know the face is hollow will usually still see it as a normal convex face.

So why does the illusion work? It seems that the common explanation is that our "prior knowledge of the shape of faces dominates perception, even when in conflict with information from binocular disparity".1 Simply stated, our past experiences of the world around us influences what we are currently viewing. Faces are normally convex, so when we see one that is concave, our brain makes an adjustment as to how that information is perceived.

As with most things relating to our brains, it isn't quite as simple as that. Information ambiguity and a "general bias toward convexity" also seem to play a part.2

1. Is prior knowledge of object geometry used in visually guided reaching?

2. The hollow-face illusion: Object-specific knowledge,general assumptions or properties of the stimulus?

Depth Perception and the Hollow Face Illusion