Hollow Face Illusion

A hollow mask that bulges

Turn a mask round so you're looking into its hollow back, where the nose points away from you, and it should look like what it is: a bowl-shaped dent. It doesn't. The hollow mask looks like an ordinary face bulging towards you, nose nearest, the opposite of the truth.

It gets stranger when you move. Walk past a hollow mask and the face seems to turn to follow you, an unsettling effect that's a staple of haunted houses. The picture above can only show the still version; to see the movement, you need a real mask, or one of the many videos of rotating hollow masks online.

Faces are expected to stick out

You've seen an enormous number of faces, and nearly all of them were convex. The brain has a very strong expectation that anything arranged like a face bulges outwards, strong enough to overrule depth cues it would normally trust, such as shading. A hollow mask's shading is actually consistent with a hollow shape, but the brain overrides it and fits the evidence to what faces are supposed to look like.

That makes the illusion one of the clearest examples of perception working from expectation as well as from the light reaching your eyes, the idea behind predictive processing. The apparent turning follows from the same mistake. As you move past a hollow surface, its shading and outline change in a way that, for a convex face, could only mean the face is rotating to keep facing you, so that's what you see.

What weakens it

Harold Hill and Vicki Bruce measured how close people could get to a hollow mask before they saw it as hollow. The illusion was stronger when the mask was lit from above, as faces usually are, and when it was the right way up. Turned upside down, the mask still fooled people to some extent, suggesting a general preference for convex shapes as well as one for faces. Viewing with two eyes rather than one also weakened it at close range, because the difference between the two eyes' views gives direct evidence of the true depth (Hill & Bruce, 1993).

There's also a striking exception. Danai Dima and colleagues found that while healthy volunteers almost always saw hollow faces as normal, people with schizophrenia could usually tell they were hollow. Brain imaging pointed to weaker top-down influence, the expectation that normally overrides the evidence (Dima et al., 2009).

Gregory's favourite illusion

Rotating hollow masks were novelties long before anyone studied them, but the British psychologist Richard Gregory made the illusion famous as evidence that perception is a kind of hypothesis-testing rather than a passive recording (Gregory, 1970; Gregory, 1997). His view anticipated today's predictive-processing accounts of vision. The Thatcher effect is another illusion that depends on how specially the brain treats upright faces, and the Ames room is another case of the brain trusting its expectations over the real geometry.