The Hollow face illusion

Long before computer scanners were able to make detailed digital masks of faces, Plaster of Paris was used to make casts of peoples faces. This runny substance, also used for plaster casts to prop up broken limbs in medicine, hardens rapidly and when spread carefully over the face can produce a prefect copy, a mask of the face. Where the real face bulges out, the mask bulges in; or to use the proper technical terms, the face is convex and the mask is concave. If you look straight on at one of these concave masks though, it doesn't look concave at all. It looks like a normal convex face. You can see the effect rather vividly in this 3D mask portrait video [Before clicking note it is a very large file]. Your brain is being deceived by an optical illusion called the hollow face illusion, but why?

There is something funny about the eyes?

Every waking second your brain is processing massive amounts of visual information streaming from your eyes. It's a massive computation to turn this raw data into what we 'see', so our brain makes assumptions to help make these computations simpler. One such simplification is that on the whole things tend to be convex rather than concave, especially faces. So when our brain sees all the data it would expect from a normal face in the concave mask, its assumption takes over. We perceive the hollow face as a normal face, even when there are other visual clues like shading and shadows that say otherwise. Spookier still if you look at the eyes on a normal convex face they look only in one direction, but the eyes in a hollow face do something very strange. They look forward when you look directly at the face, but if you move slightly to one side, they start to look even further in this direction. The effect can be quite disturbing.

Enter the Dragons Den

You may not have access to a digital scanner or Plaster of Paris, but you can see the rotating 3D video of Alla's portrait - watch those eyes! If that movie is too large to download, you can also experiment for yourself with this sort of hollow face illusion by building a paper dragon! [EXTERNAL] If you print out the diagram and follow the instructions you can build a model dragon with a hollow head, and this head will seem to follow your eyes everywhere (even up or down). There is also a video of the illusion to watch. Have a go and see how your brain copes with the perception deception of the Hollow Face illusion.