THE HOLLOW MASK ILLUSION

The hollow mask illusion is a feat of optical witchcraft that tricks your brain into thinking that the inside of a rotating mask is exactly like the outside – a face with protruding features.

The hollow-mask optical illusion is a simple test for the neurological dysfunction associated with schizophrenia. The visual senses of a person with a healthy brain will be overwhelmed by the illusion; a schizophrenic person reliably sees right through it.

What is the hollow-mask illusion? When a hollow mask is turned away from you, it will appear to be facing you; the nose, for example, will appear to be protruding from the mask. The illusion is so powerful that even if a test subject is told in advance what to look for, and even if he watches the illusion unfolding, his senses will still deceive him.

Seeing through the illusion isn’t a completely reliable indicator of schizophrenia. Some alcoholic test subjects may not be fooled by the hollow mask illusion. Being under the influence of cannabis (marijuana) may also yield a false positive.
Article:
http://www.cracked.com/article_22412_6-tiny-things-that-indicate-youve-got-huge-medical-problems.html

http://www.newscientist.com/article/dn16911-schizophrenics-see-through-hollowmask-illusion.html#.VT3B07mdK70