"The Hollow Mask Illusion
Posted on: April 14, 2009 11:19 AM, by Jonah Lehrer
Wired Science reports on a fascinating finding: schizophrenics have trouble seeing the hollow mask illusion, in which people perceive the concave inside of a mask as an actual face. The reason we're vulnerable to this illusion is that our expectations of what we'll see - we're used to seeing real faces - profoundly influence our actual sensations. Schizophrenics, however, seem to have trouble with modulating their perceptions, which might explain why they persistently believe in delusions and far-fetched conspiracy theories. (Interestingly, the only other group of people who don't see the hollow mask illusion are those drunk on alcohol or high on cocaine.)
The polite term for this mental ability is "top-down processing," a term that describes the way cortical brain layers project down and influence (corrupt, some might say) our actual sensation. After the inputs of the eye enter the brain, they are immediately sent along two separate pathways, one of which is fast and one of which is slow. The fast pathway quickly transmits a coarse and blurry picture to our prefrontal cortex. Meanwhile, the slow pathway takes a meandering route through the visual cortex, which begins meticulously analyzing and refining the lines of light. The slow image arrives in the pre-frontal cortex about 50 milliseconds after the fast image.
Why does our mind see everything twice? Because our visual cortex needs help. After the prefrontal cortex receives its imprecise picture, the "top" of our brain quickly decides what the "bottom" has seen, and begins doctoring the sensory data. The end result is we see a face where there are only concave shadows.
Sure enough, this latest experiment saw differences in activation in the frontoparietal network in schizophrenics as they looked at the illusion, suggesting that the lack of top-down processing was responsible. The irony, of course, is that the absence of top-down processing doesn't simply lead to a more literal view of the outside world. It could also be responsible for schizophrenic hallucinations. T.S. Eliot was right: "Humankind cannot bear very much reality."