Schizophrenia sufferers aren't fooled by an optical illusion known as the “hollow mask” that the rest of us fall for because connections between the sensory and conceptual areas of their brains might be on the fritz.

In the hollow mask illusion, viewers perceive a concave face (like the back side of a hollow mask) as a normal convex face. The illusion exploits our brain's strategy for making sense of the visual world: uniting what it actually sees — known as bottom-up processing — with what it expects to see based on prior experience — known as top-down processing.

"Our top-down processing holds memories, like stock models," explains Danai Dima of Hannover Medical University, in Germany, co-author of a study in NeuroImage. "All the models in our head have a face coming out, so whenever we see a face, of course if has to come out."

This powerful expectation overrides visual cues, like shadows and depth information, that indicate anything to the contrary.

But patients with schizophrenia are undeterred by implausibility: They see the hollow face for what it is. About seven out of 1000 Americans suffer from the disease, which is characterized by hallucinations, delusions, and poor planning. Some psychologists believe this dissociation from reality may result from an imbalance between bottom-up and top-down processing — a hypothesis ripe for testing using the hollow mask illusion.

In healthy viewers, the illusion is so powerful that even when aware of the illusion (see video below), they are unable to see the concave face — the mind just flips it back. Though the illusion is strong for faces, it doesn't work well with other objects, or even with upside-down faces. This bias is likely due to the special relationship we humans have with faces. Many neuroscientists believe we have brain regions dedicated to processing faces, and some brain injuries can leave patients unable to recognize faces, even though their vision and other memories remain intact.

Dima and Jonathan Roiser of University College London wanted to understand why people with schizophrenia aren't fooled. They put 13 schizophrenia patients and 16 healthy control subjects in an fMRI scanner that measures brain activity, and showed them 3D images of concave or convex faces. As expected, all of the schizophrenic patients reported seeing the concave faces, while none of the control subjects did.

Dima and Roiser analyzed the fMRI data using a relatively new technique called dynamic causal modeling, which allowed them to measure how different brain regions were interacting during the task. When healthy subjects looked at the concave faces, connections strengthened between the frontoparietal network, which is involved in top-down processing, and the visual areas of the brain that receive information from the eyes. In patients with schizophrenia, no such strengthening occurred.

Dima thinks when healthy subjects see the illusion, which is somewhat ambiguous, their brains strengthen this connection such that what they expect — a normal face — becomes more influential, overpowering the actual, though unlikely, visual information. Schizophrenia patients, meanwhile, may be unable to modulate this pathway, accepting the concave face as reality.

Schizophrenics aren't the only ones who see the concave face — people who are drunk or high can also 'beat' the illusion. A similar disconnect between what the brain sees and what it expects to see may be occurring during these drug-induced states.

Citation: "Understanding why patients with schizophrenia do not perceive the hollow-mask illusion using dynamic causal modelling" by Danai Dima, Jonathan P. Roiser, Detlef E. Dietrich, Catharina Bonnemann, Heinrich Lanfermann, Hinderk M. Emrich, Wolfgang Dillo, NeuroImage, In Press, Available online 24 March 2009.

See Also:- The Meaning of a Schizophrenic Mouse

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