Not fooled: In the hollow mask illusion, an inverted face mask (pictured) appears as a face to most people. Schizophrenics and people under the influence of cannabis include those who don't recognise the inverted mask as a face (click play, above, for image of the front of the mask).

Credit: UCL

A different perspective: The front side of the mask, revealing the real face.

Credit: UCL

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SYDNEY: Schizophrenics are able to see through the 'hollow mask' optical illusion, because they have difficulty coordinating different brain areas, says new research.

Previous studies showed that sufferers of schizophrenia are immune to this and also the 'contrast-contrast' illusion. The hollow mask (or hollow face) illusion occurs when an inverted mask is perceived by most people, incorrectly, as a face.

However, the neurological basis for schizophrenics not being fooled by this illusion was unknown. See a video illustrating the illusion here.

Connectivity issues

Now, researchers, lead by psychiatrist Danai Dima, at the Hannover Medical School, Germany, have tested the responses of schizophrenic patients and non-schizophrenic volunteers to 3-D images of normal faces and 'hollow' faces while they lay inside a functional MRI (magnetic resonance imaging) brain scanner.

Jonathan Roiser, another researcher involved with the study from University College London, in England, said that as expected, patients with schizophrenia were immune to the illusion.

"Furthermore, connectivity between different brain areas was impaired relative to healthy controls, who did perceive the illusion," he said.

Dima said that by using fMRI, they were able to show that the connectivity between the part of the brain involved in the internal production of concepts, and the part involved in sensory input from the eyes was increased in non-schizophrenic volunteers when presented with the illusion.

"However, in the patients with schizophrenia, this connectivity change did not occur," she said.

The study is published in the current issue of the journal NeuroImage.

Drug-induced psychosis

Dima said that these findings support a theory of schizophrenia called the 'dysconnectivity hypothesis' [sic], which suggests the disorder is caused by impaired connectivity between brain regions.

Roiser said that their findings also shed light on studies of visual illusions that have used drugs to mimic the symptoms of psychosis.

"Studies using... THC, the ingredient of cannabis resin responsible for its psychotic-like effects, have found that people under the influence of cannabis are also less deceived by the hollow mask illusion," he said.

"It may be that THC causes a temporary disconnection between brain areas, similar to that seen in patients with schizophrenia, though this hypothesis needs to be tested in further research."