Normal human subjects are readily fooled by a 3D representation of a face mask, but schizophrenics are not.
This visual trick is known as the hollow mask illusion and consists of a 3D representation of a hollow, concave mask of a face, viewed pointing inwards. When healthy individuals look at this, more than 99% of the time what they report seeing is a normal face that is convex. This illusion exploits the brain's system for making sense of the visual world by superimposing what it expects to see, based on past experience and memories, with what it is actually seeing. Yet patients diagnosed with schizophrenia almost never fall for it and instead report seeing a "hollow" face.
But why? To find out a joint UK and German study published in the journal Neuroimage brain scanned 16 healthy volunteers and 13 schizophrenics as they experienced the illusion, which was presented to them using a 3D headset. Hannover Medical School researcher Danai Dima, together with UCL scientist Jonathan Roiser, then compared the brain scans and found that, in the healthy controls, a region of the brain called the parietal cortex increased its connectivity to the brain's primary visual areas when the image was being presented. In the schizophrenic patients, however, this activity boost did not occur.
What this shows, the researchers explain, is that in schizophrenics there is a connectivity problem whereby they struggle to unite the contributions of different brains regions and therefore experience stimuli as their raw components. Whilst this makes them immune to visual illusions it also means that they cannot, for instance, tell internally generated stimuli, such as a voice in their head, from stimuli originating from outside the body, such as someone else's voice.
Intriguingly, it's not just schizophrenia sufferers who cannot experience this illusion. Smoking cannabis also prevents people from seeing it, suggesting one action of this drug might be to provoke a stage of dysconnectivity between different brain regions, perhaps explaining the observed link between cannabis use and an increased risk of developing psychosis.