Schizophrenia patients can see through an illusion known as the 'hollow mask' illusion, probably because their brain disconnects "what the eyes see" from what "the brain thinks it is seeing".
The observations made during the study attain significance as they may help understand why cannabis users may also be less deceived by the illusion whilst on the drug.
The study, carried out by scientists at the Hannover Medical School in Germany and UCL Institute of Cognitive Neuroscience in the UK, confirms that people with schizophrenia are immune to certain vision illusions, and that they are not fooled by the 'hollow mask' illusion.
The researchers say that this may relate to a difference in the way two parts of their brains communicate with each other, the 'bottom-up' process of collecting incoming visual information from the eyes, and the 'top-down' process of interpreting this information.
They point out that illusions occur when the brain interprets incoming sensory information on the basis of its context and a person's previous experience, so called top-down processing.
Sometimes this process can mean that people's perception of an object is quite different to reality - a phenomenon often exploited by magicians.
The researchers say that their study suggests that schizophrenia patients rely considerably less on top-down processing during perception.
During the study, three-dimensional normal faces and hollow faces were shown to schizophrenia patients and control volunteers, while they lay inside an fMRI brain scanner, which monitored their brain responses.
The researchers observed that all 16 control volunteers perceived the hollow mask as a normal face, mis-categorizing the illusion faces 99 percent of the time.
On the other hand, all 13 schizophrenia patients could routinely distinguish between hollow and normal faces, with an average of only six percent mis-categorization errors for illusion faces.
Upon a brain imaging analysis, the researchers observed that in the healthy volunteers, connectivity between two parts of the brain, the parietal cortex involved in top-down control, particularly spatial attention, and the lateral occipital cortex involved in bottom-up processing of visual information, increased when the hollow faces were presented.
The schizophrenia patients did not show that connectivity change.
The researchers say that the findings indicate that schizophrenia patients have difficulty coordinating responses between different brain areas, also known as 'dysconnectivity', and that this may contribute to their immunity to visual illusions.
They are presently studying dysconnectivity in schizophrenia further, which will hopefully advance the scientific understanding of this disorder.
Danai Dima, Hannover Medical School, says: "The term 'schizophrenia' was coined almost a century ago to mean the splitting of different mental domains, but the idea has now shifted more towards connectivity between brain areas. The prevailing theory is that perception principally comprises three components: firstly, sensory input (bottom-up); secondly, the internal production of concepts (top-down); and thirdly, a control (a 'censor' component), which covers interaction between the two first components. Our study provides further evidence of 'dysconnectivity' between these components in the brains of people with schizophrenia."
Dr. Jonathan Roiser, UCL Institute of Cognitive Neuroscience, says: "Our findings also shed light on studies of visual illusions which have used psychomimetics, drugs that mimic the symptoms of psychosis. Studies using natural or synthetic tetrahydrocannabinol (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. 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."
The study has been published in the journal NeuroImage.