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.