When people see the optical illusion known as "the hollow mask" - a concave face, like the back side of a hollow mask - they see a normal convex face, according to a Wired Science post (see photo below). But the approximately 1 percent of the population with schizophrenia see the concave face.
"Healthy participants perceive a hollow mask as a normal face, presumably due to the strength of constraining top-down influences, while patients with schizophrenia do not," the authors write in the joint UK/German study published in NeuroImage. Apparently cannabis users may also be less deceived by the illusion. More on that later.
The study examined how two parts of the brain communicate with each other: visual cues from the eyes, or the "bottom-up" process, and the brain's internal interpretation of the information, or the "top-down" process.
Illusions work because the brain interprets visual cues in the context of your previous experiences - a top-down process. A face should be convex, according to your brain, so you see a convex face. In schizophrenic patients, there is a disconnect between the top-down and bottom-up processes, the researchers hypothesize, so they are better able to see the face for what it is in reality - concave.
Researchers looked at fMRI scans of healthy controls and schizophrenic patients that were shown the hollow mask illusion. The results showed that schizophrenic patients rely more on the visual cues - bottom-up - whereas healthy volunteers used interpretation (top-down processes) more.
What about the cannabis effect? One of the authors, Jonathan Roiser, said that studies using natural or synthetic THC (responsible for psychotic-like effects) have found that subjects under the influence are less deceived by the optical illusion. THC may cause a temporary disconnection between the two brain areas, similar to schizophrenic patients, but Roiser said more research is needed to test this hypothesis.
All very interesting so far. But here's the freaky part: watch this video, which was included in the Wired post. "In healthy viewers, the illusion is so powerful that even when aware of the illusion, they are unable to see the concave face — the mind just flips it back," the Wired blogger says.
Here's the press release from the UK center (or centre, if you prefer).
*Second period, Washington Capitals v. Atlanta Thrashers, Caps up 3-2. While San Jose may have a better statistical shot at the Stanley Cup, I still maintain that the Caps are the most exciting young team in the league. Suck it, Hank.