Optical illusions are one of those things that seem fun and frivolous but actually illustrate deep insights into how brains function. I recently saw an illustration of the “hollow mask illusion” over at the Wired Magazine Blog and it made my jaw drop. In the illusion, a person viewing a concave face (like the back side of a hollow mask) perceives it as a convex face, like the front side of the mask. This illusion is so strong that even when a person is aware of the illusion they are still unable to see the concave face because the brain perceives it as a convex face.
Check out this video and see if you perceive the concave face as concave or convex, or both:
Description of video: hollow Charlie Chaplin mask is attached to a rod, rotating slowly. As the mask rotates and the concave inner side of the mask comes into view, it seems to pop outwards, becoming convex.
I have watched this video upward of ten times, and no amount of anticipation, concentration, stern looks, putting hands on hips, or even cursing has allowed me to see the concave side of the mask as concave. Every single time, the visual information goes into my brain and some filter is applied to the raw information and POOF it’s convex again. It seems that this illusion works only for human faces, as “it doesn’t work well with other objects, or even with upside-down faces.” This suggests that there is a program in the brain trying really hard to find face-like patterns in visual information – this is why we see faces in clouds, or snow covered mailboxes or even grilled cheese sandwiches. We can also recognize faces when they are distorted or compressed or otherwise obscured:
This is likely a developed ability to allow us to recognize faces quickly and unconsciously instead of having to consciously process the facial features and determine that it was a face. It was helpful to humans to be able to recognize faces of other humans, both to determine that they were human and to distinguish them from other humans.
The results of a recent study strengthen the suggestion that this is a process applied by the brain on received visual input. Two researches in London found that people with schizophrenia did not experience the illusion and reported seeing concave faces. When they used an fMRI scanner to measure brain activity on people with and without schizophrenia while viewing the rotating mask, there was a significant difference between the brain activity of people with and without schizophrenia:
[The researchers] analyzed the fMRI data using a relatively new technique called dynamic causal modeling, which allowed them to measure how different brain regions were interacting during the task. When [neurotypical] subjects looked at the concave faces, connections strengthened between the frontoparietal network, which is involved in top-down processing, and the visual areas of the brain that receive information from the eyes. In patients with schizophrenia, no such strengthening occurred.
Dima thinks when [neurotypical] subjects see the illusion, which is somewhat ambiguous, their brains strengthen this connection such that what they expect — a normal face — becomes more influential, overpowering the actual, though unlikely, visual information. [People with s]chizophrenia , meanwhile, may be unable to modulate this pathway, accepting the concave face as reality.
What I think is most interesting about all this is the clear illustration that the functioning of the brain is still largely a mystery to us. This illusion basically exploits a processing error that occurs in neurotypical brains and, incidentally, is used in a lot of the illusions at Disneyland’s Haunted Mansion ride. But there are so many other processing glitches that instead of being seen as an amusing and harmless byproduct of a neurotypical brain, are seen as problematic and annoying if they are credited at all. I work and concentrate better when I play music or I take a three minute break to play Bejeweled. Why? I have no idea. But I doubt that any supervisor asking me why I’m playing Bejeweled could explain to me why their brain perceives the concave mask as convex. Or could stop doing it if they just tried a little harder. Maybe if we had a better understanding of how brains worked it would be easier for people to understand and credit the reality of mental disabilities.
Note: clearly, this illusion is not a diagnostic test for schizophrenia and should not be treated as such.