The Brain Is Still A Giant Mystery

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:

The New York Times > Science > Image > Distorted or Blurry, the Face Shines ThroughThis 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.

By 9 December, 2009.    mental health, normality, social attitudes   


  1. I actually only saw the hollow face as not hollow when it came around for the third time. Before, it looked perfectly hollow to me. *shrug* My brain has always been weird when it comes to optical illusions. Some I never get, no matter how often people explain it to me. Others work very well. And for some I can and do switch back and forth between the two versions, which can be very annoying and distracting. For some reason, a lot of people who had me look at optical illusions (mostly teachers) seemed to think that being able to see both is a sign of intelligence. I’m not so sure.

  2. That mask thing – holy crap! No matter how hard I try, I can only see it as concave when it’s just partially turned and I can’t see the features. That’s wild!

  3. I see it as concave until the back side of the mask is full on to the camera. Then it’s convex and I can’t see it as concave any more until the next time it come around. Like Rodo some illusions swap back and forth between interpretations. Sometimes very rapidly and it’s uncomfortable.

    Oh yeah the brain is still a giant mystery. For all we think we know about how it works and how researchers interpret fMRI and other scan data it’s still very large-scale stuff and the brain works on a very very fine scale indeed. We know some of the larger structures are related to some functions because when those structures stop working some people stop being able to do some things. But not everyone. There are clearly redundancies and workarounds and we simply do not know the how of very basic stuff like memory storage. What changes physically and/or electrochemically when we make new memories? What happens with how memories are kept to make some of them so unreliable?

    We don’t know. It may be a long time before we have the technology to be able to examine a working brain on on the scale it works at in the time frames it does stuff. It would be kind of nice if neuro researchers and especially pop science writers would just be okay with that and say “We don’t know yet” instead of coming up with bullshit just-so stories that astonishingly often just happen to reinforce existing privilege structures.

  4. We know more about our oceans, which isn’t very much, than we do the human brain.

    There is a condition called Synesthesia in which the person sees sound, smells colors etc. It is really interesting.

    Synesthesia is a neurologically based phenomenon in which stimulation of one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway.

    Synesthesia is a condition in which one sense (for example, hearing) is simultaneously perceived as if by one or more additional senses such as sight. Another form of synesthesia joins objects such as letters, shapes, numbers or people’s names with a sensory perception such as smell, color or flavor. The word synesthesia comes from two Greek words, syn (together) and aisthesis (perception). Therefore, synesthesia literally means “joined perception.”

  5. The thing is that the moe ofthese illusions you see and play around with, and the more often you’ve seen them lately, the better you get at seeing what you want to see (note that there are individual differences for people there). Not all the claims people make about these illusions (note the voice in the video claiming seeingthe hollow side asconcave is impossible) are true or even logical in any way (like the one with the spinning picture where which way you see it spinning is supposed to indicate which side of your brain is dominant. I’ve had lots of testing done which indicates (if true, but I still think it’s more reliable than this) that the left side of my brain is very much dominant, but I see the picture spinning in one direction or the other at different occasions, and sometimes I see it spinning in different directions while looking at it, I mean like the bottom half will go one way and the top half another, stuff like that).

    I only saw the mask hollow side as concave if I let it be, but I can switch it at will to seeing it convex if I put in a little bit of effort. If I played with it a while I’d probably be able to see all kinds of strange stuff in that too.

  6. Oh, and sorry for the rapid posts in succession, but I forgot something…

    Just recently I saw a colourblindness test being advertised as an IQ test. This does nothing to increase my confidence about how much these tests say, especially from any online source, fun though they may be.

    (I had really interesting results from an official colourblindness test some professional once had me do out of interest, since my brother is colourblind and he just wanted to see how I’d do.)

  7. As further proof of what we don’t know about the brain: remember the fMRI of a dead fish.

    Be very wary of trusting people who claim this or that about brain structure of all people, women, folks with a particular disability, etc based on fMRI of a few subjects. Because they could have been dead fish.

  8. How cool!

    I watched it go round about twelve times, and even used the pause button to slow it down. Knowing what was coming I was able to see it as concave right until the back side was fully facing me, when it became convex, and wouldn’t go back until it was about 3/4 around. Playing with it . . . a couple of times I couldn’t do it at all, and once, just once, I was able to hold the illusion for all but a split second, again, right when the mask is facing fully away and the illusion is most complete.

    Neat stuff.

  9. I wonder how this could potentially relate to dopamine, which is the neurotransmitter connected to pattern seeking, and also, per several theories, schizophrenia?

    Thanks for an inneresting piece.

  10. Dopamine is supposedly connected to a lot of things, seems they find a sort of drug that they at least claim works on something (even if it only does a tiny fraction of the time or for reasons unrelated to the main known property of the drug) and then they claim that condition is related to the opposite of what the drug does. (Especially inaccurate if the condition in question is actually several very different conditions that bear one name due to some guy a century ago making a fairly ridiculous guess that they were all caused by the same cognitive mechanism.)

  11. It worked opposite of the posters above for me. It looks most convex when the features were visible, but it was turned slightly (it kind of looked like those busts in the Haunted Mansion at Disneyland that seem to follow you as you walk past), but when it was straight on, it looked concave. (Though I could also see it as convex if I unfocussed slightly, like a Magic Eye Puzzle, though usually I am horrible at those and can’t see the secret image.)

  12. 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.


    I wonder if anyone will realize how unteneble it is to maintain that there is only one reality, arbited by NTs, and that anyone who is not experiencing that particular reality is broken somehow. In this case, NTs are actually seeing their brain function in a way that makes them perceive something incorrectly.

    People should realize that brains are complicated things, and that we *all* have *perception.* All of us. We all have various factors influencing how we perceive the world. And there is no such thing as “The World” objectively, and us getting it straight unless we’re broken somehow (like having schizophrenia). Rather, we all have different, unique lenses through which we view the world, and we need to have the flexibility to take into consideration someone else’s lens and its equal validity to ours.
    .-= amandaw´s last blog ..Inertia =-.