The Hollow Mask Illusion: Beyond Charlie Chaplin

By Neuroskeptic | April 23, 2009 8:20 am

Update – This paper dragon is an even better illustration of the effect, and you can make your own (if you have a printer). Amaze your friends! Really, you will.

Everyone’s talking about the hollow mask illusion, a.k.a the hollow face illusion. Wired have a nice piece about this freaky visual phenomenon, complete with YouTube video so that you can see it for yourself. It’s seriously weird. Here it is on YouTube:

The illusion is a form of depth inversion. It involves a hollow (concave) object which appears to be non-hollow (convex). This happens whether the object is stationary or moving, but it’s even more striking when it’s in motion, as in the video above. When the mask of Charlie Chaplin rotates so that the inside is facing you, it suddenly appears as if it’s looking out at you – but rotating in the opposite direction. This happens even though you know what’s really going on.

The current surge of interest in the illusion was sparked by a recent fMRI study, Understanding why patients with schizophrenia do not perceive the hollow-mask illusion using dynamic causal modelling. But the fact that people with schizophrenia are generally immune to this illusion has been known for a long time. The illusion itself is even older – in one form or another, it goes back centuries.

But why exactly does it happen? That’s an important question, because if you want to understand why schizophrenics are immune to it, you really need to know why it works on “normal people”. (Notice that we normal people are the ones who are fooled while schizophrenics see reality as it is – R. D. Laing would be pleased).

Most people seem to assume that the answer is pretty simple: it’s expectation. We strongly expect things to be convex, so when we see something concave our brain tries to re-interpret it as convex. Easy! Hold on. There’s a bit more to it than that.

The intricacies of the mask illusion are discussed in a paper called The hollow-face illusion: Object-specific knowledge, general assumptions or properties of the stimulus? The authors, Hill and Johnston, start out by discussing three possible explanations for the illusion.

First off, it could be that the illusion is driven by object-specific knowledge, i.e. about faces. (I’ve previously discussed the theory that faces are “special” – that our brains are specialized to percieve human faces.) According to this account, our brains expect that faces are convex, not hollow and this “top-down” expectation is so strong that it over-rides the “bottom-up” data of our eyes, and we see what we expect to see. Presumably, we also have specific expectations about teddy-bears and pineapples, which is why we see the hollow jelly moulds (above) as convex…

But Hill and Johnston point out that there’s a second possibility – maybe our brains just expect everything to be convex. Rather than being about the specific object – a mask or face – the illusion might represent a more general expectation of convexity. There’s some evidence for this, because there have been reports that the illusion works even for objects which the viewer has never seen before.

A variant of this explanation claims that the illusion is about light. Maybe we expect that light always comes from above – because after all, it usually does. So we assume that the hollow mask is actually a convex face lit from above. This isn’t a very good theory, however, especially because in the video above, the light clearly comes from the side…

The final possible explanation considered by Hill & Johnston has nothing to do with expectation at all. Some people have claimed that the illusion occurs because the information reaching our eyes is ambiguous – it simply doesn’t tell us whether the object is convex or concave. However, as Hill and Johnston point out, this is only true of information reaching one eye. Because we have binocular depth perception, we should be able to tell that the mask is hollow. Also, this wouldn’t explain why the hollow mask never looks hollow. If it were ambiguous, it should be 50-50 whether it looks convex or concave.

The authors report the results of six different experiments investigating various aspects of the illusion. These are worth reading as they’re a good example of the ways in which even something as subjective as visual illusions can be scientifically studied. After considering all of the resuls Hill & Johnston conclude

In summary, the hollow-face illusion appears to reflect a combination of the explanations offered. Some ambiguously interpretable bottom-up data must be present. That, coupled with a general bias towards convexity, is sufficient to generate the illusion, even when this interpretation is incompatible with other, unambiguous, bottom-up data. However, familiar orientations and patterns of shading and surface-colour information can greatly enhance this effect for both faces and other familiar objects.

In other words, expectation drives the illusion in the context of ambigious information, and while the expectations in question don’t need to be about specific objects, like faces, it helps if they are.

ResearchBlogging.orgHill, H., & Johnston, A. (2007). The hollow-face illusion: Object-specific knowledge, general assumptions or properties of the stimulus? Perception, 36 (2), 199-223 DOI: 10.1068/p5523

CATEGORIZED UNDER: papers, philosophy
  • pongba

    Great post. I always thought this illusion occurs because we strongly expect a face to be convex. But now it seems a lot complicated than that..
    Thanks! :)

  • BEST

    Nice Article… I enjoyed reading it…



  • Sillysighbean

    The older I get the less I trust what my brain tells me.

  • katiedid

    Okay, the first three times I watched it I couldn’t see it at all and now I can’t UNsee it. It’s so weird.

    I also thought it was just expectation, this is very interesting.

  • Neuroskeptic

    katiedid: Yeah, that’s one of the most interesting things about illusions – the fact that they’re completely out of concious reasoning… you see them, even though you know they’re illusions.

    the explanation of the illusion in terms of “expectation” is not completely wrong, but it’s not complete. It leaves lots of unanswered questions, like what exactly is the expectation, and is it concious or “subconcious” (and what does that mean)?

  • Anonymous

    man that scary looking figure in the back left freaked me out too much 😛

  • Jez

    I couldn't see the illusion at all until I paused the video, from then on I couldn't perceive it as being anything but convex (even in motion). What does this say about me, am I borderline schizophrenic?

    The jelly mold pictures all looked convex to me and I have never been able to look at a picture of the lunar surface without seeing the craters appear as convex.

  • Jake

    I love the paper dragon illusion. Well worth downloading it and making it up. Hours of amazement in the office.

  • Neuroskeptic

    That paper dragon illusion is amazing, I strongly recommend people to follow that link, it only takes 10 minutes to make your own paper dragon and the effect is unbelievable.

    You do need to look at it with one eye closed to get the full effect.

  • aashish

    Really great post on Illusion, enjoyed reading it.
    visit my blog @ Charlie Chaplin

  • we

    Great stuff Neuroskeptic.

  • Acai Berry Select Reviews

    Keep it up!!! You are too good at your work… how do you manage to keep writing superbly each time. Is there some hidden secret which you do not want to share…

  • Anonymous

    GAH. The suited man in the corner of the room in your video was freaky enough to distract me from watching it for too long :(

  • XeeMez AsHu

    Great Post…Technologically it’s superb. I also associate myself with technology.see it here.



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About Neuroskeptic

Neuroskeptic is a British neuroscientist who takes a skeptical look at his own field, and beyond. His blog offers a look at the latest developments in neuroscience, psychiatry and psychology through a critical lens.


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