If you've ever felt like that crusty old oil portrait is watching you as you move about the room, you may not be totally off yer rocker. Researchers at Rutgers University have just published an intriguing optical illusion that shows despite our best efforts to understand the world around us, our brains insist on making assumptions that sometimes mess with our heads... or is it our eyes? Oh, whatever. You know what I'm getting at...

The optical illusion is a new twist on an established illusion - that of the hollow mask. In this traditional trick, a hollow mask that is rotated as though shaking its head "no" appears to move in the opposite direction from the way it is actually being turned. In the latest iteration of this classic trick, a concave mask was coupled with a convex torso and vice versa to see what the brain would make of the situation. The result:

The researchers dubbed it "The Exorcist Illusion" because of the creepy way the neck pivots. I can't weigh in on the veracity of this claim, having censored horror flicks from my life after losing two weeks straight of sleep following the release of Silence of the Lambs (the basement chase sequence with night goggles?! Don't remind me!) But regardless of what they call it, the researchers propose two possible explanations for why the illusion works.

The first explanation has to do with a fundamental assumption our brains make about what we are looking at. Since the beginning of sight, we have been accustomed to seeing mostly convex shapes - our prey, our predators, obstacles in our path are almost all convex (I say almost because you undoubtedly just said, "Wait, what about a hole you could fall into?!" Almost all, yes.) And, since light in our world streams in from above, we have learned that in general, the lightest part of a convex object should be at its top with the shadow falling below. Scientific illustrators (and other visual artists, of course) know this because in order to achieve the illusion of 3-dimensionality on a 2-dimensional surface, we must use the assumptions our brains make to trick our viewers into seeing dimension where none exists (on a flat piece of paper).

convex/concave illusion

This principle is beautifully illustrated by this classic illusion of two artist's palettes. The palette on the left appears to have convex paint wells. The one on the right seems to be turned over so the wells are concave. In reality, they are the same image, just rotated 180 degrees. If you don't believe me, print this page and turn it back and forth 180 degrees to see that the image on the left always looks convex whereas the image on the right always looks concave. Why does our brain perceive these two identical images differently? For the simple reason that we expect the light to be coming from the top, and as such, we read the paint wells with the highlights at the top left as bulging towards us while the ones with the highlights in the lower left recede.

The competing theory the researchers propose for their Exorcist Illusion's success (and for what it's worth, the one that seems more believable to me) has to do with another concept artists know well: foreshortening. You understand foreshortening so intuitively that chances are, if I show you the following postcard and ask what shape the tire is, you'll say...

foreshortening examples

Circular, right? If anyone says elliptical, then props to you, you have an artist's eye. But for the majority of us, our knowledge that tires are round trumps what we actually see, which is an ellipse. Likewise, you have enough knowledge of the world to know that "no parking" signs are rectangular despite the fact that the one represented in the image is actually a trapezoid. Foreshortening is this principle that objects appear shorter than they actually are when viewed in perspective. So a circle gets squashed into an ellipse, a rectangle turns into a trapezoid. But because you know that tires are round and parking signs are rectangular, when you see them as ellipses and trapezoids in perspective, your brain translates this to mean they are tilted.

In the Exorcist Illusion, when the masks and torsos are rotated, their widths become foreshortened - they appear less wide than if they were facing you full-on. If viewing a convex torso, the portion of the torso rotating towards you becomes more visible as the other half becomes more obscured. But a concave mask will do just the opposite - as it is turned, more and more of the far side of the mask will become visible as the closer side gets more obscured. Your brain reads the fact that it can see more of the receding half of the mask as evidence that it is moving towards you. And voila, the illusion.

Special thanks to Professor Thomas Papathomas, lead/corresponding author on the Exorcist Illusion, for drawing my attention to the illusion and for sharing the video and the paper describing it. The Exorcist Illusion won top-ten honors for 2012 Best Illusion of the Year contest. For more mind-bending illusions, check out the other BIOTY finalists.