John Kubie is a neuroscientist and my colleague at SUNY’s Downstate Medical Center in Brooklyn. He records from neurons in the hippocampus—the part of the brain that was the focus of this year’s Nobel Prize for Medicine or Physiology, awarded to John O’Keefe and May-Britt and Edvard Moser. Kubie thinks deeply about all kinds of neuroscience and blogs for the Society for Neuroscience. He’s come up with an idea for how to interpret the classic illusion known as the Hollow Mask. Here’s an example of this illusion from Thomas Papathomas, which won third prize in the 2008 Best Illusion of the Year Contest:
The Hollow Mask is a special case of a large family of illusions based on reverse perspective—the 3D representation of an object or scene in which the parts of the image that are nearest to the viewer appear farthest away.
Standard linear perspective was developed by Renaissance architects and artists, who determined that the perceived size and detail of objects is related to their distance from the eye. The angle between the various edges of an object, within the image projected on the retina, changes with either object size or distance. Let’s say you are walking down the street in a new suburban housing subdivision in which all the houses are cookie-cutter versions of each other. The house closest to you, and the house half-mile down the road, have different sizes on your retina, but your brain concludes, correctly, that they are the same size. Artists run rampant with this brain mechanism and do crazy illusory things like put pigments on a flat canvas that look like two identically-sized houses a half-mile apart in a subdivision, despite the fact that, in the painting, the smaller farther-looking house really is smaller.
Reverse perspective takes these shenanigans a step farther to play havoc with your brain’s model of the external world. Here’s the world’s premier reverse perspective painter—Patrick Hughes—describing his art with relation to other types of perspective:
One of the very strange effects of reverse perspective is that the images seem to follow you as you pass by them. As if, while you are observing them, the pictures are watching you back. Kubie realized that, in the case of the Hollow Mask illusion, this must have to do with how viewers track the perspective of the nose of the nose with respect to the rest of the face. As he shows in the diagram below, with normal perspective the nose blocks more and more of the face as you move from being directly in front of the face to either side, whereas with the Hollow Mask illusion, the nose does not block any part of the face as you move from directly in front of the image. The brain’s interpretation is that the face follows you as you move. Creepy!