What if you duplicate some of the features of a portrait without overlapping them completely? It is relatively easy to create images in Photoshop in which the eyes and the mouth, but no other facial features, have been doubled. The results are little short of mind-bending: as the brain struggles (and fails) to fuse the doubled-up features, the photograph appears unstable and wobbly, and observers experience something akin to double vision.
The neural mechanisms for this illusion may lie within our visual system’s specialized circuits for face perception. If you double up the eyes and mouths in a portrait, the neurons in the face-recognition areas of the brain may not be able to process this visual information correctly. Such failure could make the faces unsteady and difficult to perceive.
THE IRIS ILLUSION
This illusion, by vision scientists Jisien Yang and Adrian Schwaninger of the Visual Cognition Research Group at the University of Zurich, was one of the top 10 finalists in the 2008 Best Illusion of the Year Contest. It shows that context, such as the shape of the eyelids and face, affects the apparent distance between the irises. Consider the pair of Asian faces shown here: the distance between the left eye of the right face and the right eye of the left face seems short. In the Caucasian faces, the separation looks wider. Notice the reconstructions of the eyes and irises below each face: without the context of the face and eyelid shapes, it is clear that the irises are equally spaced. Visit http://illusioncontest.neuralcorrelate.com/2008/yangs-iris-illusion for more details.
HERE’S LOOKIN’ AT YOU, KID
Vision researcher Pawan Sinha of the Massachusetts Institute of Technology shows us with this illusion that our brains have specialized mechanisms for determining gaze direction. In the normal photograph of Humphrey Bogart (left), the actor appears to be looking to his left, but in the photo negative (right) he appears to be looking in the opposite direction. Yet Bogart’s face does not look backward; only the eyes are reversed. Why? The answer is that we have specialized modules in our brain that determine gaze direction by comparing the dark parts of the eyes (the irises and pupils) with the whites. When the face is negative, the whites and irises appear to swap position. Our knowledge that irises are light rather than dark in a negative does not change our perception of this illusion.