THE ENIGMA ILLUSION
Look at the center of the above image and notice how the concentric green rings appear to fill with rapid illusory motion, as if millions of tiny and barely visible cars were driving hell-bent for leather around a track. Neuroscientist and engineer Jorge Otero-Millan of the Barrow Neurological Institute in Phoenix created this image as a reinterpretation of Enigma by Léviant, who unknowingly combined the MacKay rays and the BBC wallboard.
But does the illusion originate in the mind or in the eye? The evidence was conflicting until we found, in collaboration with our Barrow colleagues Xoana G. Troncoso and Otero-Millan, that the illusory motion is driven by microsaccades: small, involuntary eye movements that occur during visual fixation. The precise brain mechanisms leading to the perception of the illusion are still unknown, however. One possibility is that microsaccades produce small shifts in the geometric position of the peripheral areas of the image. These shifts produce repeated contrast reversals that could create the illusion of motion. Otero-Millan's Enigmatic Eye (right), also a tribute to Enigma, reflects the role of eye movements in the perception of the illusion.
Neuroscientist and artist Bevil Conway and his colleagues at Harvard Medical School recently demonstrated that pairs of stimuli of different contrasts are able to generate motion signals in visual cortex neurons, and they have proposed that this neural mechanism may underlie the perception of illusory motion in certain static patterns.