Staring at a pattern meant to evoke an optical illusion is usually an act of idle curiosity, akin to palm reading or astrology. The dot disappears, or it doesn’t. The silhouette of the dancer spins clockwise or counterclockwise. The three-dimensional face materializes or not, and the explanation always seems to have something to do with the eye or creativity or even personality.
That’s the usual cue to nod and feign renewed absorption in the pattern.
In fact, scientists have investigated such illusions for hundreds of years, looking for clues to how the brain constructs a seamless whole from the bouncing kaleidoscope of light coming through the eyes. Brain researchers today call the illusions perceptual, not optical, because the entire visual system is involved, and their theories about what is occurring can sound as exotic as anyone’s.
In the current issue of the journal Cognitive Science, researchers at the California Institute of Technology and the University of Sussex argue that the brain’s adaptive ability to see into the near future creates many common illusions.
“It takes time for the brain to process visual information, so it has to anticipate the future to perceive the present,” said Mark Changizi, the lead author of the paper, who is now at Rensselaer Polytechnic Institute. “One common functional mechanism can explain many of these seemingly unrelated illusions.” His co-authors were Andrew Hsieh, Romi Nijhawan, Ryota Kanai and Shinsuke Shimojo.
One fundamental debate in visual research is whether the brain uses a bag of ad hoc tricks to build a streaming model of the world, or a general principle, like filling in disjointed images based on inference from new evidence and past experience. The answer may be both. But perceptual illusions provide a keyhole to glimpse the system.
When shown two images in quick succession, one of a dot on the left of a screen and one with the dot on the right, the brain sees motion from left to right, even though there was none. The visual system has apparently constructed the scenario after it has been perceived, reconciling the jagged images by imputing motion.
In an experiment originated by Dr. Nijhawan, people watch an object pass a flashbulb. The timing is exact: the bulb flashes precisely as the object passes. But people perceive that the object has moved past the bulb before it flashes. Scientists argue that the brain has evolved to see a split second into the future when it perceives motion. Because it takes the brain at least a tenth of a second to model visual information, it is working with old information. By modeling the future during movement, it is “seeing” the present.
Dr. Changizi and his colleagues hold that it is a general principle the brain applies to a wide variety of illusions that trick the brain into sensing motion.
“It’s likely that there are many different neural mechanisms involved in perceptual illusions,” said Jacob Feldman, a Rutgers psychologist. “But the idea that there may be some overarching explanation that accounts for these separate mechanisms is compelling and satisfying to some scientists.”
Timothy Hubbard, a psychologist at Texas Christian University, said the principle of perceiving the present was sound, adding, “If a person’s response to an object, to catch, hit, block, whatever, is to be optimal, that response should be calibrated to where the object would be”— not a split second earlier, when the perception occurred.
This is why identical squares arranged around the center of a spoked-wheel image appear misshapen, said Dr. Changizi, who writes about it in a book due in 2009, “The Vision Revolution.” The sides of squares closer to the center appear to bulge. The sides farther out appear shorter. The radiating lines in the pattern trick the brain into perceiving motion forward, so it projects objects forward, making those nearer the center appear closer to the eye.
The same effect can be seen by leaning forward toward a precise checkerboard. The image seems to bulge forward, this time because the eyes are moving.
Dr. Changizi says such illusions can also occur in real life. When a golf ball or baseball rolls through the grass and suddenly drops into a hole, the brain sometimes perceives a trace of the ball on the other side of the hole.
“But these are things that we don’t experience very often,” he said, “because the brain is so good at covering up its mistakes.”
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