November 17, 2017
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Optical illusion may soon help paralyzed people write with their eyes

By Emily Underwood, ScienceNOW

People “locked in” by paralyzing disorders such as Lou Gehrig’s disease have long relied on blinks or facial twitches to build sentences one letter at a time. But they soon might be able to take advantage of a simpler, faster mode of communication, new research suggests. With the help of an old optical illusion, people can train their eyes to write and draw in cursive on a computer screen as quickly as they can write with a pen. In addition to providing a new medium for self-expression, the technique challenges traditional ideas about the limits of human vision.

In 1970, illusionist and cognitive psychologist Stuart Anstis of the University of California, San Diego, was playing around with a common visual trick when he stumbled on a strange phenomenon. He was moving white dots around by hand, photographing them in different positions, and then projecting them in rapid succession to create phi motion — the visual illusion that the dots are moving. Feeling frugal, he refused to throw out the film negatives and had them made into slides as well. He accidentally slipped one of the negative slides into the projector wheel. As the dots switched to black and then back to white, their direction of movement appeared to reverse. He dubbed the odd, flickering effect “reverse phi motion.” The illusion helped reveal that when the brightness of an object changes rapidly; our brain “sees” the object moving in the opposite direction.

Four decades later, cognitive neuroscientist Jean Lorenceau of the Université Pierre et Marie Curie in Paris was idly watching a reverse phi motion illusion on a computer screen when he noticed that as his gaze drifted across the field of flickering dots, it looked as if the field was moving in the same direction as his eye movements. This enabled him to do something that had previously been thought impossible, he says: control and sustain the continuous eye motion we use to track moving objects, such as a car driving along road. Called smooth pursuit, this eye motion is different from saccadic motion, in which we rapidly shift our eyes to, say, skim lines of text or scan a crowd. One reason that smooth pursuit is so difficult to start and maintain, says Lorenceau, is that in normal tracking situations the background constantly slips away from the object in motion, inducing saccades. But in a reverse phi illusion, Lorenceau explains, the background moves with the gaze, like a wave. All a viewer has to do, he says, is learn to “surf.”

Using eye-tracking technology that recorded his eye movements with a video camera, he found that he could gain enough control over his smooth pursuit eye movements to write letters and numbers and even sign his name using only the motion his eyes created on the screen. To determine if other people could learn the technique, Lorenceau designed his own reverse-phi display with 200 disks that switch between black and white and are projected on a gray background.

Over three 30-minute sessions, he trained six volunteers to surf the illusion. “At the beginning, it is difficult to get on the wave,” he says. Even when you catch it, “at first you go directly to shore,” meaning that instead of moving smoothly, the subjects’ eyes jerked. Eventually, however, they learned to make small turns and then to fully control their eye movements, Lorenceau and colleagues reported Thursday in Current Biology. For the volunteers, who couldn’t see what they were writing, it was like writing with a pen that had run out of ink, he says.

Although Lorenceau notes that some participants had a harder time of learning to control their eye movements than others, by the end of the sessions most could freely draw legible letters and numbers on the reverse phi visual display. Contrary to current opinion, says Lorenceau, his study shows that people can indeed gain control over smooth pursuit eye movements. Anstis agrees, saying that the smooth pursuit system is “much cleverer than we used to think.” Richard Krauzlis, a neuroscientist at the Salk Institute for Biological Studies in San Diego, California, doesn’t think that’s necessarily new information, as previous studies have shown that humans do sometimes start smooth pursuit in anticipation of an object’s movement. But he agrees that the result is “very clever and very cool,” particularly as a way for people who have lost the use of their limbs to express themselves. Unlike blinking at letters on a screen, which doesn’t allow for the flourishes of a handwritten signature, smooth pursuit creates a “continuous stream of eye writing that can reflect the writer’s own character.” The next step, says Lorenceau, is to improve his training so that everyone, including paralyzed patients, can learn the technique.

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