Gaze-tracking illusion lets you draw with your eyes

IMAGINE drawing or writing on a screen with just your eyes. That's what a novel gaze-tracking software promises to do for people with locked-in syndrome.

Existing eye-writing systems involve focusing on a letter then blinking to select it. Moving your eyes smoothly enough to trace out words is hard because your eyes constantly make jerky motions known as saccades, unless you are tracking a moving object.

Jean Lorenceau at the Pierre and Marie Curie University in Paris, France, has found a way to fool your eyes into making smooth movements by using an optical illusion called "reverse phi motion".

Phi motion is the effect that turns a series of still photos into a movie, but reverse phi motion is weirder. Take a film of a moving white dot then turn the dot in every other frame black. The film will appear to run backwards. Lorenceau's system manipulates the illusion to trick you into thinking you are moving a dot around the screen with your eyes, so the motion is smooth. A gaze-tracking camera follows the right eye's movements to control a cursor on the screen. This lets you trace out numbers, letters or drawings. "It's like surfing, you move your eyes to get on the wave and once you're on you just slide with it," Lorenceau says.