You might think you control your eyes, but really they are constantly making jerky motions known as saccades that allow us to take in the surroundings. Moving your eyes in a smooth, sweeping motion is hard unless you are tracking a moving object, meaning eye-writing systems tend to involve focusing on one of a static list of letters then blinking to select it.
Now neuroscientist Jean Lorenceau at the Pierre-and-Marie-Curie University in Paris, France, has found a way to fool your eyes into making smooth movement by using an optical illusion called "reverse phi motion" – first discovered in the 1970s – to create a virtual target.
Phi motion is essentially the effect that turns a series of still photos into a movie, but reverse phi motion is a bit weirder. Take a film of a moving white dot then turn the dot in every other frame black, and the film will appear to run backwards – that's reverse phi in action.
Lorenceau's system uses a display covered in dots that flick from all-white to all-black. The reverse phi illusion means that moving your eye in any direction while looking at the screen makes it appear as if an on-screen dot is moving in that same direction. A gaze-tracking camera follows the right eye's movements to control a cursor on the screen. After practicing for around 30 minutes on three separate days it is possible to trace your gaze in the shape of numbers, letters or even whole words and drawings. "It's like surfing, you move your eyes to get on the wave and once you're on you just slide with it," he says. "Then you learn to turn right, turn left and make figures."
The ability to trace any shape makes this technique much more flexible than other eye-writing systems, but the trade-off is that users must periodically stop to check what they have written or drawn, as displaying the results in real-time would draw the eye and interfere with the reverse phi effect. "You need to have no ink in the eye pen," explains Lorenceau.
"Using this reverse phi motion effect is really novel," says Howell Istance, who researches gaze control at De Montfort University, Leicester, UK, but he says that existing systems are likely more effective for just entering text. "I could imagine people using this for artistic or creative expression, which is something that you can't do otherwise."