Science: Reverse-phi motion—the illusion that an image that alternates between black and white is moving in the direction opposite its actual motion—has been used to teach people to control “smooth pursuit” eye tracking and could form the basis of a new means of communication. Smooth-pursuit tracking is not easy to maintain because human eyes normally move in a start-and-stop manner called “saccadic motion.” Because the background in reverse-phi illusions appears to flow, it can be used to train the eye to follow that flow. Jean Lorenceau, a cognitive neuroscientist at the Pierre and Marie Curie University in Paris who developed and tested the technique, also discovered that people who have learned to control their smooth-pursuit eye motion can then use that ability to “write” with their eyes by means of eye-tracking software. That ability could allow people who have been paralyzed by disease or injury to communicate much more easily than with the systems currently available.