This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Our nervous systems are good at establishing connections. You learn your boss’s name on your first day at work and from then on associate it with her face. You enjoy a new food at a restaurant and start ordering it when you go out to eat. Repeated experiences such as these strengthen neural connections and form the bases for powerful expectations about our sensory experiences. If you have children or have interacted with very young infants, you may remember the point in time in which they appeared to discover their own hands. Babies will wave and turn around their hands in front of their faces, time and again, for minutes at a time, throughout the course of the day. Some weeks or months after, they stop playing that game: they have figured out that the hands are theirs and can move them in certain ways to achieve specific results. They have associated the visual image of their hands and the related kinematics in an enduring fashion. The association will persist, and grow stronger, through their childhood and into their adult lives. In many individuals, the link will be so robust that hand motion, by itself, will produce a visual image, even in total darkness.
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A team of scientists at the Universities of Rochester and Vanderbilt has found that study participants can see, and follow with their eyes, a ghostly image of their hand, when they wave it in front of their completely occluded eyes. Some of the experimental subjects in which the visual illusion was strongest were grapheme-color synesthetes: people who experience numbers and letters as having particular colors. You can try to replicate the experiment at home: in complete darkness, wave your hand slowly in front of your face and see if you can discern the (illusory) motion. Then have somebody else wave his or her hand in front of your face instead. The kinematics of your own hand motion may produce an illusion in the first case, but somebody else’s hand kinematics will not generate a comparable sensation, because your brain has not learned to associate your visual experience with their hand motion.