FOLLOW MY FINGER
The artists who drew these World War I recruiting posters knew something about eye tracking. No matter how you look at Uncle Sam (right) or British Field Marshal Lord Kitchener (left), the eyes and finger seem to be pointed directly at you. Today you can experience the same phenomenon in an art museum, where the painted eyes in portraits sometimes seem to follow you around the room.
Such eye tracking is not only a B-movie horror flick cliché but also a powerful illusion that continues to inspire visual science studies. In 2004 vision psychologists Jan Koenderink, Andrea van Doorn and Astrid Kappers of the University of Utrecht in the Netherlands, along with James Todd of Ohio State University, concluded that, contrary to popular belief, this compelling illusion does not require special artistic abilities on the part of the painter. Surprisingly, all that is required is that the person portrayed looks straight ahead, and the visual system takes care of the rest.
The deceptively simple explanation is that when we look at a real human face or anything else in our three-dimensional physical world, the visual information that specifies near and far points changes with our viewing angle. But when we observe a two-dimensional painting or photograph hanging on the wall or a poster such as the ones above, the visual information that defines near and far points remains unaltered by our viewing angle. The brain interprets this information as if it pertained to a 3-D object, however. That interpretation is what creates the eerie sensation that a portrait's eyes are following you.