Do you remember the doll Great-aunt Mary gave you for Christmas when you were six? You could never figure out why that doll made you uncomfortable. She was meant to be pretty and lifelike, but she stared at you from the top shelf in the bedroom and gave you the occasional nightmare. You couldn't explain what was wrong—and your mom thought you were nuts—but now science may have determined why that doll was so creepy. It was the eyes.
We've evolved to see faces in just about anything; it makes sense since the things that can eat you generally come with two eyes and a mouth. But we also have to quickly determine which of those faces is real, and thus a potential threat, and which is just a false alarm. And so when we are looking at a face that isn't alive but meant to look like it is—dolls, for example, or computer-generated characters in movies—some of them look more lifelike than others, and it isn't always easy to pinpoint why.
Scientists from Dartmouth College, who report their findings in Psychological Science, set out to discover that line in perception, the point at which we perceive life in a face. They did this by having study participants evaluate a series of images, morphing from a completely human face to that of a mannequin (see video below). "The tipping point is consistently close to the human end of the continuum," they write. "This suggests that people base animacy judgments of a face, at least in part, on how closely the structural proportions of the face fit a human prototype."
When they examined which facial features are most important for imparting that lifelike quality, the eyes, followed by the mouth, correlated most often with animacy. "Eyes convey a wealth of information, from attention to emotion to intent; therefore it is no wonder that eyes have been the Achilles heel of CGI, with renderings of eyes described as ' unnervingly without soul.'"