"What people love about the Mona Lisa is that her expression changes as you look at her, and that makes her seem alive," says Margaret Livingstone, a neurobiologist at Harvard University. She insists that the expression really does change, but the switch occurs in your eye, not in the paint. Our central vision is good at picking up small details, whereas peripheral vision processes blurry features, called low spatial frequencies. Mona Lisa's smile is painted in soft tones and muted colors that fall into those low frequencies, Livingstone finds. "You cannot see it with your central vision. It appears only to your peripheral vision when you look away from the mouth," she says.

Soft tones emphasize Mona Lisa's smile. Harder-edged features evoke an enigmatic, somber expression.
Photograph courtesy of Margaret Livingstone/Harvard University.

A similar effect is at work in the paintings of the late-19th-century pointillists, as well as in modern portraits by Chuck Close and photomosaics by Robert Silvers (often used in advertising). "Wherever you look, you see individual dots, but your peripheral vision puts it all together and blends the colors, so as you move your eyes around, what you see changes," Livingstone says. This interpretation would probably surprise the pointillists, who thought their paintings were about color mixing. Likewise, Livingstone doubts Leonardo understood the nature of his visual trick: "He wrote about a lot of things, but he never wrote about this, and he never did it again. My theory is that he saw the same wonderful, lifelike quality in the Mona Lisa that everyone else did but never understood how it worked."