For hundreds of years, art critics have mused over why the Mona Lisa's smile seems so mysterious. Now the Harvard neurobiologist Margaret Livingstone has a fascinating answer: It's because Da Vinci painted her face in colors that play tricks on the eye.

Livingstone's work has long examined the way that different cells in the visual system process different types of information -- such as form, color, depth and movement. When she analyzed the Mona Lisa, she found that Da Vinci painted her smile almost completely in low spatial frequencies, and these are best picked up in your peripheral vision. The result, as she notes on her web site, is a nifty illusion:

These three images -- [pictured above!] -- show her face filtered to show selectively lowest (left) low (middle) and high (right) spatial frequencies.

So when you look at her eyes or the background, you see a smile like the one on the left, or in the middle, and you think she is smiling. But when you look directly at her mouth, it looks more like the panel on the right, and her smile seems to vanish. The fact that the degree of her smile varies so much with gaze angle makes her expression dynamic, and the fact that her smile vanishes when you look directly at it, makes it seem elusive.

It's somewhat like the way rods in the eye are more numerous in the periphery of our retinas -- so the best way to see a faint star in the night sky is to look slightly to the side of it. Either way, this is really cool science.

Cooler than the actual Mona Lisa, really. When I visited the painting a few year ago, I found the experience incredibly underwhelming, because the painting is so ferociously guarded by security devices: A velvet rope preventing you from coming closer than 20 feet, storm troopers with tasers, and, worst of all, a plastic box that produces reflections of light so garish that they destroy any effect Da Vinci was trying to make. Seriously -- I get a more moving artistic experience when I look at a low-rez gif of the painting on Flickr. Walter Benjamin would be rolling in his grave.

(Thanks to David Dobbs for this one!)