It's a peripheral smile
We've blogged about the Mona Lisa before. Who can blame us; some 500 years after she was painted, they're still uncovering the secrets Da Vinci embedded in his masterpiece. (Secrets like this.) Now, scientists have discovered the reason why Lisa's wan smile seems to disappear when you look at it: because we see it better with our peripheral vision.
It's thanks to the way our eye sees. We've got two types of vision: foveal -- what we use when we stare at things dead-on -- and peripheral -- which isn't so good at picking up detail, like foveal is, but is great at detecting the nuances of shadowed areas. "The elusive quality of the Mona Lisa's smile can be explained by the fact that her smile is almost entirely in low spatial frequencies, and so is seen best by your peripheral vision," Harvard Prof Margaret Livingstone said.
The same principle is at work when you stare at a single letter of text; it makes it tough to see the letters around it. The Mona Lisa's smile becomes obvious only if you stare at her eyes, or elsewhere on her face. Interesting, yes, but we're still left with one lingering question: why, Leonardo?