An article in the New Scientist reports on a study into visual noise at the Smith-Kettlewell Eye Research Institute in San Francisco which seems to have discovered why Mona Lisa's expression seems so alive. Researchers manipulated a digital image of the painting to introduce different effects caused by visual noise and asked 12 subjects to rate the expression in the pictures on a scale ranging from sad to happy. "As would be expected, noise that lifted the edges of her mouth made Mona Lisa seem happier, and those that flattened her lips made her seem sadder. More surprising though, was how readily the visual noise changed people's perception of the Mona Lisa's expression." The online article includes a copy of the pictures so you can see the effects for yourself.
'Noisy secret of Mona Lisa's smile' by Philip Cohen. New Scientist, Vol 44, p.1493.