The more you look away, the more she smiles

To get a real smile from Mona Lisa, give her a sidelong glance.

Focus on the background, or her dainty hands, rather than her enigmatic lips. That way, Margaret Livingstone of Harvard University told the AAAS meeting, her mouth is seen by the viewer's low-frequency, peripheral vision and the smile appears much more cheerful.

"Hence its elusive quality - you can't catch her smile by looking at it," Prof Livingstone said. "Every time you look directly at her mouth, her smile disappears, because your central vision does not perceive low spatial frequencies very well. "Mona Lisa smiles until you look at her mouth, and then her smile fades, like a dim star that disappears when you look directly at it." The elusive smirk of Leonardo da Vinci's master piece has tantalised admirers for 500 years. Experts have proposed that la Gioconda, as the portrait is also known, after the woman's supposed family name, was either pregnant, suffering from a facial paralysis, or simply indigestion. They have also suggested Leonardo deliberately blurred her mouth to make her smile ambiguous.

Prof Livingstone argues that they may have been looking at too high spatial frequencies, or to put it another way, looking too directly. The answer was there all the time, in the corner of the viewer's eye. Peripheral vision can see things that central or foveal vision cannot, simply because it is tuned to lower spatial frequencies.

"Look at her mouth, then the background," she said. "Look at her mouth again, and then her eyes. Look back and forth between her mouth and other parts of the painting."