That's because direct vision is excellent at picking up detail, but less suited to picking up shadows. Peripheral vision, on the other hand, picks up low spatial frequencies like the blurry smile of the Mona Lisa.
"She has a mocking quality," said Livingstone, who is the author of the book Vision and Art. "When you're not looking at her, she seems to be smiling behind your back, and then you look at her and she stops."
The turbulent history of the Mona Lisa has added to its universal fame. In 1911 the painting was stolen from the Louvre by a former employee who believed it belonged in Italy. The thief walked out of the gallery with the picture under his painter's smock. He was apprehended in Florence, Italy, two years later, and the painting was safely returned.
Since then, the Mona Lisa has been frequently caricatured, dissected by psychologists like Sigmund Freud, and portrayed as a femme fatale in advertising campaigns.
"The story around the Mona Lisa [is] more famous than the painting itself," Zerner said.
To preserve the fragile work, curators many years ago enclosed the Mona Lisa behind a thick pane of glass. The barrier guards against climatic changes and camera flashes from the six million people who visit the Louvre every year.
But experts say the true Mona Lisa is difficult to see, because it has been buried under thick layers of various varnishes. Over the years, the painting has gained a dull brown-and-yellow tint from chemical changes in the varnish.
Now the wood on which it's painted is also changing.
Wood is particularly difficult to repair, because it easily absorbs and releases water, changing its dimensions and shape. There is always a chance of doing more harm than good.
To make matters worse, the experts don't really know what materials da Vinci worked with.
The Center for Research and Restoration of Museums of France will now conduct a technical study to determine what materials the painting is made of and evaluate its vulnerability to temperature changes.
Said Rosand: "I would be surprised if that painting wasn't in fairly desperate need of some sort of help."
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