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Snow, the secret of Mona Lisa's smile
Updated: 2004-06-25 09:31

The enigmatic smile of the Mona Lisa lies in a quirk of our brain which interprets "visual noise," the equivalent of the white flecks called snow which occur on a badly-tuned TV set, a pair of scientists say.

Christopher Tyler and Leonid Kontsevich of the Smith-Kettlewell Eye Research Institute in San Francisco digitally manipulated an image of the painting, overlaying it with a greyish filter of random flecks.

They showed the results to 12 volunteers, who were asked to gauge Mona Lisa's expression on a scale on one to four -- sad, slightly sad, slightly happy and happy.

As would be expected, "noise" that lifted the edges of her lips made her look happier, and "noise" that flattened her lips made her seem sadder, the British weekly New Scientist reports in next Saturday's issue.

But what was more surprising was how the flecked patterns made people change their perception of the expression as they viewed the picture.

Why this is so appears to be rooted in the way the brain interprets the sketchy and sometimes interrupted flow of light signals which is visual noise.

It means that when many people see the Mona Lisa, they believe her expression is subtly changing, the scientists believe.

"That may be part of what makes the painting so powerful," Tyler told New Scientist.

Visual noise is a major but poorly understood part of the daily input into our minds.

Tyler believes it to have many sources, including fluctuations in the number of photons hitting light receptors in the retina and randomness in the firing of neurons which carry the visual signals to the brain.

The Mona Lisa, painted by Leonardo da Vinci from 1503-1506, hangs in Paris' Louvre Museum.

In April, curators said scientists would be closely examining the legendary work for signs of apparent warping of the thin panel of poplar wood on which it was painted.

Some experts claim Mona Lisa is actually a self-portrait of Leonardo himself, or an adolescent boy.

X-ray examination shows three earlier versions hidden beneath the final painting.

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