Mona Lisa's enigmatic smile is just one of the tricks used by great artists who knew how the human eye works - knowledge that scientists are only now starting to understand, according to an American neurologist.

"I'm de-mystifying the procedures that some artists have known for years, but not debunking their art in any way," Professor Margaret Livingstone of the Harvard Medical School in Boston, told the annual meeting of the American Association for the Advancement of Science in Denver.

During a recent trip to the Musée Marmottan in Paris, France, Livingstone was able to examine Monet's 1872 painting of the port of Le Havre, known as "Impression: Sunrise," measuring the brightness of the Sun. The Sun appears in the painting as a ball of fire in a green-grey misty sky, drawing the eye. And its apparent brightness is reflected in the water near some fishing boats.

But contrary to our perception of the scene, the Sun is in fact no more luminous than the sombre sky it is set in, Livingstone said. Our visual system has two pathways: colour and luminance; due to the 'disconnect' between the two, in the colour version the Sun is only interpreted as a type of brilliance.

The brilliance - or intensity of the reflected light - is not connected to colour. For example, in black and white photos, all objects of the same brilliance are grey to the same degree.

So when the neurologist took a black and white photo of the Monet painting, the Sun disappeared in the sky, merging into its greyness - demonstrating that the Sun is of exactly the same brightness as the sky surrounding it.

Monet knew instinctively that the system of human vision is split into two functions, she said. "Artists like Monet understood this dichotomy in our visual processes and used it empirically to give the illusion of colour and space."

Today, we know that one of the functions, which all mammals have, allows us to see three dimensions, and movements -- but not colour. The other function, unique to humans and primates, allows us to recognise objects and faces, see colours and evaluate the environment.

Equally, in her study of the Mona Lisa at Paris' Musée du Louvre, published in 2000, Livingstone details the cerebral process that gives us the impression of the Mona Lisa's enigmatic smile. Leonardo da Vinci used the shadows around the subject's jawline to accentuate her mouth: her smile is most pronounced when one's eyes are fixed on her eyes or on the background of the painting.

When one looks directly at her mouth, however, the smile is less evident, Livingstone explained. "The elusive quality of her smile can be explained by the fact that her smile is almost entirely in low spatial frequencies, and so is seen best by our peripheral vision," she told reporters.

"These artists, the Impressionists - da Vinci, Chuck Close and Robert Silvers, for example - discovered fundamental truths that scientists are only now unraveling," she said.

Da Vinci painted the Mona Lisa, also known as "La Gioconda" sometime between 1503 and 1506.

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