Sideways look reveals secret of Mona Lisa

For 500 years, it has bewitched art lovers and mystified academics. Now an American scientist claims to have identified the secret behind the Mona Lisa's smile.

Its enigmatic quality arises from the hazy technique Leonardo Da Vinci used to paint the mouth of his subject. It is known as sfumato, Italian for "toned down", literally "evaporating like smoke".

But Margaret Livingstone, a neurobiologist at Harvard University, believes there is more to it than that.

She argues that, in blurring the mouth Da Vinci played a trick on the human eye which meant that the smile can only be seen properly when looked at from the side. By using a computer simulation to vary the definition of the Mona Lisa's features, she shows that the smile is only truly visible when viewed as a "coarse" image – the way we perceive it with our peripheral vision.

Ms Livingstone outlines her theory in a new book, Vision and Art: The Biology of Seeing, which is published by Abrams next week.

She writes: "This explanation goes beyond the popular idea that Leonardo blurred Mona Lisa's mouth (used sfumato) to make her expression ambiguous. That hypothesis would mean that her smile would vary depending on the viewer's imagination or state of mind, but I think its variability is more systematic than that. I suggest that her smile is more apparent in the coarse-information component of the image, and is therefore more apparent to peripheral than to central vision.

"This explains its elusive quality – you literally can't catch her smile by looking at her. Every time you look directly at her mouth, her smile disappears because your central vision does not perceive coarse image components 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."

But Donald Sassoon , author of Becoming Mona Lisa: The Making of a Global Icon, said: "Any scientist who wants to attract media attention should always claim to have resolved the mystery of the Mona Lisa. It never fails."

Mr Sassoon , an advocate of the sfumato theory, which argues that Leonardo deliberately blurred the smile to make it "indeterminate", added that it was "unnecessary" to "drag" neurobiology into the debate.

"What I tried to do in my book is to explain that the reason why people say the smile is enigmatic is because it was so described by leading opinion-makers in the arts in the 19th century," he said. "Previous to circa 1850 Mona Lisa was just a cheerful housewife."