Secret revealed of 'Mona Lisa' smile

Leonardo da Vinci exploited a biological property of the human eye – a phenomenon known as peripheral vision – when he painted the enigmatic smile of the Mona Lisa.

The famous expression of La Gioconda has notoriously perplexed art lovers down the centuries because it is unclear whether the smile is happy or sad, or even whether it is a smile.

One explanation involves the way the retina of the eye handles light. When someone looks at something directly, the light falls on a central structure called the fovea which is specialised to deal with light of high spatial frequency.

But light from objects seen out of the corner of the eye, falls on the peripheral part of the retina which is better adapted for seeing low or medium spatial frequency light.

Professor Margaret Livingstone, of Harvard University, believes the Mona Lisa's "smile" changes. "[If] your centre of gaze falls on the background or the hands – which means her mouth is being seen by your peripheral, low frequency vision – Mona Lisa's mouth appears much more cheerful. She smiles until you look at her mouth, then her smile fades, like a dim star that disappears when you look directly at it."