The Mona Lisa, painted by Leonardo da Vinci in the 1500s, has intrigued art lovers for five centuries because of its subject's mysterious smile.

The smile disappeared when it was looked at because of the way the human eye processes visual information, said Prof Livingstone.

The eye uses two types of vision, foveal and peripheral.

Foveal, or direct vision, is excellent at picking up detail but is less suited to picking up shadows.

"The elusive quality of the Mona Lisa's smile can be explained by the fact that her smile is almost entirely in low spatial frequencies, and so is seen best by your peripheral vision," Prof Livingstone said.

The more a person stares fixedly ahead, the less useful is their peripheral vision.