The smile on the face of the Mona Lisa is so enigmatic that it disappears when it is looked at directly, says a US scientist.
Professor Margaret Livingstone of Harvard University said the smile only became apparent when the viewer looked at other parts of the painting.
The theory has been presented at the American Association for the Advancement of Science's (AAAS) annual meeting in Denver, Colorado, this week.
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.
http://news.bbc.co.uk/2/hi/entertainment/2775817.stm