The Mona Lisa's enigmatic smile was created by a special painting technique which tricks the eye into thinking the expression is changing, a study has claimed.

Austrian neurologists say analysis of the masterpiece shows her face appears to shift depending where a person focuses their gaze.

If her eyes are stared at, it appears she has a subtle smile on her lips. But if the onlooker shifts their gaze to her mouth, then the smile disappears.

Professor Florian Hutzler, a psychology expert at the Centre for Neurocognitive Research in Salzburg, said Leonardo da Vinci had used clever techniques to trick the viewer.