Later, she realized what was happening. Livingstone says our peripheral vision sees blurry images while our central vision sees fine detail.
When Livingstone blurred the face with a filter, the Mona Lisa looked as if she were smiling cheerfully. But homing in on the fine detail gave her a more demure expression.
So Livingstone says that in his painting, da Vinci achieved an unusual effect: the Mona Lisa's smile changes depending on where you look.
For the American Association for the Advancement of Science, I'm Bob Hirshon.