Dr. Livingstone is an authority on visual processing, with a special interest in how the eye and brain deal with different levels of contrast and illumination. Recently, while writing on a book about art and the brain, an editor advised her to learn more about art history. ''I got a copy of E. H. Gombich's 'The Story of Art' in which he basically said, 'I know you've seen this painting a hundred times but look at it, just look at it.' And so that's what I did.''

In staring at the picture, Dr. Livingstone said she noticed a kind of flickering quality. ''But it wasn't until later when I was riding my bike home that I realized what it was,'' she said. ''The smile came and went as a function of where my eyes were.'' A scientific explanation for the elusive smile was suddenly clear. The human eye has two distinct regions for seeing the world, Dr. Livingstone said. A central area, called the fovea, is where people see colors, read fine print, pick out details. The peripheral area, surrounding the fovea, is where people see black and white, motion and shadows.

When people look at a face, their eyes spend most of the time focused on the other person's eyes, Dr. Livingstone said. Thus when a person's center of gaze is on Mona Lisa's eyes, his less accurate peripheral vision is on her mouth. And because peripheral vision is not interested in detail, it readily picks up shadows from Mona Lisa's cheekbones.

These shadows suggest and enhance the curvature of a smile. But when the viewer's eyes go directly to Mona Lisa's mouth, his central vision does not see the shadows, she said. ''You'll never be able to catch her smile by looking at her mouth,'' Dr. Livingstone said. The flickering quality -- with smile present and smile gone -- occurs as people move their eyes around Mona Lisa's face.

The actress Geena Davis also shows the Mona Lisa effect, Dr. Livingstone said, always seeming to be smiling, even when she isn't, because her cheek bones are so prominent.