They're coming in thick and fast folks... A reader alerts me to this research at Harvard, by Dr. Margaret Livingstone, Professor of Neurobiology. She looks into how the eye processes information:
We are interested in how cells in the visual system process information and in the functional organization of the visual system. We use complementary techniques going from psychophysics, functional MRI, to single unit recording.
As part of her work, Dr. Livingstone has been looking into how the eye reads paintings:
A side interest in the lab is to use what we know about vision to understand some of the discoveries artists have made about how we see. The separate processing of color and form information has a parallel in artists' idea that color and luminance play very different roles in art (Livingstone, Vision and Art, Abrams Press, 2002). 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 (Science, 290, 1299). These three images show her face filtered to show selectively lowest (left) low (middle) and high (right) spatial frequencies.
So when you look at her eyes or the background, you see a smile like the one on the left, or in the middle, and you think she is smiling. But when you look directly at her mouth, it looks more like the panel on the right, and her smile seems to vanish. The fact that the degree of her smile varies so much with gaze angle makes her expression dynamic, and the fact that her smile vanishes when you look directly at it, makes it seem elusive.
What do readers think when they see the smile in the photo on the right? Does it seem to vanish, as Dr. Livingstone suggests? Doesn't to me. Seems more obvious than ever, in fact. Still, A for effort.