|
||||||||
|
NEUROBIOLOGY Science Illuminates ArtDual Nature of Seeing Accounts for Brain's Double Take on Visual World Screaming headlines grab your attention using some of the same techniques as great works of art. Both can skillfully manipulate the biology of your visual system, playing form against feature, contrast against color, and primitive against sublime.
Computer programs now can generate images by matching the local luminance and color of low-resolution features of many small images, says Marge Livingstone, whose picture is the basis of this photomosaic. As the eye moves over the image, different parts of it flip between global and local perspectives. (Photo by Clinton M. Lipsey) Monet and other painters exploited the parallel visual processing of color and brightness. A sunset seems to shimmer, a field of poppies seems to wave, and a river seems to flow when there is a disconnect between the color and luminance pathways, according to the new book, Vision and Art: The Biology of Seeing, by Margaret Livingstone, HMS professor of neurobiology. Art implies a personal, unanalyzable creative power, but Livingstone finds plenty to analyze about how various works of art--and Impressionist paintings in particular--reflect different properties of the visual system. "Art depends ultimately on our brains," says colleague David Hubel, who has worked with Livingstone for 27 years and wrote the foreword for her book. "By understanding what goes on in our brains when we look at a work of art, we can hope to deepen our appreciation of both the art and science." (Hubel shared the 1981 Nobel Prize in medicine or physiology, in part, for revealing the functional organization of the visual system and the importance of early visual stimulation as the system develops after birth.) Vision BasicsIt is tempting to imagine that an eyeball projects a Renoir from the museum wall onto the retina, disassembles the pieces, and sends them to the back of the brain to process color, contrast, shape, position, and motion through the cortical layers. At each stage neurons become more selective for more precise features in the image. Finally, the image reassembles in the front of the brain. But the visual system does not work like that."This misperception is so common that it has a name: the homunculus fallacy," Livingstone writes. "The fallacy is the idea that when we see something, a small representation of it is transmitted to the brain to be looked at by a little man. The fact is, of course, that there is not a little man in the brain to look at that or any other image. The function of the visual system is really to process light patterns into information useful to the organism." Biologically useful information reaches the brain through two massively parallel processing systems, Livingstone said. The subdivision begins in the retina, with two major classes of ganglion cells that send signals to two different areas of the thalamus. The two sets of signals travel ultimately to areas of the cortex so distinct they are separated by several inches. The more primitive half of the visual system begins with large ganglions having big, bushy, dendritic arbors. This part perceives motion, depth, and spatial organization. Known as the Where system, it is colorblind but keenly sensitive to small differences in brightness. We share the Where system with other mammals, but only primates have evolved the more refined What system. The What system starts with smaller ganglion cells. It sees color well but not contrast. It has vision that is three times sharper than the Where system. Both systems cover the entire visual field. The Where system sees a person moving toward us; the What system tells us it is Aunt Emily. The Where system sees the forest; the What system sees the trees. Playful Patterns of LightIn the case of works by Monet and other Impressionist painters, the shimmering comes from colors that appear distinct to the dazzled What system but become shades of gray to the black-and-white Where system. In Paris, after some haggling with a dubious curator, Livingstone measured the brightness of the pulsating orange-red sun in Monet's Impression: Sunrise and found it to be the same as the gray background. In the more modern Broadway Boogie Woogie by Piet Mondrian, the What system sees the bright yellow and gray squares against an off-white background, but the Where system cannot see them. Consequently, the squares seem to move, or jitter, Livingstone said.Conversely, the different messages conveyed by color and contrast also explain why expressive, unrealistic colors of shadows still can impart normal depth in paintings such as André Derain's Portrait of Henri Matisse and Matisse's Femme au Chapeau. Photomosaic art exploits another aspect of our visual system. In 1995, Robert Silver, then a student at the MIT Media Lab, invented the technique of generating large pictures composed of tiny images. The assembled composite mosaic looks dynamic in part because our acute center of vision focuses on the small individual images while our peripheral vision picks up the global view. The Mona Lisa's elusive smile toys with the same difference in acuity across the visual system, Livingstone said. "Instead of concluding that there was something mystical and magical about this painting, I filtered images of Mona Lisa's face so that you could see what the peripheral and central vision could see," Livingstone said. "It turns out that most of the smile is in the blurry components best seen by peripheral vision. In some sense, she's coy. She smiles until you look directly at her, then she stops." "Our knowledge of visual science is rudimentary," Hubel writes. "It goes as far as three or four stages of visual cortex, whereas we know that there are at least several dozen further stages in the occipital lobes alone, none of which are yet explored. We know about some of the early building blocks of vision, much more than we did 50 years ago, but we still have no idea of what happens in the brain when we recognize a hat, a safety pin, or a boat or when we look at a painting that has intense emotional content. But we are beginning to understand some elementary things fairly well: why yellow plus blue light makes white, why equiluminant colors shimmer, why a black object remains black whether seen in dim light or on the beach." In other words, scientists cannot even begin exploring the more soul-searching questions raised by art, even if they have made a good start in explaining the Where and the What. --Carol Cruzan Morton |
