Brain may make bright decisions early.

Renaissance artists such as Rembrandt rendered a stunning but deceptive luminescence to the painted canvas by placing areas of light and dark pigment next to one another. For example, in one of the Dutch master's works, this effect-known as chiaroscuro-intensifies the apparent glow of sunlight streaming through a window while it deepens the perceived blackness of a room in which a philosopher meditates.

Scientists have now exploited this phenomenon to uncover evidence suggesting that, once visual information passes beyond the eyes, the brain immediately begins to interpret the brightness of surfaces based on light- dark contrasts, rather than solely calculating absolute levels of light.

At the very first stage of visual processing, brain cells transform signals from the retina into light perceptions that bear only an indirect relation to the pattern of illumination registered by the eye, assert neuroscientist Andrew F. Rossi of the National Institute of Mental Health (NIMH) in Bethesda, Md., and his colleagues.

Until recently, many scientists have classed neurons in the primary visual cortex as straightforward transmitters of simple visual information that play no role in the coordination of brightness, form, and color perceptions.

"Our results imply that information about surfaces, such as their brightness, is represented in the responses of the same neurons that are sensitive to [other visual characteristics]," Rossi's group contends in the Aug. 23 Science.

Rossi and his coworkers implanted electrodes into a section of the primary visual cortex of 20 awake, anesthetized cats. This allowed them to measure electrical responses of the same 160 neurons in each cat's brain.

Cats viewed on a computer screen a series of gray squares that differed in brightness. They then saw a gray square of fixed brightness displayed against a changing spectrum of light and dark backgrounds.

A large majority of the feline neurons tracked by Rossi's team showed the same electrical responses in the two trials to squares of equal perceived brightness, at least as judged by humans, rather than absolute brightness.

Although no data as yet demonstrate that cats experience a visual illusion similar to chiaroscuro, considerable similarities between cat and human visual systems suggest that the new findings apply to people as well, according to Rossi.

Moreover, he adds, the results complement studies conducted in the past several years showing that, in cats and monkeys, the perception of apparent motion, color contrasts, and surface colors also occurs in the primary visual cortex.

"Rossi's study indicates that once you move beyond the retina's photoreceptors into the earliest stages of visual processing, context effects such as the contrast between light and dark determine the perceived brightness of surfaces," asserts NIMH neuroscientist Robert Desimone.

A keen sensitivity to light-dark contrasts proves extremely useful, notes Thomas D. Albright of the Salk Institute for Biological Studies in San Diego in a commentary in Science. For instance, the shift from sunlight to shadow as the day progresses robs the world of absolute brightness, but contrasts of light to dark surfaces remain the same as dusk approaches, allowing individuals to maintain a constant perceptual portrait of their surroundings.