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DURHAM, N.C. -- Humans recognize differences in hue, saturation and brightness by using natural brain reflexes evolved from past experience, Duke University Medical Center researchers have found.

"The basic problem, recognized for several centuries, is that the image on our retinas can't specify what's out there in the world, in terms of color or any other aspect of vision," said Dale Purves, M.D., the George B. Geller Professor for Research in Neurobiology. "The light received by our retinal receptors tangles up illumination, reflectance and transmittance, the factors that go to making up a color stimulus. The only way to sort out the meaning of such ambiguous information is through trial and error."

This strategy is a means of solving the so-called "inverse problem" as it pertains to color, Purves said. Thus, as light comes in through the eye, the brain perceives color properties by generating the likely answer using a vast database of results from previous situations, the researchers argue.

The team's findings appeared this week in the early online edition of the Proceedings of the National Academy of Sciences and will be published in the journal's April 11, 2006, edition. The work was supported by the National Institutes of Health, the Air Force Office of Scientific Research and the Geller Endowment.

The study by Purves and colleagues Fuhui Long, Ph.D., and Zhiyong Yang, Ph.D., builds on their earlier research into how the brain processes information from the eyes on a wholly empirical basis, rather than by analysis of stimulus features as such.

The researchers tested their theory of hue, saturation and brightness perception on known visual quirks people exhibit when asked to judge colors. The team compared these quirks to predicted color perception based on a statistical color analysis derived from 1,600 images of natural scenes on and around the Duke campus.

The natural scene statistics support the idea that humans rely on past experience to generate percepts -- the objects of perception -- from present images. For example, the sense of brightness should scale directly with the intensity of light, such that a more intense light coming to the eye always corresponds to a stronger sensation of brightness. However, a target surrounded by an area of predominantly higher luminance (intensity) looks brighter than the same target surrounded by lower luminance. Scene statistics predict this effect.

In the past, neurobiologists explained this well-known effect on the fact that retinal neurons that send information from the eye to the visual part of the brain respond more vigorously to a patch in a dark surround than the same patch in a light surround, Purves said.

The problem with this interpretation is that scenes with exactly the same surrounds can look differently bright -- the opposite of what the retinal-firing-rate explanation of brightness predicts, Purves said.

"The visual system has evidently evolved to use the statistics of past experience to produce useful perceptions of hue saturation and brightness," Purves said. This cumulative experience is hard-wired into the visual system thanks to ancestors who made successful choices in similar circumstances, he said.