Lighting up the rainbow: color perception tied to early visual experience.

For an enlightening perspective on how primates acquire color vision, consider baby monkeys. These infants' ability to recognize basic colors in different environmental settings depends on their prior exposure to a full spectrum of colors in natural light, a new study suggests.

Although the colors in an image shift as available light intensifies or diminishes, people, as well as monkeys, usually recognize a particular hue throughout that change. For instance, an observer perceives a dog's red collar as the same color on a dark, cloudy day as on a sunny day. Scientists refer to this crucial visual adjustment as color constancy.

Prior investigations have failed to clarify whether color constancy is an innate capability of the retina's cone cells or it's acquired only with help from the brain's visual system.

Yoichi Sugita of the Neuroscience Research Institute in Tsukuba, Japan, explored color constancy in four macaque monkeys that had been raised from age 1 month to 1 year in a room illuminated by light with a highly restricted range of wavelengths, which ensured that the animals couldn't discern a normal array of colors.

After age 1, the monkeys couldn't usually identify colors they had just seen on a computer screen when the on-screen illumination of those colors changed, even after intensive training designed to overcome this problem, Sugita reports in the July 27 Current Biology. In contrast, four macaque monkeys that had been raised in a room illuminated by sunlight and fluorescent lamps recognized colors in a variety of lighting conditions.

Over 3 days of training, all the year-old animals learned to identify matching pairs of black, white, or gray rectangles. After another 3 days of training, the monkeys identified pairs of equally illuminated rectangles with common colors--blue, green, yellow, or red.

After 10 days of training and 3 weeks of further testing, however, those monkeys raised under restricted-illumination conditions still had great difficulty recognizing different shades of the same color as well as identifying the same color illuminated to varying degrees. These problems remained 9 months after the monkeys had been moved to a room illuminated by sunlight and fluorescent lamps.

"These results indicate that early visual experience is indispensable for normal color perception," Sugita says.

His report is the first clear demonstration that animals can perceive colors, which indicates working cone cells in the retina, but that they lack the capacity for color constancy, comments Stanford University vision researcher Brian A. Wandell. This implies that the brain, not the retina, assumes substantial responsibility for performing color judgments under different lighting conditions, in his view.

Brain-imaging studies are needed to pinpoint disrupted parts of the brain's visual system in monkeys without color constancy, Wandell says.