The Brain’s Rainbow: Neuroscientists Find the Key to Perceiving Color

From vibrant primary hues to subtle pastels, the colors we “see” actually are captured and perceived on patches of neurons deep in the cerebral cortex, researchers at The University of Texas Medical School at Houston have discovered.

Such advances in mapping the brain’s functions could lead some day to prosthetic aids for people who have lost their sight.

A paper in a recent edition of Nature shows how modules of neurons within the visual cortex called “thin stripes” form a rainbow-like map of color for the brain to use that matches the order of the spectrum of colors as we see them.

“This finding provides the first physiological basis for the perception of the full gamut of color,” said Daniel Felleman, Ph.D., associate professor of neurobiology and anatomy and senior author of the paper.

The thin stripes in an area of the brain known as V2 have been associated generally with processing color but little was known about how color was represented in these modules. Felleman, lead author Youping Xiao, a former postdoctoral fellow now on the research faculty of Mount Sinai School of Medicine in New York, and postdoctoral fellow Yi Wang demonstrate how the stripes are organized to represent color.

By showing macaque monkeys a series of colors and recording tiny changes in blood flow along the thin stripes, the team showed that specific portions of the stripes showed restricted regions of peak activity in connection with each specific color.

They also found that these regions of peak activity shifted systematically as the color of the visual stimulus was changed. For example, the area that showed peak activity for red would be next to portions that peaked for orange, which in turn was next to yellow, then green, blue and purple.

“We believe that the brain uses a spatial code for color such that the location of the peak activity within these color maps determines the color that you see,” Felleman said.

Do macaques reflect human experience?

“Everything we know about area V2 in the human brain is identical to the macaque. We expect the same mechanism to be operating in the human brain,” Felleman said.

In the longer term, basic knowledge of the brain’s functions could lead to dramatic help for people who have lost their sight.

“You always hope that knowing the basic mechanisms of brain coding and perceptual processing would allow you to some day have a prosthetic aid that would activate the brain in the same way that the eye normally does,” Felleman said.

A visual prosthesis could mimic the detailed information passed on to the rest of the brain by the retina. The first stop for information from the retina is the primary visual cortex, known as V1, which is located at the back of the brain. V1 gathers the separate information from the eyes and passes it on in a combined, binocular fashion. V2, the next stop for color, shape, and depth perception, is just in front of V1.

“While color vision obviously begins with the photoreceptor cones of the retina and is further processed by V1, we believe that V2 is the first location in the hierarchy of brain processing where the full range of colors is explicitly represented,” Felleman said.

Felleman continues to study the thin stripes in V2, including an exploration of their role in processing brightness. Evidence shows that this may have been the stripes’ original role, developed in a common ancestor of primates that was nocturnal.

“Color perception probably came later,” Felleman said.

Felleman’s research is supported by grants from the National Eye Institute of the National Institutes of Health. An earlier grant from the Advanced Research Program of the Texas Higher Education Coordinating Board helped initiate his work in optical recording of brain activity.

Lead author Xiao earned his doctorate from the UT-Houston Graduate School of Biomedical Sciences and worked on this project as both a graduate student and as a postdoctoral fellow.

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