By William J. Cromie
Researchers have pinpointed the part of the brain responsible for conscious experience of color. It lies near the back, behind and below your temples, next to but distinct from the area that was believed for years to be the color center.
The discovery, made by researchers at Harvard Medical School, came from careful analysis of brain areas activated when a person sees color. People looked at rotating wheels of different colors and of black-and-white while their brains were scanned using functional magnetic resonance imaging (fMRI). This technique determines which parts of the brain are most active by measuring the flow of blood and oxygen as people look at colors.
"Experiments done on monkeys in the early 1970s located their color center," explains Nouchine Hadjikhani, a research fellow. "This information was then extrapolated to the human brain. But when we used a new mapping method that inflates and flattens out human images, we found the most activity in a different, adjacent area."
This work was done at Harvard-affiliated Massachusetts General Hospital in Boston, and the results match well with studies of people who have a strange medical malady known as achromatopsia, an inability to see color. The area pinpointed by Hadjikhani and her colleagues is damaged by stroke or trauma in the brains of these people.
In related experiments, Patrick Cavanagh, a Harvard professor of psychology, and his colleagues discovered that achromats can still use visual information about color without actually seeing it.
"Color information does not flow as a single stream from the eyes to the brain's visual area; rather, it takes parallel paths to other regions that process motion, shape, and texture," Cavanagh says. "This enables achromats to detect the motion of color objects without seeing any color."
Both teams of scientists published their intriguing observations in the July issue of Nature Neuroscience.
Color Is Personal
"There is no 'absolute' color," Cavanagh notes. "What I call 'red' isn't necessarily the same color others experience as 'red.' "
Such personal differences don't have much impact on life, but complete absence of color can be strange and distressing. What people usually refer to as "color blindness" involves a genetic condition that only affects ability to discriminate red and green. Achromatopsia, however, comes from head injury or stroke and it erases all experience of color.
"Everything appears to me as [if I were] viewing a black-and-white television screen," reports an achromat in Oliver Sacks' book, An Anthropologist on Mars. "My brown dog is dark gray. Tomato juice is black. Color TV is a hodge-podge."
This 65-year-old artist found foods "disgusting due to their grayish, dead appearance." He tried closing his eyes but that didn't work, so he turned to eating mostly black and white foods -- rice, yogurt, black olives.
That's not the worst of the problem. "The brain area that deals with face recognition sits right next to the color area, so achromats often don't recognize their spouse, parents, friends, or business associates," Cavanagh points out. "This is devastating because, otherwise, they appear normal. People who know them well don't expect to be treated like strangers."
Jonathan I., the achromat described by Sacks, shunned social intercourse and found sexual intercourse impossible.
Affected people often cannot even recognize themselves. Jonathan could not bear his own appearance in a mirror. One person joked that he's not sure who he's shaving.
The difficulty in recognition often extends beyond faces. Cavanagh describes a patient who, shown a photo of Bridget Bardot and her dog, pointed to the dog when asked to identify the actress.
Yet these people can distinguish the motion of certain colored objects as effectively as people unaffected by the condition. Despite profound loss in the conscious experience of color, three achromats tested by Cavanagh "showed surprisingly strong responses to high-contrast, moving color -- equal in all respects to the performance of subjects with normal color vision."
Such results led Cavanagh and his colleagues to the conclusion that color information coming from the eyes travels different paths through the brain and reaches areas that specialize in different visual tasks. "You can think of it as a division of labor," he says. "Color information goes to areas concerned with motion, shape, texture, etc., and helps people to experience these other parts of vision."
Motion Without Color
Achromats, of course, see black-and-white motion, but that doesn't provide a complete view of what's going on. For example, in the confusion of sun and shadows in a forest, color is a more reliable clue to a moving predator or prey than black and white.
"Only in the last decade have we realized that color is an important cue for detecting motion," Cavanagh points out. "Now we've found that, under certain conditions, it can be just as good a cue for achromats as for those with normal vision."
A similar situation exists for shape. Edges and color changes at the borders of objects define shape. Achromats respond to such changes; they can sometimes see differences in color even though they don't know what the colors are.
These shape and motion distinctions only apply when color contrast is strong, say, intense green and red. When hues are faint, with little contrast, achromats can't detect color motion or borders between colors.
"Analyzing high contrasts of color is a primitive, direct brain response; both achromats and those with normal color vision do it automatically," Cavanagh explains. "But faint colors, or low levels of contrast, involve a more complex process. You must first find a pattern; achromats don't see color patterns."
No treatment exists for achromatopsia. However, studying what and how achromats see contributes to a better understand of how the brain processes what the eyes send to it.
"That understanding might hasten the day when we can build machines that see as well as people," Cavanagh says. "In the distant future, we might be able to make machines that enable blind people to see."
Copyright 1998 President and Fellows of Harvard College