Color vision strategy defies textbooks: cone cells fill in hues on black-and-white image, study suggests.
Cone cells, which can sense red, green or blue light, detect white more often than colors, researchers report September 14 in Science Advances. The textbook-rewriting discovery could change scientists' thinking about how color vision works.
For decades, researchers have known that three types of cells, or cones, in the retina are responsible for color vision. Those cones were thought to send "red," "green" and "blue" signals to the brain. The brain supposedly combines the colors, much the way a color printer does, to create a rainbow-hued picture of the world (including black and white). But the new findings indicate that "the retina is doing more of the work, and it's doing it in a more simpleminded way," says Jay Neitz, a color vision scientist at the University of Washington in Seattle who was not involved in the study.
Red and green cones each come in two types: One type signals "white" and another signals color, vision researcher Ramkumar Sabesan and colleagues at the University of California, Berkeley discovered. (The team didn't test blue cones.) A large number of cones detect white (and black--the absence of white) and create a high-resolution black-and-white picture of a person's surroundings, picking out edges and fine details. The rest of the red and green cones signal to the brain low-resolution color information. The process works much like filling in a coloring book or adding color to black-and-white film, says Sabesan, who is now at the University of Washington.
Sabesan and colleagues discerned this color vision strategy by stimulating 273 individual cones in the eyes of two men. The technological accomplishment of stimulating single cones in the retina is akin to getting people to walk on the moon, Neitz says. "It is a super technological achievement. It is an amazing thing."
Sabesan's team first used a microscope that could peer into living human eyes to map light-detecting cones in the two volunteers. To get a clear picture of the cells through the distortion of the lens and cornea, the researchers borrowed techniques that astronomers use to compensate for disturbances in the atmosphere.
With the blur from imperfections in the eye corrected, the researchers had to precisely target individual cells to hit with the laser. Because the eye is constantly jiggling, the researchers had to determine the pattern of the eye movements to predict where cone cells would be several milliseconds in the future. Over about two years, the researchers repeatedly stimulated 273 red or green cones one by one. After a flash of laser light was delivered to the cone, the men would indicate on a keyboard what color they had seen.
Of the red cones the researchers stimulated, 119 made the men see white, while only 48 flashed red. Similarly, only 21 of the green cones tested actually signaled green, while 77 registered white. Each individual cone probably signals only white or a single color, the researchers say. "It's a rather inefficient arrangement," says Donald MacLeod, a vision scientist at the University of California, San Diego. All the cones are capable of detecting color, but few actually seem to do so.
Cells surrounded by cones that detect a different color were more likely to send white signals to the brain. That finding is unexpected and runs counter to a popular idea that cones ringed by cells detecting other colors would be better at color detection, MacLeod says.
These findings could be good news for people with color blindness. The results suggest that gene therapy that adds red or green cones could work even in adults, Neitz says. Although his group gave a monkey full color vision (SN: 10/10/09, p. 14), many researchers thought human brains would never be able to incorporate additional color information even though the eye could detect it. The new findings indicate that all the brain needs to learn is that there is an additional color needed to fill in a basically black-and-white picture, a task the brain should accomplish easily, Neitz says.
Caption: Researchers have learned surprising lessons about color vision by using precisely targeted lasers (green line) to stimulate single cone cells (red, green and blue dots) in human retinas. The inset area is about 100 micrometers in diameter.
Please note: Illustration(s) are not available due to copyright restrictions.
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|Title Annotation:||GENES & CELLS|
|Author:||Saey, Tina Hesman|
|Date:||Oct 15, 2016|
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