Two retinal images show different cone ratios. Courtesy of the University of Rochester
We all see the same colors, so it's only natural to assume that our color receptors appear in similar ratios. Not so, say researchers from the University of Rochester's Center for Visual Science.
For over 200 years, scientists have believed that we have three types of cones, each type responsive to different wavelengths of light. At long last the theory has been confirmed, but the answer has raised a whole new set of questions about our vision system.
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Using a new technique called "adaptive optics," the Rochester group produced the first sharp images of the living human retina. They were surprised by what they saw: The ratio of cones that detect long wavelength (red) light, to cones that detect medium wavelength (green) light, varies hugely from person to person. In addition, the researchers found that different numbers of cones don't make for different color perceptions.
Joseph Carroll, a coauthor of the study, said scientists looking for a cohesive model of how we see color will face a challenge in reconciling the disparity between cone ratio and color perception.
"Things aren't as simple as most vision scientists previously thought," he said. "[The color vision system] has enormous variability when you put it in context compared to other sensory modalities."
The researchers imaged eight human retinas using their new method, a technique originally developed by astronomers wherein two dynamic devices cooperate. One continually measures aberrations in a person's eye, and the other acts as a deformable mirror, constantly correcting for these small imperfections. This is currently the only technique that allows for direct measurement of the numbers of the three types of cones in living humans (the third type being short wavelength, blue, cones).
In this study of only eight people, the largest ratio of long to medium cones was over 40 times the size of the smallest.
After imaging the retinas, the team analyzed color perception by performing a number of tests, including a test for "unique yellow." Carroll said every person has one wavelength he or she sees as perfectly yellow—neither a little red nor a little green. The eight subjects all identified their unique yellow to be at the same wavelength, to a precision of a few nanometers.
Lead author Heidi Hofer said the brain must find a way to compensate for differing ratios.
"It seems that our brain adapts to the environment," Hofer said. "A colleague of mine did a study showing that you can change your overall color perception if you spend a lot of time in an unusual chromatic environment.
"It seems that the reason we all tend to see the same colors is that we grow up in similar chromatic environments."