Imaging the living retina allowed Williams to shine light directly into the eye to see what wavelengths each cone reflects and absorbs, and thus to which color each is responsive. In addition, the technique allows scientists to image more than a thousand cones at once, giving an unprecedented look at the composition and distribution of color cones in the eyes of living humans with varied retinal structure.
Each subject was asked to tune the color of a disk of light to produce a pure yellow light that was neither reddish yellow nor greenish yellow. Everyone selected nearly the same wavelength of yellow, showing an obvious consensus over what color they perceived yellow to be. Once Williams looked into their eyes, however, he was surprised to see that the number of long- and middle-wavelength cones--the cones that detect red, green, and yellow--were sometimes profusely scattered throughout the retina, and sometimes barely evident. The discrepancy was more than a 40:1 ratio, yet all the volunteers were apparently seeing the same color yellow.
"Those early experiments showed that everyone we tested has the same color experience despite this really profound difference in the front-end of their visual system," says Hofer. "That points to some kind of normalization or auto-calibration mechanism--some kind of circuit in the brain that balances the colors for you no matter what the hardware is."
In a related experiment, Williams and a postdoctoral fellow Yasuki Yamauchi, working with other collaborators from the Medical College of Wisconsin, gave several people colored contacts to wear for four hours a day. While wearing the contacts, people tended to eventually feel as if they were not wearing the contacts, just as people who wear colored sunglasses tend to see colors "correctly" after a few minutes with the sunglasses. The volunteers' normal
Source:University of Rochester