While it may be a travesty to colorize Man Ray's classic black-and-white photos, imagine trying to appreciate the work of Jackson Pollock or Piet Mondrian without the benefit of color vision. Now, a study in this week's Nature dispels a widely held view of how our eyes discern the difference between, say, a red rectangle and a green one--and leaves neuroscientists questioning a half-century-old theory.
The retina in the eyes of most vertebrates, including humans, contains several types of photoreceptor cells, each stuffed with a different light-sensing protein, or pigment. The so-called cone cells sense color, with three subclasses each attuned to blue, green, or red light. When a photon hits the pigment, the energy forces the protein to change its shape, which in turn sets off a cascade of reactions culminating in an electrical impulse that is sent to the brain.
Since the 1940s, researchers have presumed that each kind of pigment has a different threshold of response: Photons in the red end of the spectrum, for example, are less energetic than those at the blue end, and should not get a blue pigment to be activated, or change shape. (Likewise, blue photons would have too much energy to turn on a red pigment.) However, heat flow within the retina is not so neat and simple. Pigments can absorb heat from their surroundings, and scientists aren't sure how this extra energy influences color perception.
Vision researchers Kristian Donner of the University of Helsinki, Ari Koskelainen of the Helsinki University of Technology, and their teams shined light on isolated frog and toad retinas and compared the electrical signals generated by photoreceptors at various temperatures, which allowed them to calculate the activation threshold for the different photoreceptors. They had expected the red-sensitive pigment to have a much lower threshold of activation than the green one. "Instead," says Donner, "they were almost indistinguishable." What's more, two green pigments, one from frogs and the other from toads, required different amounts of energy to become active. The findings suggest that factors other than a photon's energy are important in getting pigments to respond to different colors.
The results are "very interesting and completely unexpected," says neuroscientist Horace Barlow of the University of Cambridge in the United Kingdom. So just what makes photoreceptors respond to different colors? The answer, says Donner, may lie in how efficiently a cone cell absorbs heat from its environment. If red cones are more efficient than blue or green ones, they would require less energetic photons--those at the red end of the spectrum--to be turned on.