A different point of hue
- Bevil R. Conway*,†,‡ and
- Margaret S. Livingstone*
- *Department of Neurobiology, Harvard Medical School, 220 Longwood Avenue, Boston, MA 02115; and †Society of Fellows, Harvard University, 78 Mt. Auburn Street, Cambridge, MA 02138
Adistinguished line-up of scholars recently got together to stir up discussion about the physiological basis for color and have, with a simple manipulation of decades-old data, challenged one of the fundamental tenets of our current understanding of the neurobiology of color (1).
Understanding color is not easy. Newton made some headway, but his demonstration of color's physical basis provided only limited insight because, as Young (2) pointed out, there simply is not enough space for a receptor for each of the seven million or so perceivable colors at each retinal location. Young argued for a triplet color code, and we now know that such a code exists in the form of the three cone types (Fig. 1).
Neural basis of color. The three types of cones (Bottom) are sampled by neurons in the lateral geniculate nucleus (Middle). R+ indicates excitation by red cones; G- indicates suppression by green cones. Spatially and chromatically opponent neurons arise in primary visual cortex (Top).
Young's idea made color a construction of the brain, not a physical attribute, and paved the way for opponent color theory (3) in which color is determined not by trichromacy but by three opponent processes: red-green, blue-yellow, and black-white. This theory gained ground because it accounted for the fact that we are unable to see a continuous mixture of “reddish-greens” and “bluish-yellows,” which should be perceivable if color were simply trichromatic.
In the 1960s, De Valois et al. (4) discovered that many cells in the lateral geniculate nucleus (LGN) (the thalamic relay from the retina to primary visual cortex) show chromatic opponency. LGN cells inherit this property from retinal ganglion cells. Some are excited by red and inhibited by green (R+/G-); others are excited by blue and inhibited by yellow (B …
