The Neural Control of Vision
J. The Processing of Color

We can now proceed to ask how color is processed in the nervous system. One thorny problem is that there are only three kinds of cones, yet we process color by talking about red and green and blue and yellow. This comes about the way the three cone types are combined and are laid out in opposition with each other in the nervous system. Remarkably, it has been discovered that in retinal ganglion cells and in the lateral geniculate nucleus color is processed along just four axes of the color circle. Examples of this appear in Figure 44 in which the color circle is shown head on. Cells are shown that peak at blue, yellow, green and red. Extensive sampling showed only these classes, although red and green cells come in both ON and OFF subvarieties. The blue and yellow most commonly come as ON subtypes.

The blue cones shown here are now believe to connect predominantly to the koniocellular ganglion cells which in turn project to the intralaminar layers of the
LGN. The projections from these layers are predominantly to the upper layers of area V1 as depicted in Figure 10.

(Chichilinsky and Baylor (1999) Nature Neuroscience, 2, 889-893; Dacey, et al. (2003) Neuron, 37, 15-27)