Fig. 10 A simplified zone model for color vision, showing how cone outputs (the first zone) are combined in a second zone to produce the color opponent signals. S cone signals oppose L and M signals to generate a blue–yellow opponent signal. L signals (with an S input) oppose M signals to generate a red–green opponent signal. L and M signals combine additively to generate a luminance signal. The diagram is not anatomically correct, and it simplifies numerous details.