Figure 3.

Proposed circuit for producing a subclass of blue–yellow double-opponent midget ganglion cells in which S-cone signals are added to L/M opponent signals. (a) Feedback- and GABA-mediated feedforward from S-cone inputs are proposed to produce the center and surround organization as illustrated. (b) Full circuit showing S-cone and L cone inputs. In this example, the ON- and OFF-midget ganglion cells (colored light and dark gray, respectively) receive their direct center input from an L cone. This is the basis for a blue–yellow system in which the OFF bipolar with S-L inputs is responsible for the percepts of blueness. The ON-bipolar cell, responsible for yellowness, has L-S inputs. Red–green percepts are proposed to arise later in evolution by addition of M-cones to the retina and thus forming two subtypes of color opponent midget ganglion cells. L cone centers serve blue–yellow color vision and M cone centers serve red–green color vision.