Simple-opponent receptive fields are asymmetrical: G-cone centers predominate.

For quantitative models of color vision, the R-cone contribution to the r-g channel is less than half of the R-cone contribution to the V lambda channel. There is currently no explanation of how this different contribution of R cones to the two channels comes about. We propose an asymmetrical receptive-field arrangement to explain the difference in weighting. Because cones in receptive-field surrounds are weighted less than cones in centers, placing R cones predominantly in surrounds and G cones in centers provides a simple differential weighting mechanism. Electrophysiological and psychophysical evidence substantiates such an asymmetry of simple-opponent fields.