Figure 3.11.39 from Stockman, A., & Brainard, D. H. (2009). Color vision mechanisms. In M. Bass, C. DeCusatis, J. Enoch, V. Lakshminarayanan, G. Li, C. Macdonald, V. Mahajan & E. van Stryland (Eds.), The Optical Society of America Handbook of Optics, 3rd edition, Volume III: Vision and Vision Optics. New York: McGraw Hill.
Three-stage Müller zone model. First stage: L-, M- and S-cone photoreceptors (top and bottom). Second stage: L-M and M-L cone opponency (top) and S-(L+M) and (L+M)-S cone opponency (bottom). Third stage: color opponency (center) is achieved by summing the various cone-opponent second stage outputs.
Stockman, A., & Brainard, D. H. (2009). Color vision mechanisms. In M. Bass, C. DeCusatis, J. Enoch, V. Lakshminarayanan, G. Li, C. Macdonald, V. Mahajan & E. van Stryland (Eds.), The Optical Society of America Handbook of Optics, 3rd edition, Volume III: Vision and Vision Optics. New York: McGraw Hill.