Color Vision

Human photopic color perception operates via trichromatic encoding at the photoreceptor level with opponent-process encoding at the retinal ganglion cell level.

Trichromatic Stage (Cone Photoreceptors): The retina contains three distinct cone classes, each defined by an opsin photopigment tuned to a specific portion of the electromagnetic spectrum:

• S-cones (Short-wavelength): Peak sensitivity around ~420–440 nm (blue).
• M-cones (Medium-wavelength): Peak sensitivity around ~530–535 nm (green).
• L-cones (Long-wavelength): Peak sensitivity around ~560–565 nm (yellow-red).

Because an individual photoreceptor’s hyperpolarization signals only the rate of photon capture and not wavelength (the principle of univariance), a single cone cannot differentiate color from intensity. Instead, the brain determines hue by comparing the relative ratio of excitation across all three cone types, maintaining color constancy across varying levels of illumination.

Opponent-Process Stage (Retinal Ganglion Cells): Downstream retinal circuits—mediated by horizontal cells, bipolar cells, and specialized retinal ganglion cells (RGCs)—recode trichromatic cone inputs into antagonistic, center-surround opponent channels:

• Red-Green Channel (L vs. M): Carried predominantly by midget ganglion cells that contrast L-cone and M-cone activation.
• Blue-Yellow Channel (S vs. [L + M]): Carried by small bistratified ganglion cells that pit S-cone signals against combined L- and M-cone inputs (yellow).
• Luminance Channel (L + M): Carried by parasol ganglion cells that sum L- and M-cone inputs to encode spatial contrast and brightness without chromatic selectivity.