The color argument in your head The bright red of a cardinal against green summer leaves does not exist as "red" or "green" in the outside world. It exists as different wavelengths of light. The transformation into the experience of color happens within the neural circuitry of your visual system, a process confirmed by the discovery of specific cells right here at Johns Hopkins University. This work provided physiological proof for a 19th-century idea called the opponent-process theory.
In the 1870s, German physiologist Ewald Hering proposed that our vision operates through three opposing channels: red-versus-green, blue-versus-yellow, and black-versus-white. He observed that we can perceive a yellowish-red (orange) or a bluish-green (cyan), but never a "reddish-green" or a "yellowish-blue". For nearly a century, this remained a compelling but unproven psychological theory. Then, in the mid-20th century, researchers using microelectrodes to record from single neurons in the visual pathway found the proof. They located cells that were excited by one color and actively inhibited by another. A specific neuron's firing rate would increase for, say, red light, but decrease below its baseline rate for green light. This antagonistic relationship is the biological basis of Hering's theory.
These spectrally-opponent cells are found in the retina and in a relay station in the thalamus called the lateral geniculate nucleus (LGN). This discovery reconciled two competing theories of color vision. The earlier trichromatic theory correctly stated that color vision starts with three types of cone cells in the retina, each sensitive to a different range of light wavelengths (blue, green, and red). The opponent-process theory explains the next step: signals from these cones are wired together in an opponent fashion by retinal ganglion cells before the information even leaves the eye.