While the trichromatic theory makes clear some of the processes involved in how we see color, it does not explain all aspects of color vision. The opponent-process theory of color vision was developed by Ewald Hering who noted that there are some color combinations that we never see, such as reddish-green or yellowish-blue. Opponent-process theory suggests that color perception is controlled by the activity of two opponent systems; a blue-yellow mechanism and a red-green mechanism.
You can create your own demonstration of these opponent systems by observing the effect of afterimages.
- Take a small square of white paper and place it at the center of a larger red square (see image at right.)
- Look at the center of the white square for approximately 30 seconds, then immediately look at a plain sheet of white paper and blink to see the afterimage.
- What color is the afterimage? You can repeat this experiment using green, yellow, and blue.
How Does This Opponent Process Work?
The opponent color process works through a process of excitatory and inhibitory responses, with the two components of each mechanism opposing each other. For example, red creates a positive (or excitatory) response while green creates a negative (or inhibitory) response. These responses are controlled by opponent neurons, which are neurons that have an excitatory response to some wavelengths and an inhibitory response to wavelengths in the opponent part of the spectrum.