The Opponent Process Theory of Color Vision

The opponent process theory of color vision suggests that our ability to perceive color is controlled by three receptor complexes with opposing actions. These three receptors complexes are the red-green complex, the blue-yellow complex, and the black-white complex.

According to the opponent process theory, these cells can only detect the presence of one color at a time because the two colors oppose one another. You do not see greenish-red because the opponent cells can only detect one of these colors at a time.

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 people simply never see.

For example, while we often see greenish-blue or blueish-reds, we do not see 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.

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.

The opponent process theory explains the perceptual phenomena of negative afterimages. Have you ever noticed how after staring at an image for an extended period of time, you may see a brief afterimage in complementary colors after looking away?

You can see this effect in action by trying out the following demonstration.

• Take a small square of white paper and place it at the center of a larger red square.
• Look at the center of the white square for approximately 30 seconds, and 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.

So, how does opponent process theory explain afterimages? Staring at the white and red image for 30 to 60 seconds caused the white and red opponent cells to become fatigued. When you shift your focus to a blank surface, those cells are no longer able to fire, so only the opposing black and green cells continue to fire in response to visual stimuli. As a result, you will see a brief afterimage that is black and green instead of white and red.

Which theory is correct -- trichromatic theory or opponent process theory? It turns out that both theories are needed to account for the complexity of color vision. The trichromatic theory explains how the three types of cones detect different light wavelengths, while opponent process theory explains how the cones connect to the ganglion cells. These ganglion cells are where the opposing elements inhibit each other to determine how color is perceived.