The Opponent Process Theory of Color Vision

The opponent process theory of color vision is one of the theories that helped develop our current understanding of sight. The theory suggests that our ability to perceive color is controlled by three receptor complexes with opposing actions. These three receptor complexes are the red-green complex, the blue-yellow complex, and the black-white complex. (Current research suggests that the true pairings for these receptor complexes are actually blue-yellow, red-cyan, and green-magenta.)

According to the opponent process theory, our minds can only register the presence of one color of a pair at a time because the two colors oppose one another. The same kind of cell that activates when you see red will deactivate in green light, and the cells that activate in green light will deactivate when you see red—hence why you can't see greenish-red.

Opponent Process Theory vs. Trichromatic Theory

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.

How Opponent Color Process Works

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 in a cell, while green creates a negative (or inhibitory) response. When this cell is activated, it tells our brain that we are seeing red. Meanwhile, there is an opponent cell that gets a positive response to green wavelengths of light and an inhibitory response to red. In other words, these two types of cells in a red-green receptor complex can't be activated at the same time.

Example of Opponent Color Process

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 red image for 30 to 60 seconds caused the white and red opponent cells to become "fatigued" (meaning they started sending weaker signals to save energy).

When you shift your focus to a blank surface, those cells no longer have the stimuli telling them to fire. When the white and red receptor cells briefly de-activate, the opposing black and green cells fire in response. As a result, you will see a brief afterimage that is black and green instead of white and red.

Current research has updated this explanation slightly. It seems the green receptor cells do not activate because the red cells become inhibited. In fact, the afterimage seems to be generated in the brain's cortex, not the retina.

According to the complementary color theory, each receptor pairing registers complementary colors—there is no white/black pairing. When complementary colors are added together, they make white. When you were staring at the red image, your brain got used to the red and suppressed the signals it was getting from red cells. When you the shifted your gaze to the white paper, your brain saw less red light as before and mentally "subtracted" red from what it as seeing. The green cells, however, hadn't been suppressed and could send full-strength signals. White "minus" red is green, hence why you saw a flash of green.

Which Color Vision Theory Is Correct?

Although complementary colors theory is the most up-to-date, the trichromatic theory and opponent process theory do help account for the complexity of color vision.

The trichromatic theory explains how the three types of cones detect different light wavelengths. The opponent process theory explains how the cones connect to the ganglion cells and how opposing cells are excited or inhibited by certain wavelengths of light. The complementary color theory explains which wavelengths translate to which colors and how these colors are processed in the brain.