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Opponent Process Theory of Color Vision

Instructor: Christopher Muscato

Chris has a master's degree in history and teaches at the University of Northern Colorado.

The fact that we can perceive colors is pretty amazing. In this lesson, we are going to explore one of the main models explaining how color is understood by the brain and perform a brief experiment to test it.

Color Vision Theories

Color doesn't exist. Not really. Right now you're probably thinking something along the lines of: ''well, I can see color.'' This will lead you to one of two conclusions. Either I'm crazy, or you are incredibly special. Actually, there's more at work here than that. What we perceive as colors are light waves of different frequencies that bounce off of objects, enter specific receptors in our eyes, are translated into an electric signal, enter our brains, and are translated as color. It's a complex process, and scientists have developed a number of theories to explain how it works. One of these models is called the opponent-process theory.

The Trichromatic Theory

The opponent-process theory, developed by Ewald Hering, is one of the two basic models explaining how we see color. But to understand it, we have to talk about the other model first. The trichromatic theory basically states that in our eyes there are three kinds of photoreceptors called cones. Some cones can only receive light waves of short frequencies, resulting in the color blue. Others can only receive medium waves, resulting in green, and the others can only receive long waves, resulting in red. So, blue, green, and red cones receive light of different wavelengths and translate them into signals the brain can understand as color.

Ewald Hering
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The Opponent-Process Theory

The trichromatic theory is very popular, but it doesn't seem to explain everything. Enter the opponent-process theory. This model describes the relationship between the cones to explain how they form a spectrum of colors. The basic idea is that cones are linked together in opposing pairs. Only one cone in each pair can create a signal for the brain at a time. Let me ask you this: have you ever seen a color that wasn't quite blue, but wasn't quite green--a bluish-green? Sure you have. That's because the cones for blue and green are not opposing pairs. They are able to fire at the same time. However, have you ever seen a color that's sort of yellow and sort of blue? No, there is no yellow-blue, just as there is no reddish green. Red and green are opposing pairs. Only one of them can be firing at a time. You can see blue and red together, or blue and green together, but not green and red.

This theory also explains how we can see the color yellow. Yellow is a primary color, meaning it is not composed of any other combined colors. So how can we see it if there is no cone for yellow? Yellow is the opposing pair of blue, so as long as the blue cone is active, the red and green cones cannot send signals to the brain that can be translated as yellow. However, when the blue cone is inactive, the red and green cones can receive yellow light (which has a medium-long wave length) and transmit that signal. But the minute blue comes back on, this is inhibited. You can only see blue or yellow in a single spot. They can be next to each other, but not overlapping or mixing. In a similar way, the balance of signals between all three cones produces the colors of black or white, but never both simultaneously. Thus, our three opposing pairs are red-green, blue-yellow, and black-white.

Experiment

There's actually a very simply experiment you can do at home to test the opponent-process theory. For this, you will need red, blue, and white sheets of paper. Cut out a small blue square, and place it on top of the red paper, placing the white paper to the side. Now stare at that blue square. Stare at it. Really hard. For 30-60 seconds. Your eyes should start to feel sore. Then, look quickly over at the blank sheet of white paper and blink. Did you see that? For just an instant, most people will see a green sheet with a yellow square in the middle. This optical illusion is called an afterimage.

This image can be used to perform the afterimage experiment as well
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