Which of the following is not a set of opponent colors according to opponent process theory

Before I begin a discussion of web design and color, I would like to say a few things about the fundamentals of how we see color. When I was a Psychology professor, I taught classes on Sensation and Perception and Neuroscience that covered much of this, and I can’t resist the opportunity to share some of the things that I learned (and can remember) from my experience teaching these classes.

Color Theories

At one time, there were thought to be two competing theories as to how the human visual system processes color. The first, trichromatic theory, is based on the idea that the visual system is maximally responsive to three colors and that color vision is a result of the combination of differential responses of these three components. As evidence for the theory, proponents cited the fact that all the colors that can be perceived can be created by mixing three colored lights that differ in wave length. All you have to do is to differentially vary the intensity of the three lights, so long as they are different wave-lengths (colors), and you can get the three of them to mix to display virtually any color.

The second, opponent-process theory, is based on the idea that the visual system is responsive to three color pairs (green-red, blue-yellow, and black-white) and that color vision is due to the combined differential response of these three different components. Evidence that supports this is the opponent color after-image effect. When most people stare at a bright green color for several seconds and then look away at a white field they will perceive a red color, and vice versa when looking at red. The same thing occurs with blue and yellow.

It turns out that both theories are accurate; they simply apply to different levels of the nervous system. Somewhere along the line I’m sure you have heard that we have rods and cones in our eyes. These are special types of neurons called receptors, which convert a physical signal (e.g., light) into a neural signal. Receptors in the visual system are called photoreceptors. The cones are responsible for our color vision and, it so happens, for those with normal color vision, there are three specific types of cones, which are maximally responsive to three different wave lengths. For most people, these preferred wave lengths represent the colors red, blue, and green. Therefore, for visual processing in the eye (on the retina) trichromatic theory applies. Once the neural signal passes beyond the retina on its way to the brain, the nature of the cells change and, in fact, the cells respond in an opponent fashion, consistent with opponent process theory. So, for example, a green photoreceptor and red photoreceptor might each send a signal to a single blue-red opponent cell farther along in the system. Opponent cells, like all nerve cells, fire periodically just for grins, establishing a sort of base line firing rate. For a red-green opponent cell the base-line rate will decrease when it gets a green signal and increase when it gets a red signal (or vice versa). There are also blue yellow and black-white opponent cells as well. (If you'd like to know more about opponent process and trichromatic theory, Kaiser, 1997)

Light vs. Pigment

You may have heard, at some point, that white is the presence of all color. You may have also tried the experiment that I tried when I was in grade school, which was to mix together all of your paints, in which case it probably appeared to you that you had disproved this theory, because you end up with a dark brown color that gets closer to black every time you add another color. (This is one of those many things that leads us not to question what we here from grown ups. :) The problem with our grade school research model and the conclusion we drew, was that white is the presence of all color when you are mixing light, but it works the exact opposite when mixing pigment. You can demonstrate that white light contains all colors by shining it through a spectrum or watching a rainbow, both of which split white light into it’s component parts.

However, we usually see color in the form of pigments, not light. Most of the time the light that exists in our environment is some variation on white. This white contacts some pigment such as a red shirt. The shirt then sucks up all of the colors from that white light except for the red and reflects the red. So, when you were doing your ingenious experiment with your paints as a kid, you started with something like green, which sucked up everything but green from the white light reflecting on it. Then you add another colored pigment and more of the color in the light is sucked up, etc., until you end up with some muddy color. White ambient light is the constant, most of the time, and changes in our perception of color are due to pigment changes. If your teacher had given you a bunch of different lights to play with and allowed you to shine them on a white background you could have done a cooler experiment. You could have shined one colored light on the background, then another, and another and, each time you added a color the background, instead of looking more and more like mud, would have looked more and more white. Unfortunately, my teacher was barely willing to let me play with paint, much less a bunch of electrical equipment, so I only know about this process hypothetically.

If you're interested in learning more about color perception in general, see the section on Color Perception in Peter Kaiser's excellent web book referenced below.

References

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Chapter 5: Sensing and Perceiving