Color Vision: Study with Video Lessons, Practice Problems & Examples

Trichromatic Theory

So now we're going to begin talking about how humans actually see in color. We're going to go over two major theories of color vision, and I don't want you to get the impression that these theories are getting, like, pitted against each other or anything like that. Each of these theories essentially explains a different stage or type of color vision. So they basically work together to help us see in color. So the first theory that we're going to talk about is called trichromatic theory and trichromatic theory essentially, is going to be the first stage of visual perception.

So it's going to be occurring in the retina, so this is the initial detection of color. So the idea behind trichromatic theory is that we have three types of cones and each type is sensitive to a different range of wavelengths. So we have one type of cone that responds maximally to blue, one type that responds maximally to green, and one type that responds maximally to red. Now, as you can see, there is some overlap between these. We have some overlap here and here, for example.

So, to give you a kind of a sense of what might happen if we had like blue light entering the eye, Our blue cones would be the most active. They would be responding the most but we would see a little bit of activity from our red cones and our green cones there. The same would happen if we had some red light entering the eye. Our red cones would be responding the strongest but we would see some level of activity from our green cones and our blue cones as well. So basically, all of the colors that you can see is the result of the combined activity of these three cones, which is really cool to think about.

Now, even though these cones can explain all of the colors that we can see, trichromatic theory actually can't explain all color processing phenomena. So we are going to be talking about that in our next video and I will see you there.

Opponent Process Theory

Okay. So as we just talked about, trichromatic theory can't explain all of color perception. So what I would love for you to do is to just look at this image on your screen. I want you to look at it focusing on that black dot in the center for about 30 to 60 seconds. And when you're done just look away at the blank white part of your screen.

So go ahead and pause the video and do that. Alright. So I'm guessing when you did that, when you looked away, you basically saw this exact same image except now it was like a reddish or like a purplish maroon color. So obviously trichromatic theory can't explain that because there's no actual red or maroon on your screen. Right?

So your cones weren't responding to that color. So what's going on there? This phenomenon can be explained by opponent process theory, and this represents basically the second stage of visual perception. Basically, the idea behind this is that it's going to be occurring in opponent process cells in the nervous system. We have moved from your retina to your nervous system now, and the idea behind it is that colors essentially have opposites.

So those opposites include red and green, blue and yellow, and black and white. So basically opponent process cells kind of come in pairs. So you have opponent process cells that will basically fire or basically send a neural impulse in response to one color and they will turn off or kind of be suppressed in response to the opposite color. So for example, you have opponent process cells that fire when you see green and are suppressed when you see red, and then the opposite one in that pair would fire when you see red and be suppressed when you see green. So then this pattern of activation and suppression of cells is then going to be interpreted by the visual cortex as whatever color you are seeing.

Now the phenomenon that you saw at the beginning of this video is something called an after image. So, an after image is a visual illusion where basically the opposite of a color is visible after you have stared at a particular color for a long time. So when you were looking at that green for, you know, between 60 and for about 60 seconds, you ended up seeing kind of a reddish maroon color. So what's happening there is that when you are looking at that green figure, your opponent process cells that respond to green were firing that entire time, and the ones that are sensitive to red were basically suppressed the whole time you were staring at it.

And so the second you looked away, all of those cells that had been suppressed basically did like a burst of firing all at once, which caused you to see like a reddish purple color even though that was not actually present anywhere on your screen. Very, very cool, right? Now keep in mind, we are still technically on the sensation side of things. We haven't even gotten into perception yet.

So coming up later in the chapter, we'll talk about how your brain takes all of this information about what color you are seeing and then add some layers of, like, pattern recognition onto it to help you really understand what is in your visual field. Alright. I will see you guys in our next video. Bye-bye.

Color Vision Example 1

Alright, so color blindness is a condition where one or more types of cones are not functional. The most common type is called deuteranopia. So the spectrum or the image right here is going to show what colors the functioning cones in a person with deuteranopia can see. The non-functional cone is drawn in a dashed line. We're going to use this image to answer the following questions.

So part a reads, 'What colors are individuals with deuteranopia not able to see?' Looking at our image here, we can see that green is in this dashed line. So, people with deuteranopia, basically, their green cone is not functioning, so because of that, they are not going to be able to see green. Now looking again at this graph, like we talked about a bit earlier in our lesson, there is a lot of overlap between the responses of red and green cones, like a significant amount of overlap specifically between red and green compared to our blue cones. And so because of this, people with deuteranopia are also going to have some trouble perceiving red.

Even though their red cones are functional, their brain isn't getting any information about the contrast that should be there regarding the color green. So quite often, they'll have a hard time distinguishing between the colors red and green. They all kind of look blended and, like, a little bit muted. Therefore, people with deuteranopia cannot see green at all and will have some trouble distinguishing between red and green.

Now part b reads, 'What colors are individuals with deuteranopia able to see well?' And that is going to be blue. Right? There is going to be some issues here, especially with more like blue-greens, of course, but for the most part, blue color perception remains fairly unaffected with deuteranopia. And then, based on all that information, which cone is not working like we talked about, that is going to of course be the green cone in this case.

Alright. So that is how deuteranopia can affect color vision, and I will see you guys in the next one. Bye-bye.

Opponent process theory explains which phenomenon that trichromatic theory doesn’t explain.

• A. Colorblindness.
• B. Afterimages.
• C. Hallucinations.
• D. Night vision.