Hi, I’m Michael Corayer and this is Psych Exam Review. In this video I’m going to explain how color vision works. Now an important thing to begin, is that color is not actually in light and it’s not really in objects either. So for instance, if we look at this coaster here we might say, it’s tempting to say, that the cloth is red. But it’s not really red, all that we’re really saying is that the cloth reflects wavelengths of light that we perceive as being red. So it’s not really the case that the cloth is red and it’s not even really the case that the light that’s reflected is red. It’s just that the light that is reflected is a particular wavelength that we perceive as being red. OK and that brings up the point that there’s a lot of light that we don’t perceive. We aren’t able to see light that’s in certain wavelengths. There’s actually a very narrow range of wavelengths they we’re able to see, this is the visible spectrum of light, and any light that has a larger or smaller wavelength, we’re not able to see even though it is still there. So here’s a chart demonstrating this. So this just shows the spectrum of lightwaves and we can see there’s very large waves like radio waves which are the size of large buildings and you know, we don’t see these, they’re all around us, but we can’t see them. The same is true for things like microwaves so when you cook food in your microwave you’re using light to cook the food but you can’t see it. You can’t see, it looks like the food is sort of magically cooking, but that’s just because you can’t see the microwaves that are bombarding the food. Then we get a little smaller we get to infrared. We can’t see infrared light, then finally we get to the visible spectrum. It’s a pretty narrow section where we have red, if you’re familiar with the mnemonic Roy G. Biv, Red Orange Yellow Green Blue Indigo Violet, that’s the order of the spectrum from longest frequencies that we can see to shortest. and then once we get shorter than violet, we get to ultraviolet which we can’t see. Although actually some animals can see, birds and bees can see ultraviolet light. So there could be ultraviolet light that we wouldn’t perceive anything but they would be able to see it. There’s some rare cases of people who can see ultraviolet light but these are people who have usually had surgery to their eyes they’ve had surgery on their cornea or their lens and in some cases this results in them being able to perceive some ultraviolet light and it’s actually not like a superpower it’s really annoying for them and it distorts their color vision as well. But that’s an exception, most people can’t see ultraviolet light and then of course we can’t see x-rays and gamma rays and even smaller wavelengths of light. OK so within this visible spectrum we can see certain types of light. Actually I want to show you a demonstration of infrared. So we can’t see infrared light but we can create devices that would convert infrared light into visible light. In fact you have one of these in your house right now. If you have a remote control, so remote controls communicate using infrared light that’s why you don’t see anything coming out. You push the button you don’t see any light going towards the tv. But there is light being emitted. And there is a way that you can see it. You’ll have to take my word for this, because this only really works in person. I can’t actually do this demonstration via camera. So when I look at this remote, if I press a button I don’t see anything. There’s no light, it doesn’t light up or anything, it’s just emitting infrared lighwaves and I can’t see them. But if I point it at the camera you can actually perceive them. Because what’s happening is my camera on my computer is able to pick up some infrared light and it sort of accidentally converts that into visible light and that allows you to see it. So you have to take my word for it that I can’t see it now but you can see it on the camera. It looks like I’m turning on a flashlight. You can try this yourself if you have a remote. Just look at it through the camera of your phone, that will also pick up some infrared. That’s essentially how night vision works. It allows you to put on some goggles that sense the infrared light that we can’t normally see and convert it into the visible spectrum and then you’re able to see it. So let’s get back to color. We have these different frequencies we can see, what happens is we sort of divide them up, we see them as different colors and that’s our perception. That’s happening in our mind, it’s not actually, they’re just different wavelengths of light. So the question is, how do we do this process of categorizing these different wavelengths into different colors? One of the first people to propose a theory of this was a British polymath named Thomas Young and I have a picture of Thomas Young here. Young proposed that we had different types of receptors in our eye that corresponded to different wavelengths of light. And another guy who we learned about previously, Hermann von Helmholtz, added to this theory and this is now known as Young-Helmholtz Trichromatic Theory. Here’s a picture of Hermann von Helmholtz, who we saw before for his work on reaction times. The Young-Helmholtz Trichromatic Theory said that we must have three different receptor types in order to see the different wavelengths that we’re able to see. So they proposed we must have receptors that correspond to shorter wavelengths of light, receptors that correspond to more medium wavelengths of visible light, and then receptors that correspond to longer wavelengths of light. By having these three receptors any sort of light that we can see is the combination of different patterns of activation of these three types. You’ll sometimes see these called “blue”, “green”, and “red” cone types but short, medium, and long is sort of a better terminology. We’ll see why in a second. Because it turns out that Young and Helmholtz were thinking about this before we had any real knowledge of the types of cells in the retina. It was a hundred years later before we could really find that we do actually have three different cone types in the retina. And then a few decades after where we could find exactly how sensitive they were to different wavelengths of light. So based on that, we have something that looks like this. This shows that each of these lines here represents a cone type and it shows the maximum sensitivity, the wavelength that it responds most intensely, down to lesser response to different wavelengths. So we see this would be the short cone here, this would be the medium, here and this would be the long. And the reason I said that the blue green and red labels aren’t really that precise is because you’ll notice the “red” cone, the longer wavelength actually peaks its sensitivity closer to a yellowish sort of orange color rather than what we would think of as pure red. OK so the idea here is that any of the colors we see are a combination of activation of these three cone types. We can see here that yellow light is equal parts of red and green light or the wavelengths corresponding to red and green light, the cones are equally activated at the point and that’s yellow. Blue here at this peak and a point that we’ll come back to in the next video is that when someone is colorblind if we were to damage or remove one of these cone types, it’s not the case that they just wouldn’t see green anymore for instance. It’s that it would affect their perception of all these colors that overlap on the green cone. It would influence their perception of all sorts of other colors; red and orange and yellow and green and a little bit of blue, those would all be affected. So we’ll see that in the next video and we’ll go into more detail on colorblindness. Ok, so that’s the Young-Helmholtz Trichromatic Theory. This idea that we have 3 cone types and the combination of activation of each of these 3 cone types gives us all the possible colors that we can see, which is over a million different colors that we’re able to see. On a related to that is the idea that this spectrum is not all the possible colors that we can see. It’s also possible , this is just the order of wavelengths, it’s also possible to combine wavelengths in ways that aren’t shown here. We can combine red and blue and see some colors that aren’t actually appearing on this spectrum. But we can still see them. OK so there is something that isn’t really explained by this trichromatic theory. You’ve probably seen something like this before. So if you stare at the center of this flag image here and you stare for a few seconds, you can pause the video if you want to stare longer, and then you switch your vision over to the white background here you’ll see a red white and blue version of the American flag. So what’s going on here? How do we explain this? It’s not just that we have three cone types, that seems to be insufficient. So we need an additional theory which complements the trichromatic theory and this is the opponent process theory. So opponent process theory was first proposed by a guy named Ewald Hering, who was a German physiologist we see here. Hering figured out that our color vision is actually working in pairs and that they oppose one another, they’re sort of antagonistic and that when you look at green it actually inhibits red. And if you look at red it actually inhibits green. And if you look at yellow which is equal parts red and green, as we saw, that actually inhibits blue. And if you look at blue that inhibits red and green equally, or yellow. So we had these two pairs of colors, we have green and red are a pair that are opposing one another and blue and yellow oppose one another. So the idea that that happens in opponent process theory is that when we stare at one color we essentially tire out those receptors. Now I don’t really like saying they’re tired out or they’re fatigued, you can say they’re habituated. But I prefer to think of it as saying that they’re bored. If you stare at green, initially the message that’s being sent is “green green green!”. But the longer you stare at it, the firing rate slows down. It’s like “green, it’s still green…green” So the firing rate gets reduced when you stare at that color for too long. Now when you look at white light, white light is all of the wavelengths at once, so we’re seeing equal parts of red and green and blue. When you switch your vision to the white light what happens is now you’re getting equal parts green and red but the green is firing a little bit slowly right? It’s gotten bored so it’s not really paying attention, it’s just like “green…green…green” and now the red comes in and says “red red red red red!” and so what happens is the red signal temporarily overpowers the green one so instead of seeing equal parts you appear to see more red then is actually there. Then eventually you adjust, it takes only a few seconds. But the same thing can happen when you stare at blue is that, the blue message weakens “blue blue blue..ok, blue…blue…blue…” then yellow comes in “yellow yellow yellow!” and suddenly you see yellow even though it’s actually equal parts blue and yellow in the white background. So that explains this color after-image that we saw. You can also have this color afterimage, it’s not just with these neat pairs of red and green and blue and yellow. This happens to any colors in the spectrum, they all have an opposite color, that would be seen in an after image. So if you take a photograph that has many different colors in it, and you invert all the colors, you show the opposite color, so anything green becomes red, but any other shades as well all have an opposite and if you stare at the opposite colors then you switch to a black-and-white version of the photgraph you’ll temporarily see the photograph in its true colors. So here’s a demonstration of this Now if stare at the, we’ll have to wait for the negative colors to come back, you’re going to stare at the X here, I know that’s hard to do, and after a few seconds it’s going to switch back to black and white and when it does that you will temporarily see a real color version of the photograph even though that’s not actually there. It’s only the negative version and the black-and-white version. Another thing that this shows, if you stare at the X and do this, when the picture switches if you let your eyes wander you’ll see that the illusion disappears. That’s because you’re only fatiguing or boring those cells in that particular part of the retina. When you move your eyes now that part of the picture is going to a different part of the retina and the effect doesn’t work anymore. So you have to keep your vision focused on the same point for this to work. OK so that’s a negative color after-image. OK in the next video I’ll talk about colorblindness in a little more detail and we’ll see why is term colorblindness is really a misleading term, people who are colorblind can see many colors but we’ll talk about the different types of colorblindness and what causes them in the next video. So, I hope you found this helpful, if so please like the video and subscribe to the channel for more. Thanks for watching!