Now, we can see color. We need to examine, in our human vision, how come we see things a little bit differently? Because we can see red. Now, the thing about human being, human being live in the daytime as well as the nighttime. But think about that, if there are animals which they are simply nocturnal, they only operate at night. Do you think they need to see color? No, they don't need to see color. So therefore, this color sensitivity somehow is unique to animals that are functioning in the daytime. Because in the daytime, when the sunlight is bright, then we will be able to see color. And so the more the color we can see, the better we'll be able to understand the world around us. So therefore these color sensitive cones would have little use for all nocturnal animals. Now while most of the animals, in fact, in the world that they see only two color. We call those dichromatic. Human being, we can see three different color, which is trichromatic. Essentially we are seeing red, blue, and green. Why do we need to do that? Think about that, if we have something called monochromatic vision such as we see only presence of light, or absence of light, or intensity of light. Think about what the world would be. Look at the diagram on the left, this is the way that you'll see it. It's the very typical black and white photos that you often see in the past. But how about if we can only see two colors, let's say only blue and green. What would the world be like? Something like this, so this is the kind of vegetable that normally we will see. And, of course, when you look at it you may not recognize how different they are. But when you compare with what we actually see in the modern days, with a human being, we're of the trichromatic vision, this is what we see. We will be able to see the red, we'll see the blue, we will be able to see the green. And all combined together, that is exactly the world that we perceive today. Now why is that important? Because seeing red is something meaningful to us, because when they ripen they will turn the color to become red. So we want to be able to search in the vegetation amongst all different colors, we want to identify fruits which are red in color. And also it is important that, when you think about human beings, it's just animal. We want to be able to be reproductively successful. And in order to do that, we need to be able to recognize each other, and what stage they are in their lifecycle. So what exactly that is? We're referring to this trichromatic vision allowed animal to be able to detect opposite sex, whether they are in estrus or not. And it's very typical that even today, we recognize in a lot of monkeys, they still use this ability to recognize whether the females are in estrus. So that they would go through the mating so that they would be able to reproduce successfully. Now that is important for our survival. Think about the full spectrum of light, it's going from all the way from the very short wavelength in the gamma ray and all the long wavelength. They are really a wide spectrum, but actually what kind of wavelength can we see with our photoreceptors in our eye? We are only seeing a very narrow range of it, which is coming from between the UV light and infrared, this is the range of color that we normally see. Now, with that in fact we are using three different types of photoreceptors, each of them carry a different type of photo pigment. These will be the short wavelength one, and the medium wavelength, and the long wavelength one. Essentially, you can separate them into the long wavelength is seeing the red color, the medium wavelength is seeing the green color, and short wavelength is seeing the blue color. So that's how we are capable of seeing the different color, think about that. With this full spectrum, in fact we will be able to perceive a world with all the colors that we are used to. So, if you look at this particular picture, you may see these two birds. They have the yellow, green, blue, and the red color, and we find them they are beautiful. But on the other hand, think about that, a lot of the other animals such as dogs and cats, they have only two photopigment. And then they don't have the pigment to be able to tell apart the red from the green, so what they see? What they see is just the blue and green area, and so what they see is that this is the kind of the world they would perceive. So when they see these birds, these are very grayish birds, that they are not really as interesting as what we see. Now in fact what it tells you is that the more photopigments that we'll be able to have, and the specificity of this pigment to detect light coming in with a different wavelength. It allows to perceive the world in a much broader spectrum. And with this, in fact, we would have individuals in fact they can have so called the tetrachromatic ability. What it means is that they are having more than just three pigments,but include pigments in the other end. So what happened to these individuals? A lot of time these are insects. Insects, they can detect wavelengths which is all the way towards the UV light range, so that they can perceive the UV light range. Compared with human beings, just seeing the range which we call the visible light, but it's really a concept for human being only, when we say visible. The most surprising is part is that, I don't know whether you have come across this kind of shrimp, mantis shrimp. It's one of the animals that, in fact, it would make a very delicious Chinese cuisine! This particular animal, in fact, it has 16 different photo pigments. And what happens is that they would have 8 of them for detecting the so-called the visible light range within the human visual spectrum. They have 4 which is in the ultraviolet, and they have 4 which are for analyzing polarizing light. So what I mean is that, well, you can't really imagine if you are a mantis shrimp and you see the outside world, what it would be like with all these different colors stimulating your eyes. I believe that it probably would be much, much more colorful than what we have. Now, having this Trichromatic Theory, we need to understand how we perceive color. We always say that there are two well established theories to try to understand what it is. The first one talks about a Trichromatic Theory, what it does is that it tells us that we have three different focal receptors. Each of them they perceive, or they would be excited by, certain wavelengths. And when they are excited they are going to trigger the signal to be transmitted and sent to your brain. So therefore, you have the red, you have the green and you have the blue. Combined together, you will have the mixture of different kind of intensity of signals sent to your brain, and that's how you perceive it. Now, that's very simple to understand. But in fact, it's even more interesting that we find that sometimes when we look at things, there are a phenomenon that we find a certain color. We simply don't see them exist together. For example, think about that, are you able to see red and green at the same time? Probably not. Have you ever seen blue and yellow at the same time? Well you need to think about it, but I'm sure you have never seen something which is black and white at the same time. All right, so what it mean is that whenever we see one color, in fact, we don't see the other. So therefore, in this [INAUDIBLE] proposal, that there's something called opponent process theory. So what happen is that this opponent color theory tells us that these color in fact when they come to our eyes, they generate some sort of opposite effect, and this needs to be processed by some of our cells. Let me remind you, some of the cells in our retina, these are the so-called interneurons. They help to process all the signal, they begin to process them in an antagonistic way. So that means if one excites it, the other one would not be able to do so. So as a result, we'll be able to tell the color apart. So essentially, it is something like that, red and green, they are opposite. So what you have is that, well if this particular neuron, it is excited by one of the photoreceptors, it would not be activated by the other. And when you activate it by the other, it will not be activated by the original one. So therefore it either see green or see red. The same thing is for the blue and yellow, it either see blue or yellow. At the same time, another group, they would process whether it's white or black. Now, you may not see what's the meaning of it, so let's take a look at it. These are the color, we say they are opposite, so, this is yellow with the blue. The green with the red, and white and black. And, usually, you'll see that when you see the so-called after-image. After you stimulate your photo receptors and your neurons in your eye. And, when I turn it off, you'll see something different. So let's do an experiment. So this experiment, I want you to focus on this screen with a blue background. In it, I have a yellow mango. So what I want you to do, is try to stare in at this particular picture, focusing your eye on this mango. And then focus, stare at it for a period of time, let's say five second, ten second. And after I turn off this particular picture, I want you to blink your eye and see what you observe. Are you ready? Blink your eye, do you see something? Do you notice that the mango now turns into an image, an after-image which is blue in color? Now, let's try again. In this particular diagram, we have another picture with a green background, and we have a red pepper in front of it. We do the same thing, you try to focus your eye on the red pepper. Stare at it continuously and then focus your attention on that. When I say blink your eyes, when I switch the slide, blink your eyes. Do you notice that? In the after-image, you see a green pepper. So what it tells you is that when this opponent process theory is at work, so what you have is that, in fact your neurons when they activated by one of the color. For example, if you look at the one which is red, you focus on it, it gets activated. But why after that when you switch it to a blank white background? So basically, your ability to see the red has to be desensitized immediately. What you perceive is the green color that comes after it, so that's what the after-image is like. Now with that, in fact, this opponent process theory tells us the following. In fact what it is, is that we rely on this interneuron, or the processing neuron, or sometimes we used the comparator neuron. That they tried to measure the input from one photoreceptor versus other photoreceptor. And ask actually, what's the signal, intensity, and they need to trigger one versus the other. When they activate one, they would not process the other. So as a result, you see one color but not the other, essentially, it is simply like this illustration. So sometimes when we say that we see yellow in color, what exactly it is? We see this two particular pigments, we are comparing which of these cone cells, with the different pigment molecules, which one would be activated more than the other. If one is activated more than the other, we see yellow. But if one is way more than the other, then in that case, we see red. So that's how this opponent process theory is at work. And, in fact, if you bear that in mind, you may now understand why sometimes people put certain colors together and they don't put certain color mixtures in another setting.
