 Hi, and welcome back to this video course, Biological Psychology. In this video, video 3.2, we're going to take a look at sensation and perception. Now, what is sensation? Sensation is stimulation of sensory receptors. So for example, if to see, light falls through the lens in my eye, onto the retina in the back of my eye, where there are photoreceptors, neurons that respond to sensory cells that respond to light. Now, this is sensation. These are receptors that respond to, in this case, light. There's other forms of sensation, for example. There are receptors in my skin that respond to touch. There are receptors in my ear that respond to sound, etc. That's a form of sensation. Now, all that sensory information is transferred to the brain where it is processed. And what then happens is that essentially the end product of all that, of the processing of all that sensory information is perception. So you could say that perception is our brain's interpretation of the information that our brain receives from the senses. So sensation would be a more low-level physiological construct, whereas perception would be a more high-level psychological construct. That's the distinction that we usually make. Now, it's important to realize that we don't perceive everything that we sense. So imagine that I would be in front of a classroom now. I'm not, of course. Maybe at best I'm in front of a virtual classroom. But imagine that I would be in front of a real classroom. Then information from all those students, all those faces, would fall onto my retina. I would, in a sense, all those faces in front of me. But I would be able to consciously attend to only a few people in the crowd at the time. So my perception would be dominated by only a very small subset of all the sensory information that I receive. And this is related to the concept of attention, selective attention, that we selectively attend to only a little bit of everything that we can possibly attend to. And we will get back to that in the lecture on attention and consciousness. Now, conversely, and perhaps a little bit more surprisingly, not everything that we perceive corresponds to something that we sense. So our brain can interpret the world in such a way that it does not match what our senses actually tell us. And if that happens, then what we essentially have is misperception, or, for example, an optical illusion, which is a form of misperception. Right? So consider, for example, this optical illusion right here, which is called the cafe wall illusion. Now, what you see here, I'm pretty sure, is what you perceive, I should say, is a wall with lines that are slightly tilted. Right? And they don't seem to be perfectly straight. Due to, for some reason, which I don't fully understand, due to the way that this visual information is configured, we perceive the lines as being slightly slanted. Whereas, of course, the lines are, if you would measure them with a ruler, perfectly parallel. So our sensation, what we sense, are perfectly parallel lines. But what we perceive are slightly slanted lines. So this is, of course, a disconnect between perception and sensation. Fortunately, of course, in daily life, these kinds of disconnects don't often happen. Right? In daily life, usually our perception corresponds pretty well to what is out there in the real world. But we can create situations, optical illusions, in which perception does not correspond to what is out there in the world. Okay. Now, how many senses do we have? I think many people believe that we have only a few very well-defined senses, namely the five classic senses, right? That you see here, hearing, vision, taste, smell, and touch. Now, of course, these senses do exist, but the reality is much messier. So you can think of the body as a biological machine that just has a lot of different ways to sense the world. And you can categorize them, these senses, in various ways. But any categorization of our senses is, in a sense, slightly arbitrary. So here is my categorization of the senses. So we have vision. We can see the world, definitely. That is a sense, generally considered to be one sense. But if you think about it hard, then there are actually two senses in there. Because we have conscious vision, if I look at something, if I look at it right now, for example, I'm looking at the camera, I can consciously perceive the camera. I'm aware that I'm looking at the camera. That is one purpose of vision, conscious vision. But at the same time, vision also supports ambient light sensation. So our body has a biological clock. We have a sense of day and night. And the sense of day and night is regulated by vision, by the fact that sometimes it is dark and at other times it is bright. Now, and these are largely different senses, you could say. And it's also possible to have impaired conscious vision while having a preserved biological clock any other way around. So you can, it makes sense in a way to make sense, no pun intended, to subdivide vision into two different senses, conscious vision and unconscious light sensation that supports our day and night rhythm. Then we have hearing. Let's keep that as one sense. Maybe a hearing expert will be able to tell you that there are actually many different senses in there, but to me, that's kind of one sense. We have touch. And touch is not even really a sense. There are many different ways in which we feel our skin. We have a sensation. We can perceive pressure. We can perceive temperature. And we can perceive pain. All of these, we can feel with our skin. There are all forms of touch in a way. But I think introspectively, it's already quite clear that pain perception is not really the same as a pleasant stroking perception or temperature perception. They're really qualitatively different ways in which we perceive the world through our skin. Then we have taste. We have smell. We have balance. Let's subdivide balance again. So balance, we have a sensation of orientation. We know right now, I know that up is there and down is there. I sense that orientation. We can feel movement. We can feel acceleration of the body. For example, when you're in an airplane and you have this nasty sensation in your tummy of the airplane seeming to fall down. That is your sense of acceleration fooling you, essentially. We also have a perception of limp position, proprioception. So right now, I know I can feel that I have my head upright. I can feel the difference through proprioception between holding my head like this and holding my head, for example, like this. I have a sensation, I have some perception of the position in which I hold my limbs. So this is one way, my way, in which we can categorize the many senses that we have. But the main point, I think, is to realize that sensation is a very fuzzy and messy affair and that our body just uses a lot of different clever tricks to sense the world around us and to perceive eventually the world around us. Now, with that, let's move on to video 3.3 in which we're going to take a look at vision.