Parts of the Eye: How Vision Works (Intro Psych Tutorial #43)

Hi, I’m Michael Corayer and this is Psych Exam Review. In this video I’m going to cover the parts of the eye and we’ll get a better understanding of how visual transduction occurs. So you might remember that transduction refers to going from a physical signal in the world, in this case light, into neurons firing in the brain. So how do we go from lightwaves to brain activity? So to understand this we’re going to have to look at the parts of the eye and the function of each of these parts. So here’s a very simple diagram of an eye and as the light comes towards the eye here the first area that it’s going to reach is this outer layer covering the front of the eye and it’s clear. This is called the cornea. So the cornea serves two purposes. The first is that it protects the eye, it’s very sensitive to touch and you’re probably familiar with this because if you get a piece of dust in your eye it feels like a boulder. So your cornea is very touch sensitive and if something touches the cornea, there’s a reflex that causes your eyelid to close. So the cornea is related to protecting the eye, but it also helps to focus the light that’s coming in. So it acts as the first lens for focusing light coming into the eye. After light gets through the cornea we get to the next part of the eye and this is the colored part of your eye and this is called the iris. This comes from the Greek messenger of the gods, Iris, and she was also considered the personification of the rainbow and so the reason for this is, you look at the iris, you see many colors in there, it’s not as simple as I’ve drawn it here; that’s why it’s called the iris. And the iris actually contains a bunch of muscle and this means that the iris can move and it can change the shape of the center of the eye. So in the center you’ll notice this black spot and you probably already know that this is called the pupil. So the pupil is actually a hole in the eye, it lets light in. The iris changes the size of the pupil, so it’s this aperture that can make the pupil larger or smaller. So why do we want to change the size of the pupil? Well, the answer is, we want to control the amount of light that gets into the eye. So when we’re in a dark situation we want to get the pupil as large as possible, we want to catch as much light as we can that’s out there in the world so that we can see. We need to have light in order to see so we want to expand our pupil as large as possible to catch as much as we can. In contrast, if we’re in a bright situation or if somebody shines a flashlight at your eyes what happens is you become dazzled, your eyes can handle that much light at once. It’s possible to have too much light entering your eye and in that case we want to shrink the pupil down, we want to constrict down to a smaller opening. This will limit how much light is actually getting into the eye. If too much gets in we become dazzled and we don’t want that to happen. So that’s why we change the size of the pupil; it controls how much light are we letting in. Now you might wonder, “why is the pupil black?”. Well, the pupil appears black because light gets into the eye, but once the light gets in it’s going to get absorbed and that means no light is coming back of the eye. Since there’s no light coming out of the pupil that’s why it’s going to appear black. The light is getting absorbed. Now there is one exception to this and this is the red eye effect that you see in photographs. So what causes this red eye effect? What red eye is, you’re actually seeing light coming out of the pupil. It’s light that enters the eye, gets reflected off of the interior of the eye called the fundus, and because of the blood vessels in here the light is going to appear red when it reflects back out. So why does this occur in photographs? Usually it happens, you’ll notice, in situations where you take pictures where it’s dark. What that means is that your pupil is very large, right, because you’re in a dark bar or something, at a party, so your pupil is quite dilated to allow you to see better in this dark environment and then suddenly there’s this bright camera flash what that does is it allows too much light into the eye, so some of that light gets reflected back out and the picture just happens to capture that moment where the light is reflecting out of the eye. So how do we stop this from occurring in photographs? Well, you already know the answer is you have a light before the flash. So you have one light and then you have the flash. So why does that stop the effect? What that first light does is it changes the pupil size, it shrinks it down it says “hey, even though you’re in a dark room, now you’re looking at this bright light” your pupil is going to contract a bit, this means that less light is going to get into your eye and that means less light is going to get reflected back out of your eye. That’s how we stop that red eye effect. OK so once light gets into the pupil it hits the next structure. It’s a little hard to draw this cross-section here, but behind the the pupil is a disc called the lens. It’s called the lens because it acts as a lens. What it does is it focuses the light that’s coming into the eye. So light comes in and the lens helps to direct that light onto the back of the eye. And the lens is surrounded by muscle called ciliary muscles. This would be, of course, all around the entire disc, but this is a cross section so you can imagine. OK so we change the shape of the lens by using this muscle to pull and push on the lens and this changes the shape and this changes our focus. It changes whether we’re focused on something close to us or far away. So you can test this or get a feel for what this is like by focusing on something close to you then without moving your eyes, change your focus to something farther away. And then back. What you’re doing when you do that is you’re changing the shape of the lens. Now as you get older you’ll find that your lens becomes stiff. It becomes less and less flexible and this means you can’t do this as well. You can’t change your focal point as easily and this explains why you see older people doing something like this when they go to read something. They do this sort of thing. What are they doing? What happening is their lens is stuck at a particular position and they can’t move it very easily. So instead of moving their lens they have to move the thing they’re trying to focus on and get it to, “OK, here it’s in focus, if I move it closer it’s blurry, if I move it farther away it’s blurry, but right here, my lens is ok for that setting”. This also why you see people with glasses they can see one thing over there over the top of their glasses, if they use reading glasses, that’s because their lens is set for that focal point, but then they look through the glasses, that’s going to change it, and that gives them essentially two focal points. If they have bifocals, they actually have three now. They can look over the top of the glasses they can look through the first lens of the glasses, or they can look through the smaller lens on the bottom and that gives them three different distances they can focus on. Ok, that’s the lens, it focuses the light that’s hitting the back surface of the eye. Along the back surface of the eye here, this is where vision is actually occurring. This is the retina. This comes from the word for “net” so this is sort of catching the light that’s coming into the eye. So the lens focuses it onto the retina. This where these cells are actually going to be stimulated by the light. These are where we have photoreceptors and I’ll talk about types of photoreceptors in a future video. So they get cast onto the retina and this is where vision actually occurs. You’ll notice I drew a little dent in the retina here, this is the fovea and this comes from the word for “pit”. So the fovea is this little bowl-shaped indentation in the retina and this is where your vision is the sharpest. So when you focus on an object, you’re really trying to see it clearly, what you’re doing is you’re adjusting your eye and changing your lens so that that object is being cast onto the fovea. That’s where you can see it most clearly. When it’s cast over here you can see it, but not all that well. But put it on the fovea and you can see it pretty clearly. So all of this information gets collected by all these cells in the retina and then they get sent out through the optic nerve. So they all get collected and sent out to the brain here at the optic nerve. Now, you might be wondering, it’s kind of strange, our eyes seem kind of backwards, right? We don’t actually see with the front of our eye, we see with the back of our eye. The light has to go through all this other stuff before we see it. A really interesting part of this is that all these cells need nutrients, they need oxygen and so at the same point here where we have the optic nerve exiting the eye, we also have stuff coming into the eye. We have things coming in and these are the blood vessels that supply oxygen and nutrients to the cells of the eye. You might be wondering, “that seems weird, why would we want these in the way of what we’re trying to see? How come we don’t notice all this?” so in the next video I’ll talk about some related ideas here, why they don’t see the blood vessels in our eye, also this point where they enter. There’s a point where we can’t have photoreceptors, right? Because we need the blood vessels to get in. This gives us a blindspot. But again, you don’t really see this, so in the next video I’ll talk about why it is you don’t see your blindspot. I’ll also talk about these things that are floating around in here. There’s this jelly-like substance, the vitreous humor, inside the eye holding it in shape. And it’s got little bits of things in it that you can see in certain conditions but you generally don’t notice. So I’ll talk about those in the next video. OK so that’s an overview of all the different parts of the eye that you should be familiar with and this process of how we get light into the eye, onto this back retina, the back surface of the eye, and in a future video I’ll talk about how it gets processed by these photoreceptors and then sent to the brain for more processing. OK, I hope you found this helpful, if so, please like the video and subscribe to the channel for more. Thanks for watching!


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