[MUSIC] Okay, so now let's talk about central vision and central vision depends at the, at the point at which we're fixating, the point that we're looking at, we're trying to get the finest detail. And this shows a cat fixated on this spot. And at that point, light from this spot is going to a particular point in the in the, in the retina, which is this area right here called the fovea or the area centralis. And the fovea is actually named because if we, if we made a cross section like this the fovea would actually have this kind of a an appearance. It looks like a pit. So the pit is the fovea. Light from less than my thumb's width at arm's length is going into the fovia. But aerious intralus or the macula another term that you'll hear. Probably the most commonly used term is macula is a wider area. About two thumbs width or so at at arm's length. And in this area we have very high acuity vision. So what kind of photoreceptors do you think are packed into the fovea and to the, in, in the area centralis in the macula? Well, it's cones. And in fact in the fovea itself, this very central part, this pit area, there are only cones. There are no rods. So if we, if we go over to the board for one moment, what we see is if we draw the the extent of the retina. And we have the fovea right there the concentration of cones is extremely high at the fovia and drops off as you get out in, into the, this area which is the peripheral retina which is going to see peripheral visual information. And in contrast the concentration, of cone, of rods, excuse me, there are no rods at the fovea and their concentration peaks at about 15 degrees off of the point of fixation. So what that means for you is if you're out on a dark night and you want to see stars, don't look at the star that you want to see, look 15 degrees to the left or to, to the right, a spot where you have a lot of rods. So if you're looking for something dim, look 15 degrees off of it. If you want to see something, and you want to see the high detail in it you're going to, you want to use your cones as much as possible, you want to look directly at it. You want to fixate at it. And another word for fixate is to foveate. So when we look at we look at something, we are fixating on it, or foveating on it. And light from a point that we are fixating on is going to be focused onto our retina. Now so this just shows an illustration. If you, and this is essentially an eye chart, a version of an eye chart. If you fixate on this red dot that's right there in the center. And you can try this at, at, at home on your computer. You'll see that you can very easily see these very small letters. This is an M. But as you go out into more peripheral parts of the retina and per, peripheral parts of the visual world letters need to be much, much bigger in order for you to be able to detect them. So if I wrote an M at the same size as that and you're fixating here, there's no way you could read that M. You couldn't read that M fixating here, but you could read this K so the M has to be essentially that big. It would have to be this big for you to be able to read it while fixating here. So the concentration of cones is very packed in the center of retina, and then it gets more and more. It, the concentration goes lower and lower as you get into the periphery. Okay so, that brings up another point, which is if you look at this retina. And we're looking down on a, on a living retina. Here's the fovea. And you see this, this white area here? That's actually the, called the optic disc, and it's, it is, it has no rods or, and no cones, no photoreceptors at all. It is the point where all the ganglion cells, those retinal ganglion cells which are the output form the retina, they're all sending their axons from all over the retina to this point here. So axons are coming from all over the retina to this point and they're going to go out through the optic nerve to go towards the thalamus. So right here you don't see anything. You have what's called a blind spot. This shows you the location of the blind spot and what this shows you is we're back to that image of our binocular vision field. So this is the area that both eyes see, and this is the, this is the left eye, this is the right eye's, blind spot. This is the left eye's blind spot. And, if you're, if both eyes are open you don't see either of these because the other eye supplies the information. But what's remarkable is even if you have only one eye open you still don't see your blind spot. You don't see your blind spot because it's, it's very small, and this is an exaggeration, it's very small and it gets filled in. Alright, so we're very we're very good at, at filling in information from the surround. Now in contrast if we had a blank spot in, in the area in the retina that did not have, was not working and that was closer to the macula, we would have problems. And in fact there is such a condition and that's called macular degeneration. And my mother has one version of this, it's the wet macular degeneration. And this was a picture that I asked her to draw of my father. And when she, she first told me she had this problem. And I asked her to draw the, the, this drawing she says well everything is fine I can see everything. And I said yes you can see everything because you have both eyes open. Close your good eye. So she closed her good eye and then she looked at my father. She kept fixating here and you can see that her central vision is highly disrupted but the peripheral vision is, is fine. So this is this is the, the the appearance of macular degeneration, which is an age-related condition, is unfortunately far more common than we wish it were. The, most of the people who have macular degeneration have what's called the dry type. This is the wet type. It gets worse. My mother er, started with having one eye affected but she has now both eyes. But luckily she's been, she's respond very well to treatment and she no longer has this this view of the world and she can, she, she gets shots into her eye that have completely fixed it for her, which is great. In any case what this illustrates is the importance of and the difference between central vision and peripheral vision. Ok, so what we'll do in the next segment is look at color vision. [MUSIC]
