Hi, I’m Michael Corayer and this is Psych Exam Review. In this video I want to go over some things I mentioned in the last video, this is the idea that we have blood vessels inside our eye that we generally don’t see, there’s little things floating around in their eyes that were usually not aware of, and we have a blind spot in each of our eyes where the blood vessels enter the eye and we can’t actually see at this spot and yet we don’t notice this. So I wanted to go over each of these and add a little more explanation. So the first thing is this idea that we have blood vessels in our eye, we need to have a blood supply to the cells of the eye and yet we don’t see this right? I said this idea that our eyes seem to be almost backwards, right? So the light enters the eye but we don’t actually see until it hits the back surface, the retina, and yet we have all this stuff in the way. So why is it that we don’t see our blood vessels? They’re there the whole time. Well the reason that we don’t see them is because they’re always there and they’re always in the same spot. These blood vessels don’t move around. So that means that we have learned not to see them. When light comes into the eye, these blood vessels to cast shadows onto the retina but these shadows are always in the same place and so we’re able to filter them out and ignore them. But there is a way that you can see them. So what you can do is you take a sheet of paper and just punch a hole in it, like this, using a pen or something, so you have a small hole and I recommend using a colored sheet of paper. So a white sheet of paper won’t work quite as well and maybe even darker than this would be better. What you want to do is hold this right up to your eye, really close, look through the hole and you want to look at a plain white surface that’s illuminated. The best way to do this is probably just use your computer and open up a paint program or something and just have a plain white canvas, so you’ve got plain white light coming into your eye and looking through this hole and then as you look through the hole you just want to jiggle the paper back and forth. Not a lot, just enough to get some movement going in the paper, sort of short, fast sort of unpredictable movement. When you do that what you’ll find is that you’ll see these little vein-like shadows. Those are the shadows of the blood vessels in your eye and what you’re doing by jiggling the sheet of paper around is you’re getting those shadows to sort of jump around in unpredictable ways, in ways that your brain can’t ignore them like it usually does when you’re looking out into the world. You’re getting them to suddenly fall onto a new spot of the retina for just an instant and you’ll see it. Of course, your brain will quickly adjust and you won’t see it but that’s why you have to keep moving the paper. So that’s a way that you can see the blood vessels in your eye. Another way you can do this is shining a light into your eye and jiggling the light around. Don’t do this with a bright light like an LED on your phone, that’s too bright. You don’t want to overwhelm your eye. But if you have a dim flashlight you can try doing that. OK the other thing I mentioned is that the inside of the eye has this jelly-like substance that holds its shape and this is called the vitreous humor and this gives the eye its structure. But this vitreous humor has little bits of debris in it. It has little bits of protein and cell debris and those have been there since you were born. They’re stuck in the vitreous humor, they’re always going to be there. And you generally don’t notice them but you’ve probably noticed them before. If you stare at a monochromatic surface, a solid color, you might notice you seem to see little things moving around. That’s these little bits in your vitreous humor. These are called “floaters” or if you prefer the Latin term, moscae volitantes, that just means “flying flies”. So what you’re seeing is actually you’re seeing the shadow of these little bits falling onto your retina. Unlike your blood vessels these can move around. So you don’t have to jiggle a paper around or something to see them. As you stare at a plain single-colored surface, you might sort of see them moving around. Now, generally they’re going to settle towards the bottom of your eye. They’re going to be around, but they’re going to mostly be down here. This explains why you see them more vividly if you lay down on the ground and you look at a clear blue sky. So what happens when you lay down is now they going to start falling to the back of your eye. And most importantly they’re going to fall over the fovea which is where you have the sharpest vision. So when you lay down on your back and you look at a clear blue sky some of those floaters are gonna float that way and they’re going to pass by the fovea you’re going to see them more clearly. So that’s why you can see those when you lay on the ground and look at a clear blue sky. That’s when they’re most noticeable, that’s when people think oh my god is there something wrong with my eyes? It’s totally normal and everybody has these little bits of debris in their vitreous humor. OK the last thing I want to discuss is this blind spot where the blood vessels enter the eye. So where they’re coming in you can’t have photoreceptors in the retina because you need to have a space for the blood vessels to enter. So how come we don’t notice this space? Well, there’s two reasons we don’t notice this blind spot. The first is that we have two eyes and the blind spots cover for each other. So the blind spot of one eye isn’t in the same place as the blind spot of the other eye. So that helps, but you might say “well, Mike, that’s not a full explanation because if I close one eye I still don’t have a gaping hole in my vision somewhere. Where is my blind spot? I don’t think I have one”. Well you do. The reason you don’t see it is that your brain knows that it’s there and your brain just fills in whatever is around the blind spot. So if you’re looking at the wall here and the blind spot is in the center of the wall, well all around it is the same color and so the brain just says, “that’s probably just wall too”. I will perceive that i’s a solid color when in fact I can’t actually see that one part of it. You can do a demonstration to find your blind spot. You can try this on the computer screen, it might be easier to do on a piece of paper. It’s very simple to draw. All you need to do is draw a line and then a gap in that line and then draw the rest of the line below, a small gap, maybe half an inch or so. maybe smaller. Then inside that gap you can put a different colored dot. You can do the same color but it’s easier to demonstrate with different colors. And then what you want to do is a couple inches over from that you want to draw a cross. Now the side that the line is on is the eye that you’ll test. So this would be designed for testing your left eye. What you’re doing is you’re focusing your vision on the cross and you have to close one eye because otherwise the blind spot of the other eye will correct this. Then you want to move the distance while focus on the cross. What you’re doing is you’re putting the cross on your fovea here and then the blind spot is going to be off to the side here and at a certain distance depending on exactly how far apart you’ve drawn this, you’ll find that as you move the sheet of paper towards you, at a certain distance the red dot will disappear and you’ll just perceive a solid blue line. So what you’re doing in that case is essentially you’re finding the point where this is your blind spot right here. So your brain will see “OK there’s the blue line here, there’s the blue line here I don’t know what’s in here but I will just assume that it’s a solid blue line”. So when you find that point where the red dot disappears, that’s the point of your blind spot. OK so I encourage you to try these out. If you want try this blind spot exercise with your right eye, just draw the line on the other side of the cross or if you have it on a sheet of paper just turn the paper upside down, it will be on the paper side, and then you can test and find the blind spot of your right eye. It will probably be in a pretty similar place as the one in your left eye. OK so I hope you found this helpful, if so please like the video and subscribe to the channel for more. Thanks for watching!