Okay lecture two week three. for this one I want to give you a real kind of concrete feel of that magic that I told you about. About the notion of there's all this energy out in the world how do we get it in. now of course we do that with every sense we have. but just to give you a good feel, I'm going to tell you the story with respect to vision. Which is probably, you know, the most important or at least the one we rely on most as a species to get information from the world. a lot to talk about, let's get going. Week three, lecture two. So I call it Point of Contact because that's really what I want to talk about is that point where us touches the world, really and lets the world in. How does that work? OK, so we're going to focus on the eye. I'm not going to go into detail but I want you to have a sense of the optics to begin with. here's an eye. This colored part around here is called the iris. The iris is actually a muscle. It's a muscle that essentially makes this black spot, the pupil larger or smaller. The pupil is where the light is going to come into the eye. It'll come through this pupil and ultimately it'll strike the retina at the back which you could almost think of like a movie projector screen. And what the pupil is really doing or what the iris is really doing I should sya is controlling how much light is getting in. so you know just quick experiment find someone that you're, that you're pretty comfortable being close too. sit face to face with 'em, turn the lights out wait a little while. Okay. And what'll happen then by the way and you've all noticed this is the room at first will seem really dark but then it'll seem brighter and brighter and brighter. Why is it getting brighter? Well, because your pupils are dilating, are opening up. And as they open up, they let more light in and you'll see the room become lighter. Eventually you'll be able to see each other quite well and hopefully you can now stare into that other person's eyes and flip the light switch on but keep staring into their eyes and keep watching their pupils. And what you should see and what they should see in your eyes is that pupil will begin large and will slowly shrink. Down okay because now there's a lot of light in the room. Too much light so it shrinks the pupil and allows less light in. Okay so that's really the main function of these parts of the eye. We see the sclera by the way's just kind of holding the structure of the eye giving some nutrients that's the white part of the eye. But this is really where most of the action is because really, what the eye is all about is taking light energy from the external world and focusing it on the retina. We see things along the way like lenses, you know and by the way why most of us need reading glasses when we get a little older is because these lenses in the eye, this is what we bend when we squint. And when we're young we're really good at bending this and we can, we can focus light on our eyes really well. But as we get older this lens gets harder, more brittle. It won't bend as easily. And eventually we just can't bend it enough to focus on the eye and then we need another lens. literally glasses or something like that to help us with the focusing process. But for a young, healthy eye, the light gets focused thanks to this lens onto the retina and the retina is really the point of contact. Okay. This is the point where right up until now it's light energy but at this little magical area at the back of the eye that light energy is going to be translated into a neural signal and then sent out the optic nerve. Let's go there. These are the two critters that really make the action happen. Now there's actually three kinds of these cones. there are ones that relate to red light, green light and blue light. Okay, so they react to different things. I sometimes try to think of these guys almost like they're vampires. Because this is kind of what happens. They go through a process called bleaching. What that means is when light of the right type hits them, it causes this chemical reaction. So I think of a vampire you know, dissolving into a bunch of pieces. Well, these don't dissolve into a bunch of pieces but a chemical reaction happens and it's that chemical reaction that literally produces. So they, they are what's called photosensitive. So, light causes this reaction. And it's the reaction which stimulates the nucleus and starts an action potential. Okay? So these are just, you know, light hits them. And if enough of the right kind of light hits them, that's what kicks off the action potential. Now, what do I mean by the right kind? Well, for these kinds, these rods, it's really just brightness of light that matters. Anytime there's a suitably bright light, they will cause their little chemical reaction and will start a neural signal. But you're going to see these are kind of like a shades of gray thing. These, these are what we use at night. I don't know if you noticed. But when I asked you to turn the lights down. If you ever try that again. Turn the lights down and then look around you. And what you'll see is if there's no bright light anywhere everything kind of looks shades of grey. But when there's lots of light, that's when we see color. And that's what these rods do. They react to colored light. As long as it's sufficiently bright. So they're really good in the daytime but they're kind of useless at night. but if it's sufficiently bright, they allow us to kind of take that energy and not only sense the energy but also sense the wavelength of that energy. Was it, which is ultimately what we experience as color. There's a whole lot to this story and I'm going to have to go kind of quick. but I want you to know the players and get a feel for the process. So these are really the ones that start things happening but here's this kind of interesting process. So when we look at how these rods and cones are distributed on the eye this, this isn't really looking at an eye. This is more like looking at the back of the screen of, of the eye. But if you're focusing on something and you're looking at it. That area where you're focusing we call the fovea, okay, that's the sweet spot of the eye. And what you see is that sweet spot has a whole lot of rods and cones. That's what's depicted by these red, green and blue things. There's a lot of rods and cones in there. and so what that means is when you really focus your attention, when you really look at something very carefully. Your casting that light on your fovea and you're seeing rich detailed color. information about what you're looking at. So you can really see it really well. around the outside of the fovea you know, where you're not focusing so much. More at what we call your periphery that's where a lot of the rods are. They're really useful in, in the night when all of these things become kind of useless. If there's not enough light, the ro-, the cones are, are useless and the rods take over. And it's the rods that give us a, a sense of night vision, as it were. an ability to kind of detect shades all be them kind of shades of grey, but we still can kind of see things in the light. So, you know if you look at what, what this fancy thing here is. If you look at the eye and how these cells respond, the cones are responding very heavily to information that's presented right at the fovea. and the, and the rods aren't there's no rods at the fovea. But outside on each side of the fovea as we go out, that's when the rods really do their thing. Notice here there's optic disc. There's a point on the retina where there's neither rods nor cones. There's, you actually have a black spot on your retina at all times. But what the brain does is it kind of fills it in, it knows what's on either side of that black spot and it fills it in for you. And you don't even notice that you have that which is you know, another whole story. It's kind of cool. Hey, I tell you, everything's cool [LAUGH] about this stuff, it really is. alright, so we have all these rods and cones. They pick up the light. They turn it into a neural signal. But then other cool stuff happens and, and I want to tell you about this because I want you to understand that a lot of our sensory perception sometimes happens before we even get to the brain okay. At the sensory organ itself and the eyes are like this so for example and this is a really strange setup that the eye uses. We have light coming in here, to the eye. And so, this is like, the back of the eye. that's the optic nerve going out. And so, light, when it strikes the retina, it actually does this, it goes through this layer of nerve cells. It goes through these neurons and it goes through the rods and cones as well. Sorry, it goes through the ganglion cells, goes through the bipolar and then through the rods and cones and then strikes the back of the retina here. And causes these photo receptors to bleach and starts the signal, which then goes through the rods and cones, through the bipolar cells, through the ganglion cells and then out the optic nerve on its way to the brain. So its kind of a weird thing the light goes all the way through these things without stimulating anything. Hits the back and then starts a process that comes back through this way. Why I don't know, that just seems to be how the eye is designed. but you know its kind of interesting and weird. So what do all, what are all these things doing here? We know that the rods and cones, they're the things that are actually picking up the light and turning it into a neural signal. and then they send it along, of course, but it's been coded a little bit now, right? The code, the, the, the rods are saying here's how much brightness is out there especially in the periphery. And the cones are saying, here's how much red, green and blue is out there at the fovea. And here's exactly where all the red, green and blue is. So they've kind of got the color already coated in a rough sense. And they've got the position, you know? Which colors are at what position coated because that depends on where it falls on the retina that tells it position. So now passes this information on. The bipolar cells and the ganglion cells clean it up. They do some pre-processing. The bipolar cells are really good at edge detecting. So any time there's an edge in our world they kind of accent that edge. make it sharper for us. And, and I'm going to let you feel this in a second. So they sharpen the edges, which is really important because edges often define the beginning and ending of objects. And so, if we're ultimately, if our ultimate goal is to see what's out in the world, that is to see objects then those edges around them are very important and the bipolar cells emphasize that. The ganglion cells sharpen the color contrast. They do almost the same thing that bipolars do with edges, but they do it with colors. And they do it in really interesting way that, that gives rise to a bit of a perceptual illusion. So I want to go to the ganglion cells and give you a taste of what they do. Both to show you the kind of complexity of nerve cells and the neat things they can do. But also so that I can then show you this neat illusion that you will understand. And the illusion will make sense of bipolar, it'll show you what the bi-polar cells are doing as well as the ganglion in a very palapatable way you'll feel it. so let me tell you about the ganglion cells. The ganglion cells use these so-called center surround setups. Now, I can't give you all the details but I, but I can give you this sense. What they do is they use what's called opponent processes. So we usually think of a nerve cell as either firing or not firing. But these cells do something more interesting. They fire all the time at a moderate rate. So think of them kind of firing [SOUND] like this. That's when they're sitting around doing nothing. Now, the advantage of having some medium rate of firing meaning nothing is that you can now signal two things. You can signal the presence of one thing by speeding up and now we go back to our natural. Or you can signal the presence of another one by slowing down. That's really bad. Okay, so if your normal rate is medium now you can either speed up or you can slow down and that's what these cells do. They will speed up when, one color hits them. So, for example in this bipolar cell. When red hits the bipolar cell, it's rate of firing speeds up. So think of this as the average rate. Its rate of firing speeds up. But if green hits it, its rate of firing slows down. So it's almost like. Its, its accentuating differences between red and green by highlighting or by altering the rate of firing in that extreme way. So if there is ever a red within a green it will really accentuate the difference by causing a big height and a big trough between the two now. And There. There's a really kind of cool thing. There's a. There's another one that does this with blue and yellow by the way. accentuates the difference between blue and yellow. You might have noticed that I never talked about yellow cones because there are no yellow cones. The weird thing is when something is in the world is yellow, it excites both the red and the green equally. But because of this opponent process if something's trying to both speed up and slow down the nerve cell at the same time, it ultimately does nothing. So, yellow excites red and green equally, which actually does nothing to the red-green ganglion cells. But, when both red and green are present. That signals to the yellow blue ganglion cells that yellow is there. Very complicated but, but really kind of cool. so we end up getting four colors out of three rods. If you're interested in that, Google that a little bit, and you'll find out a lot more. the point I want to make though is that these things have this neat opponent process. So when red's present, it speeds up. When green's present, it slows down but there's also another feature. If red is present, it speeds up and then suddenly disappears. What the cell wants to do is go back to the average level but what it actually does is overshoot a little. It goes if it reduces the firing too much and it goes past the medium and it goes back below and what you get then is the feeling that there's a green thing there okay. There was a red thing but now I just took the red thing away but suddenly where the red thing was you see green. You see a green thing that isn't there. Its called an after image. and I'm going to let you see one of these afterimages. one of the interesting things about the afterimage if I go back a slide, is the afterimage is generated by the ganglian cells. That's what I've been talking about. And that means you see it without it going through the bipolar cells. The bipolar cells crisp the edges. So what you'll notice is the afterimage, not only will it be the opposite color thanks to the opponent process of the ganglion cells but it'll also have less sharp edges because it's not processed by the bipolar. So this afterimage kind of shows you what both of these cells are doing, okay? A whole lotta talking, let's get to it. I'm Canadian, here's a Canadian maple leaf. what I would like you to do with this slide is right now start staring right at the center of this x. I'll move my mouse here and I'll get it right off the screen. So stare at the center of that x and keep staring it. And you, you'll feel weird things happen by the way. as you're staring let me just mention, the eyes are not really used to fixing on a thing very long. The eyes tend to move all around space. So when you do fix on something, sometimes weird phenomenon happens. Sometimes you can feel like the world's closing in on you. That's normal. Don't freak out. If you feel that, that's cool. But keep staring at it. Keep staring. Now, as you continue to stare at it, let me just tell you what's going to happen. I'm going to click a, a button. The screen's going to go totally white other than me up here in the corner, of course, but don't be looking at me. Look where the thing is. The screen will go totally white but you won't. You, you will see a green maple leaf. And if you kind of out of the periphery right now, look at how sharp the edges are on this maple leaf that you're looking at right now. And I, and I purposely picked one with sort of jagged around there, kind of get a sense of how crisp of a maple leaf it is. Then when you see that green afterimage notice that it's not nearly as crisp. that's because right now when you're looking at this one the image is going through your bipolar cells. That's crispening out but when you see the after image, the after image will be coming right from your ganglion cells. It won't go through the cleanup station and you'll feel that. All right, you ready? Here goes. Okay, right now I can hear the whole globe going whoa, that's so cool man. [LAUGH] I, I hope so anyway. you know, I, I wasn't even really staring it and I see a bit of a green after image. So, so I'm certainly hoping that, that you do too. Notice if you move your eyes around the after image kind of moves with it and again notice that the edges of it are not nearly as sharp because they're not going through the bipolar cells. Cool I don't know how long they'll let you stare at this green maple leaf. I will move on you can go back to it any time you want. so here's just a couple of videos, first another one about rods and cones. These are both from this interactive biology series. I find it kind of good the guy explains things pretty well so. every now and then I like to use that. So, the rods and cones will kind of go through what we've gone through. But maybe fill in some of the gaps because I had to go kind of quick. and then sound in the ear I did this one to let you know that, you know, there's a story like this for every sensory system we have. so you know maybe auditions the next most important sound in the year so this one will walk you through the auditory process so you can see its similar but different. You know, obviously the mechanics are different its sensitive to a different kind of energy, acoustic wave forms. But, the story kind of is, is similar at least at some level and so this will give you that sound. with respect to reading, here is a general reading on the visual process but if you like that after-image stuff. I've just done a quick little search on Google for you where you can see a bunch of images that produce that same after-image effect. All you want to do if you want to play with this is paste one onto a white background like I did. And then make another slide if you have PowerPoint or something like that make another slide fall on it where its just the white and put a little x somewhere near the middle have people stare at the x for a little while. And then go to the white one. By the way one of the most popular illusions among some circles is what's called the Jesus illusion. You can Google that. it's this thing that just sort of looks like white and black un, until you stare at it that way. And then when the afterimage really looks a lot like, like the face of Jesus and it kind of seems to come out of the screen at you. So, it's kind of a nice freaky. If you want to really kind of freak out your friends with After Effects, that's a good one to use because it's kind of got this whole feel to it. Alrighty? So cool, that gives you an idea about how we get information in from the world. And now we're going to take the story a little further into the brain itself and talk about some of the things the brain does with that information. All right, so I will see you on the other side of lecture two.
