So as I said before, this is not a course about the anatomy and physiology of the visual system, as you might find in most courses on vision, or might expect in most courses on vision. It's really about perception, and how the visual system works to generate what we actually end up seeing. And what that means about the brain and the way the brain works, which, as I said before, is not really known. So, nonetheless, we do have to have some information, some vocabulary about the basic things that we're going to be talking about. And so in this third topic, I'm going to be telling you about the organization of the visual system. And we'll start with the eye, obvious place to begin any description of the entire visual system. So here's what the eye looks like, of course, and it may surprise you, and I think it surprises most people who think about vision. How much of the important function that's being carried out is done before the stimulus ever gets to the retina. And I'll talk about the retina shortly, but I want to begin by talking about the non-neural parts. I think you're probably aware, or should be aware, that images are not really out there in the world. When you look at a photograph, of course, you're seeing an image, but that image has been created by a camera, in much the same way that the eye has to create an image for us to get going. Individual processing that ends up with letting us see something about the world that's out there. So a basic function of the eye, of course, is to form an image, an image on the retinas which is where the nervous system begins. The retinas we'll talk about in a minute, is really part of the brain, it's an outcropping of the brain, embryologically part of the brain. And, of course, because you can see it, it's the most accessible part of the brain. But let's understand first what goes on before light reaches the retina and forms an image. Let's begin with the cornea, and the understanding that the photons that are the radiation that's out there on the world, and we only see, as we said before, a small part of that electro-magnetic radiation. The generation of an image begins with the cornea and it begins with the refractive abilities of the cornea. What refraction means is, the ability of any material to alter the speed and, depending on the curvature of the object, if it's a curved cornea or a lens, to alter the direction of the light rays. And the cornea does that because its refractive index, its density, is far greater than air and so when light hits the curved surface of the cornea, it's bent. And that's the first step in forming the image that follows eventually on your retina. Now you might think that the lens is what's doing that, and the lens, of course, is critically important, but it's not contributing the major fraction of the refractive power that's generating the image that falls on the retina. The lens is attached by the fibers that you see here, the zonule fibers so-called, and these muscles, the ciliary muscles either relax or contract to change the shape of the lens. And the lens is also doing some refraction. But its main function is to change in a much finer way, the direction of the light rays that are coming into the eye. So that they fall precisely on the retina and not a little bit in the front, or a little bit in the back. If they fall in front or in back, that's why most of us, who've grown up doing a lot of early visual work, reading and so on, have to wear glasses. Glasses are worn by the majority of people in developed countries where we do a lot of early visual work, and that affects the development of the eye and changes the distance of the length of the eyeball. That's a whole other story that's interesting. But the point of what I'm telling you is that the lens does that fine refraction that causes the light waves to fall precisely under the neural control of the muscles, the ciliary muscles that are here to fall precisely on the surface of the retina. So, the combination of the cornea and the lens are the refractive elements, the non neural elements that go into the first step in making an image. They take the light rays that are coming into the eye that are going in all directions. What's out there in the world is just a mess of photons going in every direction, reflected off the surfaces of objects that are out there in the world that eventually compose the scene in the image. The cornea, the lens are making that first step possible. There's, of course, a lot of other stuff in here, the eyeball is not empty, it has material in it, that's called vitreous humor, but that's not important in the generation of the image. So let's turn now to the retina and what an ophthalmologist would see looking into your eye through an ophthalmoscope. And ophthalmoscope is the instrument that the ophthalmologist has. And many of you, perhaps a majority of you, have had the experience of having gone to an ophthalmologist to have your eyes checked. And to have had drops put in your eyes that dilate the pupil, and that's because if the pupil is constricted by a bright light the ophthalmologist can't really see what's shown in this picture of the so called fundus of the eye. The retina and all the details that are in it and let me describe those details to you. So this is exactly what the ophthalmologist would see when looking through an ophthalmoscope, through the pupil that's been dilated. And the key features, first of all, the optic disk, or the optic nerve head papilla, and that's where the neural axons that we'll talk about in a minute, in the retina exit the eye and form the optic nerve that's carrying information to the rest of the brain. What's really critical, in understanding the retina and what it's doing and what our vision is doing for us at this first stage, is that there is a region called central vision that consists of the macula lutea, that I have circled here. And you can see it as a region that's a little bit different in color and absent in blood vasculature from the rest of the retina. In the middle of that macula is the so called fovea. The fovea is the region that we use to see with greatest detail. It's the region that has the greatest sensitivity to detail and light, that has the finest resolution. The point of telling you that, and I'll show you more and describe that more in a minute, is that we have to align our eyes by conjugate movements of the two eyes to a point of fixation such that the object that we're interested in seeing in detail, whatever it might be falls on the fovea, falls on that region in central vision that is the key in seeing detail in images and for a For us in seeing a color. Of course there are other features of the retina that you can see here. There are blood vessels. This is a branch of the ophthalmic artery. There are ophthalmic veins. The veins are the big blood vessels, the arteries are the smaller ones. And they obviously are important because the retina has a high degree of metabolism and needs to have a vigorous and constant blood supply. And of course these blood vessels, as you may know are important in disease. A lot of people over the age of 65, about 15%, have some degree of macular degeneration. These are disorders of the blood vessel in the region of central vision that make it hard for people to see and are a form of blindness that is the most common form of blindness in the United States and other developed countries. And that's a very serious problem, that's a problem
