So the neocortex isn't all the same everywhere. In certain regions, and you can see from the artist's drawing that you have different cell types in different layers, and it looks different depending on what kind of stain you use to visualize the cells. It depends on the function of, well, let me rephrase that. Cytoarchitechtonics is kind of a big word, but what it really means is how any region of this cortex is put together. In that what kinds of neurons are there and how many of them are there, and the degree of myelination within the layers. So if I have a particularly dense area or a less dense area, these are all different ways of looking at how the cortex is put together. This was done by a man named Corbinian Brodmann, back at the turn of the century. And what Dr. Brodmann did was amazing. He took the entire human cortex, sectioned it, put it on a slide, looked at it with a microscope, and came up with all of these different areas. So he looked at the cellular density and the kinds of cells that were there, and he came up with what is known as Brodmann's map. And that is what is pictured here. All of these different regions represent different cytoarchitecture within the brain. So area 4 is right here, and that is your motor cortex. This is where our upper, Motor, Neurons live, okay? The area right behind our post central gyrus, areas 3, 1, and 2, is our primary somatosensory. Is our primary somatosensory cortex, okay? So all of those fibers that we were talking about from our anterolateral system and our medial lemniscus system that go up the spinal cord into the brainstem and into the thalamus, then they go to this area of the cortex. And quite literally, this is where you feel. This is where, when that mosquito bite hits you, that's where you feel it, right there. Area 17 is primary visual cortex. And that is quite litterally where you see. If somebody came along in the middle of the night, scooped out area 17, you would be blind, even if you had perfectly good eyeballs. Area 41 and area 42 is where you hear, that is primary auditory, Cortex. So when Brodmann first described all of these different areas, he didn't know quite what it mean and it waited until we had neurophysiology, basically. So a couple of decades later before people could actually record what neurons were doing. And then they started to correlate and realize that, the cellular structure of the brain reflects what it does, and it's different based on the job it has. So that was quite the major breakthrough and still holds true today. Primary sensory cortex is where a sensory modality comes to perception. Just beyond the primary sensory cortex is the secondary sensory cortex, [LAUGH]. And yeah that sounds obvious, but what happens is. So if primary sensory cortex were vision, let's just use vision, because it's easy, is right here, primary visual. It's not enough to just perceive that you can see something, you have to process it and figure out what it is. And that's what happens as you go out further and further away from that primary sensory cortex. You have secondary, tertiary, and then what you get into are these big areas out here in the edge of the parietal lobe and the edge of the temporal lobe that are called association cortex. So what happens out here in the association cortex is information, let me erase and redraw, information from visual system, information from hearing, information from somatosensory cortex all comes together in the association cortex. And this is where if it walks like a duck and it talks like a duck, it's probably a duck. That's where you put it together. And you learn this over time. You learn what a dog feels like, you learn what a cat feels like. You learn all the sensory information that you learn. It's coming in, going through the primary, up through these levels of cortex, and then you put it together and learn, and create an image of your external environment. Humans have a ton of association cortex. Other mammals, the non-human primates have quite a bit. But things like other animals, like rats, mice, cats, not so much. So one of the things that makes mammals special is six layered neocortex. And what makes humans and non-human primates special is the large amount of association cortex that they have. In the last part of this lecture, in the very last segment, we'll talk about some of the things that lie deep to our neocortex, some of that deep gray matter that's part of the brain.
