thalamic subdivision: just as the geniculate projects to the primary visual cortex, so other parts project to the other areas.
In the same photograph, X indicates the part of area 17 that receives information from the foveas, or centers of gaze, of the two retinas. As we move from X, in the left hemisphere, toward the arrowhead, the corresponding point in the right half of the visual field starts in the center of gaze and moves out, to the right, along the horizon. Starting again from X, movement to the right along the border between areas 17 and 18 corresponds to movement down in the visual field; movement back corresponds to movement up. The arrowhead marks a region about 6 degrees out along the horizon. The visual field farther out than 9 degrees is represented on the part of area 17 that is folded underneath the surface and parallel to it.
To see what the cortex looks like in cross section, we have cut a chunk from the visual cortex on the right side of the photograph on page 23. The resulting cross section, as in the photomicrograph on this page, is stained with cresyi violet, a dye that colors the cell bodies dark blue but does not stain axons or dendrites. With the photomicrograph taken at this low power, we cannot distinguish individual cells, but we can see dark layers of densely aggregated cells and lighter layers of more thinly scattered ones. Beneath the exposed part of the cortex, we see a mushroom-shaped, buried part that is folded under in a complicated way, but these two parts are actually continuous. The lightly stained substance is white matter; it lies under the part of the cortex that is exposed to the surface, separating it from the buried fold of cortex, and consists mainly of myelinated nerve fibers, which do not stain. The cortex, containing nerve-cell bodies, axons, dendrites, and synapses, is an example of gray matter.
For anatomical richness, in its complexity of layering, area 17 exceeds every other part of the cortex. You can see an indication of this complexity even in this low-magnification cross section when you compare area 17 with its nextdoor neighbor, area 18, bordering area 17 at d. What is more, as we look along the cross section from the region marked a, which is a few degrees from the foveal projection to the cortex, toward the region marked b, 6 degrees out, or toward c, 80 to 90 degrees out, we see very little change in the thickness or the layering pattern. This uniformity turns out to be important, and I will return to it in Chapter 6.
LAYERS OF THE VISUAL CORTEX
A small length of area 17 appears at higher magnification in the photomicrograph on this page. We can now make out the individual cell bodies as dots and get some idea of their size, numbers, and spacing. The layering pattern here is partly the result of variations in the staining and packing density of these cells. Layers 4C and 6 are densest and darkest; layers 1, 4B, and 5 are most loosely packed. Layer i contains hardly any nerve cells but has abundant axons, dendrites, and synapses. To show that different layers contain different kinds of cells requires a stain like that devised by Camillo Golgi in 1900. The Golgi stain reveals only occasional cells, but when it does reveal a cell, it may show it completely, including its axons and dendrites. The two major classes of cortical cells are the pyramidal cells, which occur in all layers except i and 4, and the stellate cells, which are found in all layers. You have seen an example of a pyramidal cell and a stellate cell in Chapter 1(page 3). We can get a better idea of the distribution of pyramidal cells witnin the cortex in another drawing from Ramon y Cajal's Histologie (on this page), which shows perhaps i percent of pyramds instead of only one or two cells.
The fibers coming to the cortex from the lateral geniculate body enter from the white matter. Running diagonally, most make their way up to layer 4C, branching again and again, and finally terminate by making synapses with the stellate cells that populate that layer. Axons originating from the two ventral (magnocellular) geniculate layers end in the upper half of 4C, called 4C alpha; those from the four dorsal (parvocellular) geniculate layers end in the lower half of 4C (4C Bata). As you can see from the diagram on this page, these subdivisions of layer 4C have different projections to the upper layers: 4C alpha sends its output to 4.B; 4Q Bata, to the deepest part of 3. And those layers in turn differ in their projections. Seeing these differences in the pathways stemming from the two sets of geniculate layers is one of many reasons to think that they represent two different systems. Most pyramidal cells in layers 2, 3, 4A, 5, and 6 send axons out of the cortex, but side-branches, called "collaterals", of these same descending axons connect locally and help to distribute the information through the full cortical thickness.
The layers of the cortex differ not only in their inputs and their local interconnections but also in the more distant structures to which they project. All layers except i, 4A, and 4C send fibers out of the cortex. Layers 2 and 3 and layer 4B project mainly to other cortical regions, whereas the deep layers project down to subcortical structures: layer 5 projects to the superior colliculus in the midbrain, and layer 6 projects mainly back to the lateral geniculate body. Although we have known for almost a century that the inputs from the geniculate go mostly to layer 4, we did not know the differences in outputs of the different cortical layers until 1969, when Japanese scientist Keisuke Toyama first discovered them with physiological techniques; they have been confirmed anatomically many times since.
Ramon y Cajal was the first to realize how short the connections within the cortex are. As already described, the richest connections run up and down, intimately linking the different layers. Diagonal and side-to-side connections generally run for 1or 2 millimeters, although a few travel up to 4 or 5 millimeters. This limitation in lateral spread of information has profound consequences. If the inputs are topographically organized—in the case of the visual system, organized according to retinal or visual-field position—the same must be true for the outputs. Whatever the cortex is doing, the analysis must be local. Information concerning some small part of the visual world comes in to a small piece of the cortex, is transformed, analyzed, digested—whatever expression you find appropriate—and is sent on for further processing somewhere else, without reference to what goes on next door. The visual scene is thus analyzed piecemeal. The primary visual cortex cannot therefore be the part of the brain where whole objects—boats, hats, faces—are recognized, perceived, or otherwise handled; it cannot be where "perception" resides. Of course, such a sweeping conclusion would hardly be warranted from anatomy alone. It could be that information is transmitted along the cortex for long distances in bucket-brigade fashion, spreading laterally in steps of i millimeter or so. We can show chat this is not the case by recording while stimulating the retina: all the cells in a given small locality have small receptive fields, and any cell and its neighbor always have their receptive fields in very nearly the same place in the retina. Nothing in the physiology suggests that any cell in the monkey primary visual cortex talks to any other cell more than 2 or 3 millimeters away.
For centuries, similar hints had come from clinical neurology. A small stroke, tumor, or injury to part of the primary visual cortex can lead to blindness in a small, precisely demarcated island in the visual field; we find perfectly normal vision elsewhere, instead of the overall mild reduction in vision that we might expect if each cell communicated in some measure with all other cells. To digress slightly, we can note here that such a stroke patient may be unaware of anything wrong, especially if the defect is not in the foveal representation of the cortex and hence in the center of gaze—at least he will not perceive in his visual field an island of blackness or greyness or indeed anything at all. Even if the injury has destroyed one entire occipital lobe, leaving the subject blind in the entire half visual field on the other side, the result is not any active sensation of the world being blotted out on that side. My occasional migraine attacks (luckily without the headache) produce transient blindness, often in a large part of one visual field; if asked what I see there, I can only say, literally, nothing—not white, grey, or black, but just what I see directly behind—nothing.
Another curious feature of an island of localized blindness, or scotoma, is known as "completion". When someone with a scotoma looks at a line that passes through his blind region, he sees no interruption: the line is perfectly continuous. You can demonstrate the same thing using your own eye and blind spot, which you can find with no more apparatus than a cotton Q-tip.
The blind spot is the region where the optic nerve enters the eye, an oval about 2 millimeters in diameter, with no rods and cones. The procedure for mapping
The main connections made by axons from the lateral geniculate body to the striate cortex and from the striate cortex to other brain regions. To the right, the shading indicates the relative density of Nissl staining, for comparison with the illustration on page 24.
A cross section of the striate cortex taken at higher magnification shows cells arranged in layers. Layers 2 and 3 are indistinguishable; layer 4A is very thin. The thick, light layer at the bottom is white matter.
This cross section through the occipital lobe was made by cutting out a piece as shown in the photograph on page 23. It is what we would see if we were to walk into the groove and look to the left. The letter a corresponds to a point halfway between X and the arrowhead. The Nissi stain shows cell bodies only; these are too small to make out except as dots. The darker part of the top and the mushroomshaped part just below are striate cortex.
The three letter d's mark the border between areas 17 and 18.