Eye, Brain, and Vision
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In three-dimensional view, the oculardominance columns are seen to be, not Greek pillars, but slabs perpendicular to the surface, like slices of bread.

Luckily, neuroanatomical methods have been invented in breathtaking succession in the past decade, and by now the problem has been solved independently in about half a dozen ways. Here I will illustrate two.
The first method depends again on axon transport. A small amount of an organic chemical, perhaps an amino acid, is labeled with a radioactive element such as carbon-14 and injected into one eye of a monkey, say the left eye. The amino acid is taken up by the cells in the eye, including the retinal ganglion cells. The ganglion-cell axons transport the labeled molecule, presumably now incorporated into proteins, to their terminals in the lateral geniculate bodies.
There the label accumulates in the left-eye layers. The process of transportation takes a few days. The tissue is then thinly sliced, coated with a photographic silver emulsion, and allowed to sit for some time in the dark. In the resulting autoradiograph, shown on the next page, we can see the three lefteye layers on each side, complementary in their order, revealed by black silver grains.
To see this geniculate pattern requires only modest amounts of radioactivity in the injection. If we inject a sufficiently large amount of the labeled amino acid into the eye, the concentration in geniculate layers becomes so high that some radioactive material leaks out of the optic-nerve terminals and is taken up by the geniculate cells in the labeled layers and shipped along their axons to the striate cortex. The label thus accumulates in the layer-4C terminals in regular patches corresponding to the injected eye. When the autoradiograph is finally developed (after several months because the concentration of label finally reaching the cortex is very small), we can actually see the patches in layer 4C in a transverse section of the cortex, as shown in the photograph on the next page. If we slice the cortex parallel to its surface—either flattening it first or cutting and pasting serial sections—we can at last see the layout, as though we were viewing it from above. It is a beautiful set of parallel stripes, as shown on page in in a single section (top) and a reconstruction (bottom). In all these cortical autoradiographs, the label representing the left eye shows up bright, separated by dark, unlabeled regions representing the right eye. Because layer 4 feeds the layers above and below mainly by up-and-down connections, the regions of eye preference in three dimensions are a series of alternating left- and right-eye slabs, like slices of bread, as shown in the top diagram on page 27.
Using a different method, Simon LeVay succeeded in reconstructing the entire striate cortex in an occipital lobe; the part of this exposed on the surface is shown in the bottom illustration on page 27. The stripes of the pattern are most regular and striking some distance away from the foveal representation.
For reasons unknown, the pattern is rather complex near the fovea, with very regular periodicity but many loops and swirls, hardly the regular wallpaperlike stripes seen farther out. The width of the stripes is everywhere constant at about 0.5 millimeter. The amount of cortex devoted to left and right eyes is nearly exactly equal in the cortex representing the fovea and out to about 20 degrees in all directions. LeVay and David Van Essen have found that owing to the declining contribution of the eye on the same side, the ipsilateral bands shrink to 0.25 millimeter out beyond 20 degrees from the fovea. Beyond 70 or 80 degrees, of course, only the contralateral eye is represented. This makes sense, because with your eyes facing the front, you can see with your right eye farther to the right than to the left.
















 

 

 

 

A second method for demonstrating the columns reveals the slabs in their full thickness, not just the part in layer 4. This is the 2-deoxyglucose method, invented by Louis Sokoloff at the National Institutes of Health, Bethesda, in 1976. It too depends ultimately on the ability of radioactive substances to darken photographic film. The method is based on the fact that nerve cells, like most cells in the body, consume glucose as fuel, and the harder they are made to work, the more glucose they eat. Accordingly, we might imagine injecting radioactive glucose into an animal, stimulating one eye, say the right, with patterns for some minutes—long enough for the glucose to be taken up by the active cells in the brain—and then removing the brain and slicing it, coating the slices with silver emulsion, and exposing and developing, as before. This idea didn't work because glucose is consumed by the cells and converted to energy and degradation products, which quickly leak back out into the blood stream. To sidestep the leakage, Sokoloffs ingenious trick was to use the substance deoxyglucose, which is close enough chemically to glucose to fool the cells into taking it up: they even begin metabolizing it. The process of breakdown goes only one step along the usual chemical degradation path, coming to a halt after the deoxyglucose is converted to a substance (2-deoxyglucose-6-phosphate) that can be degraded no further. Luckily, this substance is fat insoluble and can't leak out of the cell; so it accumulates to levels at which it can be detected in autoradiographs. What we finally see on the film is a picture of the brain regions that became most active during the stimulation period and took up most of this fake food. Had the animal been moving its arm during that time, the motor arm area in the cortex would also have lit up. In the case of stimulating the right eye, what we see are the parts of the cortex most strongly activated by that stimulus, namely, the set of right


   
 
Seen here in LeVay's reconstruction are the ocular-dominance columns in the part of area 17 open to the surface, right hemisphere. Foveal representation is to the right. (Compare right side of photograph on page 23.)
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Top: A single section through the domeshaped cortex is made parallel to the surface. It cuts through layer 4 in a ring. Bottom: A reconstruction of many such rings from a series of sections—the deeper the section, the bigger the ring—made by cutting out the rings and superimposing them.
(Traces of the rings can be seen because it was difficult to get all the sections exposed and photographed equally, especially as I am strictly an amateur photographer.)


In this autoradiograph through the striate cortex, the white segments are the labeled patches in layer 4 representing the injected left eye; these patches are separated by unlabeled (dark) right-eye regions.
 
 
 
 
 
These sections through the left and right lateral geniculate bodies show autoradiographic label in the three left-eye layers on each side. The left eye had been injected with radioactive label (tritiated proline) a week earlier. The labeled layers are the dark ones.

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Here are three different ways that a surface can be partitioned off into two kinds of regions: the possible patterns are a checkerboard, stripes, and islands in an ocean. In this case, the surface is the cortex, and the regions are left-eye and right-eye.