Eye, Brain, and Vision

Luckily, neuroanatomical methods have been invented in breathtaking suc-
cession in the past decade, and by now the problem has been solved indepen-
dently 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 transporta-
tion takes a few days. The tissue is then thinly sliced, coated with a photo-
graphic 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 left-
eye 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 wallpaper-
like 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 develop-
ing, 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