Eye, Brain, and Vision

THE PRIMARY VISUAL CORTEX
After Kuffler's first paper on center-surround retinal ganglion cells was pub-
lished in 1952, the next steps were clear. To account for the properties of the
cells, more work was needed at the retinal level. But we also needed to record
from the next stages in the visual pathway, to find out how the brain inter-
preted the information from the eyes. Both projects faced formidable difficul-
ties. In the case of the brain, some years were required to develop the tech-
niques necessary to record from a single cell and observe its activity for many
hours. It was even harder to learn how to influence that activity by visual
stimulation.

TOPOGRAPHIC REPRESENTATION
Even before further research became possible, we were not com-
pletely ignorant about the parts of the brain involved in vision: the geography
of the preliminary stages was already well mapped out (see the illustration on
the next page). We knew that the optic-nerve fibers make synapses with cells
in the lateral geniculate body and that the axons of lateral geniculate cells
terminate in the primary visual cortex. It was also clear that these connections,
from the eyes to the lateral geniculates and from the geniculates to the cortex,
are topographically organized. By topographic representation, we mean that the mapping of each structure to the next is systematic: as you move along the
retina from one point to another, the corresponding points in the lateral genic-
ulate body or cortex trace a continuous path. For example, the optic nerve
fibers from a given small part of the retina all go to a particular small part of
the lateral geniculate, and fibers from a given region of the gcniculate all go to
a particular region of the primary visual cortex. Such an organization is not
surprising if we recall the caricature of the nervous system shown in the figure
on page 6, in which cells are grouped in platelike arrays, with the plates
stacked so that a cell at any particular stage gets its input from an aggregate of
cells in the immediately preceding stage.

In the retina, the successive stages are in apposition, like playing cards
stacked one on top of the other, so that the fibers can take a very direct route
from one stage to the next. In the lateral geniculate body, the cells are obvi-
ously separated from the retina, just as, equally obviously, the cortex is in a
different place from the geniculate. The style of connectivity nevertheless re-
mains the same, with one region projecting to the next as though the succes-
sive plates were still superimposed.
The optic-nerve fibers simply gather into a bundle as they leave the eye, and
when they reach the geniculate, they fan out and end in a topographically
orderly way. (Oddly, between the retina and geniculate, in the optic nerve,
they become almost completely scrambled, but they sort out again as they
reach the geniculate.) Fibers leaving the geniculate similarly fan out into a
broad band that extends back through the interior of the brain and ends in an
equally orderly way in the primary visual cortex. After several synapses, when
fibers leave the primary visual cortex and project to several other cortical re-
gions, the topographic order is again preserved. Because convergence occurs
at every stage, receptive fields tend to become larger: the farther along the path
we go, the more fuzzy this representation-by-mapping of the outside world
becomes.
An important, long-recognized piece of evidence that the pathway is topo-
graphically organized comes from clinical observation. If you damage a certain
part of your primary visual cortex, you develop a local blindness, as though
you had destroyed the corresponding part of your retina.
The visual world is thus systematically mapped onto the geniculate and
cortex. What was not at all clear in the 1950s was what the mapping might
mean. In those days it was not obvious that the brain operates on the informa-
tion it receives, transforming it in such a way as to make it more useful. People
had the feeling that the visual scene had made it to the brain; now the problem
for the brain was to make sense of it—or perhaps it was not the brain's prob-
lem, but the mind's. The message of the next chapters will be that a structure
such as the primary visual cortex does exert profound transformations on the
information it receives. We still know very little about what goes on beyond
this stage, and in that sense you might argue that we are not much better off.
But knowing that one part of the cortex works in a rational, easily understood
way gives grounds for optimism that other areas will too. Some day we may
not need the word mind at all.