Eye, Brain, and Vision

THE ARCHITECTURE
OF THE VISUAL CORTEX

The primary visual, or striate, cortex is a far more complex and elaborate
structure than either the lateral geniculate body or the retina. We have already
seen that the sudden increase in structural complexity is accompanied by a
dramatic increase in physiological complexity. In the cortex we find a greater
variety of physiologically defined cell types, and the cells respond to more
elaborate stimuli, especially to a greater number of stimulus parameters that
have to be properly specified. We are concerned not only with stimulus posi-
tion and spot size, as we are in the retina and geniculate, but now suddenly
with line orientation, eye dominance, movement direction, line length, and
curvature. What if anything is the relation between these variables and the
structural organization of the cortex? To address this question, I will need to
begin by saying something about the structure of the striate cortex.

ANATOMY OF THE VISUAL CORTEX
The cerebral cortex, which almost entirely covers the cerebral
hemispheres, has the general form of a plate whose thickness is about 2 milli-
meters and whose surface area in humans is over i square foot. The total area
of the macaque monkey's cortex is much less, probably about one-tenth that
of the human. We have known for over a century that this plate is subdivided
into a patchwork of many different cortical areas; of these, the primary visual
cortex was the first to be distinguished from the rest by its layered or striped
appearance in cross section—hence its classical name, striate cortex. At one
time the entire careers of neuroanatomists consisted of separating off large
numbers of cortical areas on the basis of sometimes subtle histological distinc-
tions, and in one popular numbering system the striate cortex was assigned the

number 17. According to one of the more recent estimates by David Van
Essen of Caltech, the macaque monkey primary visual cortex occupies 1200
square millimeters—a little less than one-third the area of a credit card. This
represents about 15 percent of the total cortical area in the macaque, certainly a
substantial fraction of the entire cortex.
A rear view of the brain of a macaque monkey is seen in the photograph on
the next page. The skull has been removed and the brain perfused for preserva-
tion with a dilute solution of formaldehyde, which colors it yellow. Blood
vessels normally form a conspicuous web over the surface, but here they are
collapsed and not visible. What we see in this rear view is mostly the surface of
the occipital lobe of the cortex, the area that is concerned with vision and that
comprises not only the striate cortex but also one or two dozen or more
prestriate areas. To get a half-millimeter-thick plate of nervous tissue that is the
area of a large index card into a box the size of the monkey's skull necessitates
some folding and crinkling, the way you crinkle up a piece of paper before
throwing it into the waste basket; this produces fissures, or sulci, between
which are ridges, or gyri.
The area behind (below, in this photograph) the dotted line is the exposed
part of the striate cortex. Although the striate cortex occupies most of the
surface of the occipital lobe, we can see only about one-third to one-half of it
in the photograph; the rest is hidden out of sight in a fissure.
The striate cortex (area 17) sends much of its output to the next cortical
region, visual area 2, also called area 18 because it is next door to area 17. Area
18 forms a band of cortex about 6 to 8 millimeters wide, which almost com-
pletely surrounds area 17. We can just see part of area 18 in the photograph,
above the dotted line, the boundary between 17 and 18, but most of it extends
down into the deep sulcusjust in front of that line. Area 17 projects to area 18
in a plate-to-plate, orderly fashion. Area 18 in turn projects to at least three
postage-stamp-size occipital regions, called MT (for medial temporal), visual
area 3, and visual area 4 (often abbreviated V3 and V4). And so it goes, with
each area projecting forward to several other areas. In addition, each of these
areas projects back to the area or areas from which it receives input. As if that
were not complicated enough, each of the areas projects to structures deep in
the brain, for example to the superior colliculus and to various subdivisions of
the thalamus (a complex golfball-size mass of cells, of which the lateral genic-
ulate forms a small part). And each of these visual areas receives input from a