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
position 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 millimeters 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 distinctions, 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 preservation 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 completely 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 geniculate forms a small part). And each of these visual areas receives input from a
Radioactive amino acid injected into the left eye has been transported through the lateral geniculate body to layer 4C, where it occupies a series of half-millimeter-wide patches; these glow brightly in this darkfield picture. (The continuous leaflet in the middle is white matter, containing the geniculo-cortical fibers.)
