ANATOMY OF THE VISUAL CORTEX
A large part of the cerebral 
cortex on the right side has 
been exposed under local 
anesthesia for the 
neurosurgical treatment of 
seizures in this fully conscious 
human patient. The surgeon 
was Dr. William Feindel at the 
Montreal Neurological 
Institute. The scalp has been 
opened and retracted and a 
large piece of skull removed. (It 
is replaced at the end of the 
operation.) You can see gyri and 
sulci, and the large purplish 
veins and smaller, red, less 
conspicuous arteries. The 
overall pinkish appearance is 
caused by the finer branches of 
these vessels. Filling the 
bottom third of the exposure is 
the temporal lobe; above the 
prominent, horizontally 
running veins are the parietal 
lobe, to the left, and frontal 
lobe, to the right. At the 
extreme left we see part of the 
occipital lobe. This operation, 
for the treatment of a particular 
type of epilepsy, consists of 
removing diseased brain, which 
is only permissible if it does not 
result in impairment of 
voluntary movement or loss of 
speech. To avoid this, the 
neurosurgeon identifies 
speech, motor, and sensory 
areas by electrical stimulation, 
looking for movements, 
sensations related precisely to 
different parts of the body, or 
interference with speech. Such 
tests would obviously not be 
possible if the patient were not 
conscious. Points that have 
been stimulated have been 
labeled by the tiny numbered 
sterile patches of paper. For 
example, stimulation of these 
regions gave the following 
results: (1) tingling sensation 
in the left thumb; (2) tingling 
in the left ring finger; (3) 
tingling in the left middle and 
ring finger; (4) flexion of left 
fingers and wrist. The regions 
labeled 8 and 13 gave more 
complex memory-like 
sensations typically produced 
on stimulation of the temporal 
lobe in certain types of 
epileptic patients.

    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 1 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.