each of them, the visual cortex breaks up into three subregions, called areas
17, 18, and 19 of the cortex. These areas are still universal, in the sense that
they can be located in the brain of any normal individual in any of the three
species. Within each area you can go still further, reaching the "columnar"
organization of the visual cortex. Perpendicular to the surface of the
cortex, moving radially inwards towards the inner brain, visual neurons are
arranged in "columns"-that is, almost all connections move along the
radial, columnar direction, and not between columns. And each column
maps onto a small, specific retinal region. The number of columns is not
the same in each individual, so that one can't find "the same column".
Finally, within a column, there are layers in which simple neurons tend to
be found, and other layers in which complex neurons tend to be found.
(The hypercomplex neurons tend to be found in areas 18 and 19 predom-
inantly, while the simple and complex ones are found mostly in area 17.)
It appears that we run out of isomorphisms at this level of detail. From here
on down to the individual neuron level, each individual cat, monkey, or
human has a completely unique pattern-somewhat like a fingerprint or a
signature.
One minor but perhaps telling difference between visual processing in
cats' brains and monkeys' brains has 10 do with the stage at which informa-
tion from the two eyes is integrated to yield a single combined higher-level
signal. It turns out that it takes place slightly later in the monkey than in the
cat, which gives each separate eye's signal a slightly longer time to get
processed by itself. This is not too surprising, since one would expect that
the higher a species lies in the intelligence hierarchy, the more complex will
be the problems which its visual system will be called upon to handle; and
therefore signals ought to pass through more and more early processing
before receiving a final "label". This is quite dramatically confirmed by
observations of the visual abilities of a newborn calf, which seems to be born
with as much power of visual discrimination as it will ever have. It will shy
away from people or dogs, but not from other cattle. Probably its entire
visual system is "hard-wired" before birth, and involves relatively little
cortical processing. On the other hand, a human's visual system, so deeply
reliant on the cortex, takes several years to reach maturity.
Funneling into Neural ModulesA puzzling thing about the discoveries so far made about the organization
of the brain is that few direct correspondences have been found between
large-scale hardware and high-level software. The visual cortex, for in-
stance, is a large-scale piece of hardware, which is entirely dedicated to a
clear software purpose-the processing of visual information-yet all of
the processing so far discovered is still quite low-level. Nothing approach-
ing recognition of objects has been localized in the visual cortex. This means
that no one knows where or how the output from complex and hyper-
complex cells gets transformed into conscious recognition of shapes,(^346) Brains and Thoughts