This result showed clearly 
that one function of the corpus 
callosum is to connect cells so 
that their fields can span the 
midline. It therefore cements 
together the two halves of the 
visual world. To imagine this 
more vividly, suppose that our 
cortex had originally been 
constructed out of one piece 
instead of being subdivided into 
two hemispheres; area 17 would 
then be one large plate, 
mapping the entire visual field. 
Neighboring cells would of 
course be richly 
interconnected, so as to 
produce the various response 
properties, including 
movement responses and 
orientation selectivity. Now 
suppose a dictator (the deity, 
evolution, or whatever) decides 
this will no longer do: half the 
cells must henceforth go to one 
hemisphere and half to the 
other. What to do about all those 
rich connections between the 
cells that must now be pulled 
apart? The connections, we 
suppose, are simply dragged 
across and form part of the 
corpus callosum. To avoid 
delays in having the signals 
travel so great a distance (in 
humans, perhaps 5 to 6 inches) 
we speed conduction by putting 
a myelin coating on the axons. 
Of course, in evolution nothing 
like this ever really happened, 
since the brain had two 
hemispheres long before the 
cerebral cortex evolved.

    This experiment of 
Berlucchi and Rizzolatti 
provides the most vivid example 
I know of the remarkable 
specificity of neural 
connections. The cell 
illustrated to the left, and 
presumably a million other 
callosally connected cells like 
it, derives a single orientation 
selectivity both through local 
connections to nearby cells and 
through connections coming 
from a region of cortex in the 
other hemisphere, several 
inches away, from cells with 
the same orientation 
selectivity and immediately 
adjacent receptive-field 
positions--to say nothing of all 
the other matching attributes, 
such as direction selectivity, 
end-stopping, and degree of 
complexity. Every callosally 
connected cell in the visual 
cortex must get its input from 
cells in the opposite 
hemisphere with exactly 
matching properties. We have 
all kinds of evidence for such 
selective connectivity in the 
nervous system, but I can think 
of none that is so beautifully 
direct.