Visual fibers such as these 
make up only a small proportion 
of callosal fibers. In the 
somatosensory system, 
anatomical axon-transport 
studies, similar to the 
radioactive-amino-acid eye 
injections described in earlier 
chapters, show that the corpus 
callosum similarly connects 
areas of cortex that are 
activated by skin or joint 
receptors near the midline of 
the body, on the trunk, back, or 
face, but does not connect 
regions concerned with the 
extremities, the feet and 
hands.

    Every cortical area is 
connected to several or many 
other cortical areas on the 
same side. For example, the 
primary visual cortex is 
connected to area 18 (visual 
area 2), to the medial temporal 
area (MT), to visual area 4, and 
to one or two others. Often a 
given area also projects to 
several areas in the opposite 
hemisphere through the 
callosum or, in some few cases, 
by the anterior commissure. We 
can therefore view these 
commissural connections 
simply as one special kind of 
cortico-cortico connection. A 
moment's thought tells us these 
links must exist: if I tell you 
that my left hand is cold or that 
I see something to my left, I am 
using my cortical speech area, 
which is located in several 
small regions in my left 
hemisphere, to formulate the 
words. (This may not be true, 
because I am left handed.) But 
the information concerning my 
left field of vision or left hand 
feeds into my right hemisphere: 
it must therefore cross over to 
the speech area if I am going to 
talk about it. The crossing takes 
place in the corpus callosum. In 
a series of studies beginning in 
the early 1960s, Roger Sperry, 
now at Cal Tech, and his 
colleagues showed that a 
human whose corpus callosum 
had been cut (to treat epilepsy) 
could no longer talk about 
events that had entered 
through the right hemisphere. 
These subjects provided a mine 
of new information on various 
kinds of cortical function, 
including thought and 
consciousness. The original 
papers, which appeared in the 
journal Brain, make 
fascinating reading and should 
be fully understandable to 
anyone reading the present 
text.