This view of a human brain 
seen from the left and slightly 
behind shows the cerebral 
cortex and cerebellum. A small 
part of the brainstem can be 
seen just in front of the 
cerebellum.

    The brain is different. Even 
today large parts of it are 
question marks, not only in 
terms of how they work but also 
in terms of their biological 
purpose. A huge, rich subject, 
neuroanatomy consists largely 
of a sort of geography of 
structures, whose functions are 
still a partial or complete 
mystery. Our ignorance of these 
regions is of course graded. For 
example, we know a fair 
amount about the region of 
brain called the motor cortex 
and have a rough idea of its 
function: it subserves 
voluntary movement; destroy it 
on one side and the hand and 
face and leg on the opposite side 
become clumsy and weak. Our 
knowledge of the motor cortex 
lies midway along a continuum 
of relative knowledge that 
ranges all the way from utter 
ignorance of the functions of 
some brain structures to 
incisive understanding of a 
few--like the understanding we 
have of the functions of a 
computer, printing press, 
internal combustion engine, or 
anything else we invented 
ourselves.

    The visual pathway, in 
particular the primary visual 
cortex, or striate cortex, lies 
near the bone or heart end of 
this continuum. The visual 
cortex is perhaps the best-
understood part of the brain 
today and is certainly the best-
known part of the cerebral 
cortex. We know reasonably 
well what it is "for", which is 
to say that we know what its 
nerve cells are doing most of 
the time in a person's everyday 
life and roughly what it 
contributes to the analysis of 
the visual information. This 
state of knowledge is quite 
recent, and I can well 
remember, in the 1950s, 
looking at a microscopic slide of 
visual cortex, showing the 
millions of cells packed like 
eggs in a crate, and wondering 
what they all could conceivably 
be doing, and whether one 
would ever be able to find out.

    How should we set about 
finding out? Our first thought 
might be that a detailed 
understanding of the 
connections, from the eye to 
the brain and within the brain, 
should be enough to allow us to 
deduce how it works. 
Unfortunately, that is only true 
to a limited extent. The regions 
of cortex at the back of the 
human brain were long known 
to be important for vision partly 
because around the turn of the 
century the eyes were 
discovered to make 
connections, through an 
intermediate way station, to 
this part of the brain. But to 
deduce from the structure alone 
what the cells in the visual 
cortex are doing when an 
animal or person looks at the 
sky or a tree would require a 
knowledge of anatomy far 
exceeding what we possess even 
now. And we would have trouble 
even if we did have a complete 
circuit diagram, just as we 
would if we tried to understand 
a computer or radar set from 
their circuit diagrams alone--
especially if we did not know 
what the computer or radar set 
was for.