The visual pathway, from eyes 
to primary visual cortex, of a 
human brain, as seen from 
below. Information comes to 
the two purple-colored halves 
of the retinas (the right halves, 
because the brain is seen 
upside down) from the opposite 
half of the environment (the 
left visual field) and ends up in 
the right (purple) half of the 
brain. This happens because 
about half the optic-nerve 
fibers cross at the chiasm, and 
the rest stay uncrossed. Hence 
the rules: each hemisphere gets 
input from both eyes; a given 
hemisphere gets information 
from the opposite half of the 
visual world.

It was also clear that these 
connections, from the eyes to 
the lateral geniculates and 
from the geniculates to the 
cortex, are topographically 
organized. By topographic 
representation, we mean that 
the mapping of each structure 
to the next is systematic: as you 
move along the retina from one 
point to another, the 
corresponding points in the 
lateral geniculate body or 
cortex trace a continuous path. 
For example, the optic nerve 
fibers from a given small part of 
the retina all go to a particular 
small part of the lateral 
geniculate, and fibers from a 
given region of the geniculate 
all go to a particular region of 
the primary visual cortex. Such 
an organization is not 
surprising if we recall the 
caricature of the nervous 
system shown earlier in 
Chapter 2, in which cells are 
grouped in platelike arrays, 
with the plates stacked so that a 
cell at any particular stage gets 
its input from an aggregate of 
cells in the immediately 
preceding stage.