In the monkey cortex, the 
cells that receive the input 
from the geniculates, those 
whose fields have circular 
symmetry, are also like 
geniculate cells in being 
monocular. We find about an 
equal number of left-eye and 
right-eye cells, at least in parts 
of the cortex subserving vision 
up to about 20 degrees from the 
direction of gaze. Beyond this 
center-surround stage, 
however, we find binocular 
cells, simple and complex. In 
the macaque monkey over half 
of these higher-order cells can 
be influenced independently 
from the two eyes.

    Once we have found a 
binocular cell we can compare 
in detail the receptive fields in 
the two eyes. We first cover the 
right eye and map the cell's 
receptive field in the left eye, 
noting its exact position on the 
screen or retina and its 
complexity, orientation, and 
arrangement of excitatory and 
inhibitory regions; we ask if 
the cell is simple or complex, 
and we look for end stopping 
and directional selectivity. Now 
we block off the left eye and 
uncover the right, repeating all 
the questions. In most 
binocular cells, we find that all 
the properties found in the left 
eye hold also for the right-eye 
stimulation--the same position 
in the visual field, the same 
directional selectivity, and so 
on. So we can say that the 
connections or circuits 
between the left eye and the 
cell we are studying are present 
as a duplicate copy between the 
right eye and that cell.