We need to make one 
qualification concerning this 
duplication of connections. If, 
having found the best stimulus-
-orientation, position, 
movement direction, and so on-
-we then compare the 
responses evoked from one eye 
with the responses evoked from 
the other, we find that the two 
responses are not necessarily 
equally vigorous. Some cells do 
respond equally to the two eyes, 
but others consistently give a 
more powerful discharge to one 
eye than to the other. Overall, 
except for the part of the cortex 
subserving parts of the visual 
field well away from the 
direction of gaze, we find no 
obvious favoritism: in a given 
hemisphere, just as many cells 
favor the eye on the opposite 
side (the contralateral eye) as 
the eye on the same side (the 
ipsilateral). All shades of 
relative eye dominance are 
represented, from cells 
monopolized by the left eye 
through cells equally affected 
to cells responding only to the 
right eye.

    We can now do a population 
study. We group all the cells we 
have studied, say 1000 of them, 
into seven arbitrary groups, 
according to the relative 
effectiveness of the two eyes; 
we then compare their 
numbers, as shown in the two 
bar graphs to the left. At a 
glance the histograms tell us 
how the distribution differs 
between cat and monkey: that 
in both species, binocular cells 
are common, with each eye 
well represented (roughly 
equally, in the monkey); that 
in cats, binocular cells are very 
abundant; that in macaques, 
monocular and binocular cells 
are about equally common, but 
that binocular cells often favor 
one eye strongly (groups 2 and 
5).