It was obvious soon after the 
first recordings from monkeys 
that every time the electrode 
entered the cortex 
perpendicular to the surface, 
cell after cell favored the same 
eye, as shown in the 
illustration to the left. 

Ocular dominance remains 
constant in vertical 
microelectrode penetrations 
through the striate cortex. 
Penetrations parallel to the 
surface show alternation from 
left eye to right eye and back, 
roughly one cycle every 
millimeter.

If the electrode was pulled out 
and reinserted at a new site a 
few millimeters away, one eye 
would again dominate, perhaps 
the same eye and perhaps the 
other one. In layer 4C, which 
receives the input from the 
geniculates, the dominant eye 
seemed to have not merely an 
advantage, but a monopoly. In 
the layers above and below, and 
hence farther along in the 
succession of synapses, over 
half of the cells could also be 
influenced from the 
nondominant eye--we call 
these cells binocular.

    If instead of placing the 
electrode perpendicular to the 
surface, we introduced it 
obliquely, as close to parallel to 
the surface as could be 
managed, the eye dominance 
alternated back and forth, now 
one eye dominating and now 
the other. A complete cycle, 
from one eye to the other and 
back, occurred roughly once 
every millimeter. Obviously, 
the cortex seen from above 
must consist of some kind of 
mosaic composed of left-eye and 
right-eye regions.