THE PHYSIOLOGY OF STEREOPSIS
If we want to know how 
brain cells subserve stereopsis, 
the simplest question we can 
ask is whether cells exist 
whose responses are 
exquisitely dependent on the 
relative horizontal positions of 
images falling on the retinas of 
the two eyes. We should begin 
by discussing how cells in the 
visual pathway respond when 
the two eyes are stimulated 
together. We are now talking 
about cells in area 17 or 
beyond, because retinal 
ganglion cells are obviously 
monocular, and geniculate 
cells, because of the left-eye, 
right-eye layering, are for all 
intents and purposes 
monocular: they respond to 
stimulation of either one eye or 
the other, but not both. In area 
17 roughly half the cells are 
binocular, responding to 
stimuli in the left eye and to 
stimuli in the right eye. When 
tested carefully, most of these 
binocular cells seem not to be 
greatly concerned with the 
relative positions of the stimuli 
in the two eyes. Consider a 
typical complex cell, which 
fires continuously if a slit 
sweeps across its receptive field 
in either eye. When both eyes 
are stimulated together, the 
cell fires at a higher rate than 
it does to separate eyes, but it 
generally does not matter much 
if at any instant the slits in the 
two retinas fall on exactly the 
same parts of the two receptive 
fields. The best responses occur 
if the slit enters and leaves 
both eyes' receptive fields at 
about the same time, but if it 
enters one a little before or 
after the other, it doesn't 
matter very much. A typical 
curve of response (say, total 
number of spikes per pass) 
versus difference in slit 
position in the two eyes is 
shown to the left. The curve is 
rather flat, clearly indicating 
that the relative position of the 
slit in the two eyes is not very 
important. This kind of cell 
will fire well to an 
appropriately oriented slit 
whether it is at the distance 
someone is looking, or is nearer 
or farther away.

When both eyes are stimulated 
together by a vertical slit of 
light moving leftward, an 
ordinary binocular cell in area 
17 will have similar responses 
to three different relative 
alignments of the two eyes. Zero 
disparity means that the two 
eyes are lined up as they would 
be if the monkey were looking 
at the screen onto which the 
stimuli are being projected. The 
exact alignment makes little 
difference in the response of 
the cell.