The second cell (to the left) 
fires only when the object is 
farther away than that 
distance. 

For this cell, a "far" cell, 
objects closer than the screen 
evoke little or no response; at 
about zero disparity (screen 
distance), a small shift of the 
screen has a very large 
influence on the effectiveness 
of the stimulus. The response 
rises sharply to a plateau for 
distances farther away than 
where the animal is looking. 
Beyond a certain point the two 
receptive fields no longer 
overlap; in effect, the eyes are 
being stimulated separately. 
Response then falls to zero.

Still other cells respond only 
when the stimulus is nearer. As 
we vary disparity, these two 
cell types, called near cells and 
far cells, both show very rapid 
changes in responsiveness at or 
near zero disparity. All three 
kinds of cells, called disparity-
tuned cells, have been seen in 
area 17 of monkeys. It is not yet 
clear just how common they are 
or whether they occur in any 
special layers or in any special 
relation to ocular-dominance 
columns. Such cells care very 
much about the distance of the 
object from the animal, which 
translates into the relative 
positions of the stimulus in the 
two eyes. Another 
characteristic feature of these 
cells is that they fail to respond 
to either eye alone, or give only 
weak responses. All these cells 
have the common 
characteristic of orientation 
specificity; in fact, as far as we 
know, they are like any 
ordinary upper-layer complex 
cell, except for their additional 
fussiness about depth. They 
respond very well to moving 
stimuli and are sometimes end 
stopped.

    Gian Poggio at Johns Hopkins 
Medical School has recorded 
such cells in area 17 of alert 
implanted monkeys trained to 
keep their eyes fixed on a 
target. In anesthetized 
monkeys, such cells, although 
certainly present, seem to be 
rare in area 17 but are very 
common in area 18. I would be 
surprised if an animal or 
human could assess and 
compare the distances of 
objects in a scene 
stereoscopically if the only 
cells involved were the three 
types--tuned excitatory, near, 
and far--that I have just 
described. I would have guessed 
that we would find a whole 
array of cells for all possible 
depths. In alert monkeys, 
Poggio has also seen tuned 
excitatory cells with peak 
responses not at zero but 
slightly away from zero, and it 
thus seems that the cortex may 
contain cells with all degrees of 
disparity. Although we still do 
not know how the brain 
reconstructs a scene full of 
objects at various distances 
(whatever "reconstructs" 
means), cells such as these 
seem to represent an early stage 
in the process.