Compared with this cell, the 
cells whose responses are 
shown in the illustration here 
and next are very fussy about 
the relative positions of the two 
stimuli and therefore about 
depth. The first cell (to the left) 
fires best if the stimuli to the 
two eyes fall on exactly 
corresponding parts of the two 
retinas. The amount of 
horizontal malpositioning, or 
disparity, that can be tolerated 
before the response disappears 
is a fraction of the width of the 
receptive field. It therefore 
fires if and only if the object is 
roughly as far away as the 
distance on which the eyes are 
fixed. 

For this "tuned excitatory" 
cell, it makes a lot of difference 
whether the stimulus is at the 
distance the animal is looking, 
or is nearer or farther away. 
The cell fires only if the slit is 
roughly at the distance the 
animal is looking. In these 
experiments, the direction of 
gaze of one eye is varied 
horizontally with a prism, but 
bodily moving the screen 
nearer or farther away would 
amount to the same thing.