Despite this constancy 
process, however, information 
about depth from retinal 
disparity surely becomes 
increasingly less important at 
greater distances, unless the 
distance between objects is 
very great, as, between trees 
several hundred yards apart, 
for example. Stereopsis is 
hardly indispensable because, 
as has already been noted, the 
impression of the distance of 
things from us and the depth 
between them remains when 
we close one eye. (Many a 
person blind in one eye has 
excellent depth perception.)

    Why does binocular 
disparity lead to depth 
perception? Horace Barlow, 
Colin Blakemore, and John 
Pettigrew recently discovered 
neurons in the brain that 
discharge rapidly when a 
contour stimulates a certain 
magnitude of disparity between 
corresponding retinal regions. 
Such a neural mechanism may 
explain how the perceptual 
system "knows" that disparity 
exists between the two retinal 
images, signaling that there is 
more or less of such disparity. 
But it does not tell us how the 
perceptual system interprets 
the disparity thus signaled. As 
will be seen, such an 
interpretation is subject to 
learning. Moreover, the 
disparity-detector mechanism 
does not tell us how the 
perceptual system decides 
which points of stimulation in 
noncorresponding regions of 
the retina match with one 
another--that is, which derive 
from the same contour in the 
outer world.