EVIDENCE FROM ANIMALS 
DEPRIVED OF NORMAL VISION    The 
next best kind of data, it might 
be thought, would come from 
animals reared in the dark and 
tested when they achieve 
maturity. Such experiments 
have been performed, but, as 
we have seen, rearing in the 
dark prevents the normal 
maturation of the visual 
nervous system.

    Even animals reared under 
conditions in which they are 
exposed to homogeneous but 
unpatterned light during part 
or all of each day do not mature 
normally. This finding grows 
out of research following up 
David Hubel’s and Torsten 
Wiesel’s discoveries in the 
1960s of detector mechanisms 
in the brain that were most 
responsive to contours 
stimulating a given region of 
the retina in one particular 
orientation. These detector 
mechanisms were assumed to 
be present at birth, but later 
research revealed that the 
nature of the detector 
mechanisms found depended on 
the animal’s earliest 
environment. For example, if 
only vertical contours are 
present in the early 
environment, then only 
detectors for that orientation of 
a retinal contour can later be 
found. If few or no contours are 
present in the early 
environment, then few 
detectors for any contour 
orientation can later be found.