Amblyopia is a partial or 
complete loss of eyesight that is 
not caused by abnormalities in 
the eye. When we sewed closed 
a cat's or monkey's eye, our aim 
was to produce an amblyopia 
and then to try to learn where 
the abnormality had arisen in 
the visual path. The results of 
the kitten experiment amazed 
us. All too often, an experiment 
gives wishy-washy results, too 
good to dismiss completely but 
too indecisive to let us 
conclude anything useful. This 
experiment was an exception; 
the results were clear and 
dramatic. When we opened the 
lids of the kitten's eye, the eye 
itself seemed perfectly normal: 
the pupil even contracted 
normally when we shined a 
light into it. Recordings from 
the cortex, however, were 
anything but normal. Although 
we found many cells with 
perfectly normal responses to 
oriented lines and movement, 
we also found that instead of 
about half of the cells 
preferring one eye and half 
preferring the other, none of 
the twenty-five cells we 
recorded could be influenced 
from the eye that had been 
closed. (Five of the cells could 
not be influenced from either 
eye, something that we see 
rarely if ever in normal cats.) 
Compare this with a normal cat, 
in which about 15 percent of 
cells are monocular, with about 
7 percent responding to the left 
eye and 7 percent to the right. 
The ocular-dominance 
histograms for the cat, shown 
in the graph to the left, allowed 
us to see the difference at a 
glance. Clearly something had 
gone wrong, with a vengeance.

A kitten was visually deprived 
after having its right eye closed 
at about ten days, the time at 
which the eyes normally open. 
The duration of closure was 
two-and-a-half months. In this 
experiment we recorded from 
only twenty-five cells. (In 
subsequent experiments we 
were able to record more cells, 
and we found a small 
percentage that were 
influenced from the eye that 
had been closed.)