The Questions of Developmental Biology

the ipsilateral eye. The situation becomes even more complicated as neurons from the lateral
geniculate nuclei connect with the neurons of the visual cortex. Over 80 percent of the neural
cells in the cortex receive input from both eyes. The result is binocular vision and depth
perception.
Another remarkable finding is that the retinocortical projection pattern is the same for
both eyes. If a cortical neuron is stimulated by light flashing across a region of the left eye 5
degrees above and 1 degree to the left of the fovea, it will also be stimulated by a light flashing
across a region of the right eye 5 degrees above and 1 degree to the left of the fovea. Moreover,
the response evoked in the cortical cell when both eyes are stimulated is greater than the response
when either retina is stimulated alone.
Hubel, Wiesel, and their co-workers (see Hubel 1967) demonstrated that the development
of the nervous system depends to some degree on the experience of the individual during a
critical period of development. In other words, not all neuronal development is encoded in the
genome; some is the result of learning. Experience appears to strengthen or stabilize some
neuronal connections that are already present at birth and to weaken or eliminate others.
These conclusions come from studies of partial sensory deprivation. Hubel and Wiesel (1962,
1963) sewed shut the right eyelids of newborn kittens and left them closed for 3 months. After
this time, they unsewed the lids of the right eye. The cortical cells of such kittens could not be
stimulated by shining light into the right eye. Almost all the inputs into the visual cortex came
from the left eye only. The behavior of the kittens revealed the inadequacy of their right eyes:
When only the left eyes of these animals were covered, the kittens became functionally blind.