The Questions of Developmental Biology

Because the lateral geniculate neurons appeared to be stimulated from both right and left
eyes in these kittens, the physiological defect appeared to be in the connections between the
lateral geniculate nuclei and the visual cortex. In rhesus monkeys, where similar phenomena are
observed, the defect has been correlated with a lack of protein synthesis in the lateral geniculate
neurons innervated by the covered eye (Kennedy et al. 1981).
Although it would be tempting to conclude that the blindness resulting from these
experiments was due to a failure to form the proper visual connections, this is not the case.
Rather, when a kitten or monkey is born, axons from lateral geniculate neurons receiving input
from each eye overlap extensively in the visual cortex (Hubel and Wiesel 1963; Crair et al. 1998).
However, when one eye is covered early in the animal's life, its connections in the visual cortex
are taken over by those of the other eye (Figure 21.19). Competition occurs, and experience plays
a role in strengthening and stabilizing the connections from each lateral geniculate nucleus to the
visual cortex. Thus, when both eyes of a kitten are sewn shut for 3 months, most cortical cells can
still be stimulated by appropriate illumination of one eye or the other.
The critical time in kitten development for this
validation of neuronal connections begins between the
fourth and the sixth week after birth. Monocular
deprivation up to the fourth week produces little or no
physiological deficit, but after 6 weeks, it produces all
the characteristic neuronal changes. If a kitten has had
normal visual experience for the first 3 months, any
subsequent monocular deprivation (even for a year or
more) has no effect. At that point, the synapses have
been stabilized.
Two principles, then, can be seen in the patterning of the mammalian visual system.
First, the neuronal connections involved in vision are present even before the animal sees.
Second, experience plays an important role in determining whether or not certain connections
remain. Just as experience refines the original neuromuscular connections, so experience plays a
role in refining and improving the visual connections. It is possible, too, that adult functions such
as learning and memory arise from the establishment and/or strengthening of different synapses
by experience. As Purves and Lichtman (1985) remark:
The interaction of individual animals and their world continues to shape the nervous system
throughout life in ways that could never have been programmed. Modification of the nervous
system by experience is thus the last and most subtle developmental strategy.