Childhood is the
time for learning. Many cognitive and motor skills are gained quickly during
childhood and are not mastered as easily if the learning process begins
later. Developmental neurobiologists are keenly interested in this “window”
of opportunity. The early development of the brain-the birth and differentiation
of neurons, their migration to proper brain regions, the growth of their
axons to roughly appropriate targets, and the generation of synapses between
interacting neurons-relies primarily on intrinsic factors within the CNS
and is largely independent of environmental events. However, environmental
factors such as drugs, alcohol, and viral illnesses are able to disrupt
normal brain development during critical periods of CNS development.
Environmental factors become critically
important during later stages of brain maturation. In humans and other mammals,
the number of synapses increases dramatically after birth. The specificity
of neuronal connections is then |
refined during early
postnatal life. Experimental data have shown conclusively that neuronal
activity is critical for the elaboration of synaptic territories, as well
as for making proper synaptic connections. Thus, once the initial circuitry
of the CNS is guided by intrinsic factors into roughly correct patterns,
after birth environmentally derived activity takes over to refine connections
between neurons.
In the 1960s and 1970s, Torsten Wiesel
and David Hubel conducted an influencial series of experiments on this topic,
for which they received the Nobel Prize in 1981. Their work demonstrated
that the organization of the adult visual cortex relies heavily on early
visual experiences. The primary visual cortex receives input from the two
eyes via a relay in the thalamic visual area (the dorsal lateral geniculate
nucleus). Like all cortices, the primary visual cortex is a layered structure,
with visual input forming synapses on neurons in layer 4.
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| Figure 1.
The development of ocular dominance columns in the cat. At 2 weeks, there
is a continuous band of synaptic connections in layer IV that represent
the input to the visual cortex from geniculocortical afferents. At 6 weeks,
fluctuations in the intensity are already apparent. By 13 weeks, the pattern
of cortical striping is similar to that seen in the adult. The segregation
of synaptic inputs within the cortical layer depends on synaptic activity
from postnatal visual experiences. Adapted from Ocular dominance columns
and their development in layer IV of the cat’s visual cortex: a quantitative
study, LeVay S, Stryker MP, Shatz CJ, Journal of Comparative Neurology,
179:223-244, 1978; copyright © 1978, Wiley-Liss, Inc. Reprinted by
permission of Wiley-Liss, Inc., a subsidiary of John Wiley & Sons, Inc.
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In
the adult, the input from the right and left eyes is separated into alternating
bands within layer 4, which Hubel and Wiesel called ocular dominance columns.
Interestingly, early in development, the right- and left-eye inputs in layer
4 are not segregated, but overlap extensively. The development of ocular
dominance columns in the cat over the first 3 months of life is shown in
Figure 1.
Hubel and Wiesel demonstrated that the
normal segregation of inputs that is present later in life requires visual
activity during a circumscribed window of time in the postnatal period.
For example, if an animal is raised with visual input only from the right
eye, the right-eye inputs to the visual cortex will occupy most of the territory
in layer 4. The left-eye inputs will occupy very little territory. Importantly,
this ability to reorganize the pattern of inputs is temporally limited.
Restricting vision to only one eye in adult animals has little effect on
the organization of inputs to the primary visual cortex. Moreover, a return
to normal, binocular visual experience in the adult cannot repair the abnormal
organization of inputs to the visual cortex resulting from early visual
deprivation. The period during which abnormal visual activity can lead to
a disruption of the normal pattern of alternating right- and left-eye columns
is called the “critical period.”
Dramatic deficits are seen in the visual
cortex of humans and experimental animals if they do not experience normal
vision during early critical periods. A clinical example of this occurs
in children born with a congenital cataract. If the cataract is not diagnosed
and removed during the first years of life, the child will remain permanently
blind in that eye. Even if the cataract is removed in adulthood, normal
vision will never be established. This is in marked contrast to cataracts
that develop in adults. Even after many years, normal vision returns once
the cataract is removed.
Although the visual cortex is the best-studied
example of how early experiences have a critical effect on sculpting the
cortex, it is reasonable to speculate that similar mechanisms and temporal
restrictions influence the development of other regions of the brain. Recent
work on the acquisition of language has underscored
how profoundly early neuronal activity can influence the organization of
the brain. Functional magnetic resonance imaging scans have been used to
determine the spatial relationship of language centers in individuals who
have learned native as well as |
second languages.
If a child learns a second language early in life, both the native and second
language are represented in the same cortical region. In contrast, when
a second language is acquired in adulthood, a new language center that is
clearly separated from the native language center is established in the
cortex. Although these findings do not yet explain why young children are
able to learn a new language more easily than older individuals, they do
support the findings that early experiences affect the way the brain develops.
Additional Readings:
Hubel D (1988), Eye, Brain, and Vision. Scientific
American Library Series. New York: WH Freeman
Katz L, Shatz C (1996), Synaptic activity and the
construction of cortical circuits. Science 274:1133-1138
Kim K, Relkin H, Lee K, Hirsch J (1997), Distinct
cortical areas associated with native and second languages. Nature 388:171-174
Purves D (1994), Neural Activity and the Growth
of the Brain. Cambridge, England: Cambridge University Press
Shatz C (1990), Impulse activity and the patterning
of connections during CNS development. Neuron 5:745-756
Wiesel T (1982), Postnatal development of the visual
cortex and the influence of environment. Nature 299:583-591
Accepted March 13, 1998.
Dr. Hockfield is Professor, Neurobiology, and Dr. Lombroso is Associate
Professor, Child Study Center, Yale University School of Medicine, New
Haven, CT.
Correspondence to Dr. Lombroso, Child Study Center, Yale University School
of Medicine, 230 South Frontage Road, New Haven, CT 06520; e-mail: paul.lombroso@yale.edu
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