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March 31, 1998
Competing Interests
Well before a newborn baby opens its eyes for the first time, its brain
is already primed to see and understand the world. This priming occurs
as a result of the finely tuned wiring of neural networks within the
brain's visual system, say researchers at the Howard Hughes Medical
Institute at the University of California, Berkeley.
In the March 28 issue of the journal
Science,
a research team led by
Hughes investigator Carla Shatz proposes a model for brain development
that confirms the old adage, "use it or lose it."
Even before birth, the brain is abuzz with activity as it begins to wire
itself. Shatz's lab was among the first to demonstrate that early
spontaneous neural activity promotes the organization of billions of
nerve cells within animal and human brains. Their latest research shows
that if that spontaneous neural activity is blocked, correct neural
circuits in the visual system are not established and vision cannot
occur. "This shows that early brain activity generated spontaneously is
necessary to help organize detailed connections between the eyes and
brain," Shatz says. "This is a lifelong process. Experience helps
change connections in the brain."
The research has implications for understanding fetal development, says
coauthor Anna Penn, a neurobiologist at the University of California,
Berkeley. "There is growing recognition that the pattern of spontaneous
neural activity could affect fetal development in humans," says Penn.
"Some illicit drugs, and even nicotine, can interfere with necessary
synaptic transmission."
To help explain her team's findings, Shatz likens the prenatal wiring of
the visual system to the workings of a telephone system that connects
Boston (the brain), Manhattan (the left eye) and the Bronx (the right
eye). Since much of the developing brain grows in a predetermined way,
major nerve connections between parts of the brain are established
early. In other words, the "trunk line" between the New York area and
Boston develops as a matter of course, Shatz says.
Once the trunk line is established, detailed connections between the
brain and the eyes must be made specific buildings in Manhattan and the
Bronx must be wired for telephone service. Shatz discovered earlier that
this "wiring of individual buildings" occurs in the visual system when
spontaneous waves of activity sweep across the retinas. This is like
autodialing, Shatz says. The neighborhood in the eye places phone calls
to the brain. The spontaneous nerve activity is routed through the
thalamus, in an area called the lateral geniculate nucleus, and a
connection is made. "This allows millions of nerve fibers that go from each
eye to the brain to be organized into layers of incredibly precise
conductivity," says Shatz.
It is clear, Shatz says, that if the neural activity that organizes
connections is blocked, different sections of the brain never "wire up"
properly. Another important point of the research, she says, is that it
shows that the eyes compete with each other for neural connectivity. "If
Manhattan is blocked, the Bronx makes more phone calls and receives more
connections. This shows that the fetal brain is not just a miniature
version of an adult brain, but that it is a synaptic structure with
changing connections."
Neuroscientist
Larry Katz
, a Hughes investigator at Duke University
Medical Center, agrees: "This work shows definitively that an endogenous
pattern of spontaneous activity is a crucial organizing force for neural
circuitry in the developing nervous system," he said. "It's probably the
clearest demonstration yet of competitive interactions giving rise to
specific patterns of neural connections."
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