Studying the developing brains of newborn ferrets, researchers at Duke University Medical Center have made significant contributions to the debate over nature versus nurture.
Post-doctoral fellow Justin Crowley and Howard Hughes Medical Institute Investigator Lawrence Katz concentrated on the ferrets' visual systems to determine how much neural wiring is inborn, and how much is affected by later experience. "[The] visual is a model for the contributions of nature versus nurture," Katz said.
Rather than developing a visual system only as it grows, Crowley and Katz found that a newborn ferret already has visual wiring present as a foundation for its future adult system. That visual foundation can then be refined and modified by experience and environment.
The scientists found that a ferret's visual system is programmed with a very good idea of what to expect, even before the retina can detect light. This pre-programming predicts normal experience, like use of both eyes, so that even ferrets receiving visual information from only one eye showed the same visual cortex development as those receiving it from both eyes.
"We find that the setup of the [visual] system is hard-wired," explained Crowley, and first develops with no reference to the environment.
A ferret that has a normal visual experience will keep its programmed structure, but any abnormal experience immediately after birth will modify the system. This evidence greatly reduces the degree that environment was previously thought to affect the brain's wiring. "The brain is not a blank slate," said Katz, "[It has a] pre-existing network of neural connections."
Ferrets were used because their neural wiring is very similar to that of other mammals in the fetal-stage. Katz and Crowley used an innovative surgical technique to study ocular dominance columns-features in the visual cortex used to detect light from one eye or the other; existence of the columns indicates that the brain has set up a set of connections.
Once thought to be created only in reaction to visual experience, the "ocular dominance columns not only appear much earlier than previously thought, but they emerge at a markedly different stage of cortical development," state Crowley and Katz's in the Nov. 17 Science.
Although there are no immediate plans to extend the research to more complex mammals or other areas of the brain, Katz and Crowley stress that work in the field is just beginning. The next big step, said Katz, is to look for the molecules setting up the connections. Developments like the Human Genome Project, and increased funding and interest in genetics and neurobiology are creating opportunities to do so.
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