for that species. Although such a model for the emergence of the human brain does not lend itself to replication in the laboratory, it can indeed be tested—for example, by comparing it with evidence from paleoanthropological fossils or by computer simulations. Within the span of an individual lifetime, too, the novel connections of an area x can have implications for the unique circuitry of one person 's brain. For example, the cortex of someone who is congenitally blind might include a less-developed area 17 and, perhaps within it or nearby, a hybrid area with novel connections and the possibility of novel functioning.

In neuroscience, study of the formation and development of the human brain holds a special place; many lines of investigation converge here. New methods in molecular biology may now make it possible to uncover specialized genes, for example, that may control cell production in the ventricular zone or regulate the deployment of cell adhesion or cell recognition molecules along migratory pathways. Another set of genes under investigation may initiate the synthesis of neurotransmitters, receptors, and second messengers, and fix the timing for their emergence. Scientists are working, too, on the genes responsible for programmed cell death.

The study of brain development poses its own constraints and appears at times to offer only the most tentative conclusions because of the large number of variables that may all be operating at once. Nevertheless, it also offers a unique vantage point from which to observe the interaction of these same variables—from nutrition all the way down to genetic coding— in their countless possible combinations. At the same time, through its multistage model, this field of inquiry seeks to explain well-known disorders of the brain or nervous system in terms of disturbances at particular points in development. Developmental neuroscience thus forms connections with all its neighboring areas and beyond, much like the system it is observing. No wonder that some of the researchers who specialize in this area think that learning about how the human brain takes shape is “the ultimate study of mankind. ”

ACKNOWLEDGMENT

Chapter 6 is based on presentations by Pasko Rakic.



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