ment in children who suffered focal brain damage in the first months of life, revealed the extensive capacity for recovery of language functioning in these children (Bates and Roe, in press). Finally, among children whose medical conditions have required that their brains be studied, positron emission tomography has revealed metabolic patterns consonant with synaptic growth and pruning occurring in early development (Chugani and Phelps, 1986). (See Appendix B, as well as Nelson and Bloom, 1997, for a fuller discussion of technologies for studying the developing human brain.)

WHAT DEVELOPS IN EARLY BRAIN DEVELOPMENT?

The development of the brain has a long trajectory, beginning within a few days after conception and continuing through adolescence and beyond. The nervous system undergoes its most dramatic development during the first few years of life. Yet the processes that establish the structure and functioning of the brain, made possible by the developing networks of synapses that interconnect nerve cells and by the progressive fine-tuning of the neurons for the roles they will play within their synaptic networks, continue well into adolescence. The milestones of brain development from the prenatal period until school entry involve the development and migration of brain cells to where they belong in the brain, embellishments of nerve cells through the sprouting of new axons or by expanding the dendritic surface; the formation of connections, or synapses, between nerve cells; and the postnatal addition of other types of cells, notably glia. Fascination with the earliest stages of brain development is understandable. During this period, the spinal cord is formed, nearly all of the billions of neurons of the mature brain are produced, the dual processes of neural differentiation and cell migration establish the neuron's functional roles, and synaptogenesis proceeds apace. These processes represent an elaborate interplay between gene activity and the surrounding environments both inside and outside the child.

There have been significant changes over time in the aspects of brain development that have captured public attention. Twenty years ago, people were fascinated by the ability to measure developmental changes in the degree to which neurons in different areas of the brain become wrapped in the white, fatty matter—myelin—that insulates nerve cells and affects the speed with which nerve impulses are transmitted from one cell to another. Myelination is, in fact, affected by the young child's behavioral experiences and nutrition, as discussed below. Today, the public is more focused on information, not all of it new, about the rate of synapse development, particularly on studies showing that there is a tremendous burst of synapse formation early in life, followed by a decline in synapse number, apparently extending into adolescence in some areas of the brain. Combined with



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