Human development and behavior unfold through a complex and highly interactive process in which both biological regulation and experiential influences are substantial. Although most attention in the field of early childhood intervention has been focused on the role of experience, the contribution of the biological substrate requires equal scrutiny. To achieve a balanced understanding of the transaction between nature and nurture, it is essential that we examine the normal development of the central nervous system (CNS) and explore the effects of specific abnormalities and injuries on its functioning.

Extensive research conducted over the past few decades has resulted in a dramatic increase in our knowledge about the normal development of the brain (Sarnat, 1996; Nelson, this volume). This research shows that the biology of neuromaturation is controlled by genetic mechanisms whose timing is regulated precisely and whose unfolding is sensitive to a variety of environmental influences. As our understanding of the mechanisms involved in the evolution of brain structure and function has become more sophisticated, nowhere has this been more striking than in the identification of genes that program cellular growth, differentiation, and maturation. Indeed, the elucidation of cellular and intracellular signal mechanisms has not only resulted in greater knowledge of overall brain development but it also has reflected a breakthrough in our understanding of operational mechanisms of brain function. Significant progress in the fields of neuropsychology, neuroanatomy, neurophysiology, and neural-imaging has also added remarkably to the base of our knowledge (Nelson & Bloom, 1997).