development thus far come from a small number of postmortem studies and a larger number of in vivo, or live, brain imaging studies. The scientific value of postmortem studies is limited by the quality and number of tissue samples that are usually available and by the capability to study only a small number of brain regions (Lewis, 2002). In contrast, in vivo imaging has proved to be an important tool for studying postnatal brain development in humans across the life span (Marsh, Gerber, and Peterson, 2008), although thus far it has provided information about brain structure and function mainly at a macroscopic level of brain organization, revealing little molecular or cellular information (Peterson, 2003b).

Understanding of the molecular and cellular development of the human brain is therefore gleaned largely from studies of animal models, extrapolated to the maturational timeline of humans. Although a great deal has been learned from those animal models across a wide range of species, how well those findings relate to the development and function of the human brain is not fully known. Moreover, as noted earlier, the molecular bases of the highest-order functions of the human brain cannot be studied easily in animals.

Despite limited data from human and nonhuman primates, the consistency in findings across species suggests that the general features of brain development in animal models are likely to apply to humans as well. Those findings indicate that the wiring of neural architecture is neither fixed nor static. Instead, it is a dynamic entity that is shaped and reshaped continually throughout development by processes that have their own maturational timetables within and across brain regions. These processes are described briefly here and summarized in Figure 5-2.

Overview: Complexities of Brain Development

At the visible anatomical level, the human brain develops during gestation into a complex structure having distinct anatomical regions and a highly convoluted surface. Similarly, at the level of cellular architecture, the human brain is a highly complex, layered structure made up of many distinct kinds of cells that have highly specific interconnections. During fetal brain development, undifferentiated precursor cells need to divide and multiply. The resulting cells must then differentiate into the correct cell types, migrate to the correct place in the brain, and connect properly with other cells. These links among cells must then be organized into functional circuits that support sensation, perception, cognition, emotion, learning, and behavior. In a healthy intrauterine environment, this series of complex and interrelated neurodevelopmental events is initially under the predetermined control of regulatory genes (Rhinn, Picker, and Brand, 2006). In contrast, much of the fine detail of brain organization—how the brain is “wired”—develops

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