SUMMARY

8-1 Three Perspectives on Brain Development

Nervous system development entails more than the unfolding of a genetic blueprint. Development is a complex dance of genetic and environmental events that interact to sculpt the brain to fit within a particular cultural and environmental context. We can approach this dance from three perspectives: (1) correlating emerging brain structures with emerging behaviors, (2) correlating new behaviors with neural maturation, and (3) identifying influences on brain and behavior.

8-2 Neurobiology of Development

Human brain maturation is a long process, lasting as late as age 30. Neurons, the units of brain function, develop a phenotype, migrate, and, as their processes elaborate, establish connections with other neurons even before birth. The developing brain produces many more neurons and connections than it needs and then prunes back in toddlerhood and again in adolescence and early adulthood to a stable level maintained by some neurogenesis throughout the life-span. Experiences throughout development can trigger epigenetic mechanisms, such as gene methylation, that alter gene expression.

8-3 Using Emerging Behaviors to Infer Neural Maturation

Throughout the world, across the cultural spectrum, from newborn to adult, we all develop through similar behavioral stages. As infants develop physically, motor behaviors emerge in a predictable sequence from gross, poorly directed movements toward objects to controlled pincer grasps to pick up objects as small as pencils by about 11 months. Cognitive behaviors also develop through stages of logic and problem solving. Beginning with Jean Piaget, researchers have identified and characterized four or more distinct stages of cognitive development. Each stage can be identified by specific behavioral tests.

Behaviors emerge as the neural systems that produce them develop. Matching the median timetables of neurodevelopment with observed behavior infers the hierarchical relation between brain structure and brain function. Motor behaviors emerge in synchrony with maturating motor circuits in the cerebral cortex, basal ganglia, and cerebellum, as well as in the connections from these areas to the spinal cord. Similar correlations between emerging behaviors and neuronal development accompany the maturation of cognitive behavior as neural circuits in the frontal and temporal lobes mature in early adulthood.

8-4 Brain Development and the Environment

The brain is most plastic during its development, and neuronal structures and their connections can be molded by various factors throughout development. The brain’s sensitivity to factors such as external events, quality of environment, tactile stimulation, drugs, gonadal hormones, stress, and injury varies over time. At critical periods in the course of development, beginning prenatally, different brain regions are particularly sensitive to different events.

Brain perturbations in the course of development from, say, anoxia, trauma, or toxins can alter brain development significantly; can result in severe behavioral abnormalities, including intellectual disability; and may be related to such disorders as ASD or SIDS. Other behavioral disorders emerge in adolescence, a time of prolonged frontal lobe change.

8-5 How Do Any of Us Develop a Normal Brain?

The brain has a substantial capacity to repair or correct minor abnormalities, allowing most people to develop normal behavioral repertoires and to maintain brain function throughout life.