1999; Sowell, Peterson, et al., 2003). These are higher cortical areas that contribute to attentional processes and the regulation of thought and behavior. The decline in cortical gray matter may represent a synaptic pruning in adolescence and young adulthood that could produce more efficient processing in the neural pathways that support improvements in these cognitive processes, which constitute a vitally important feature of adolescent development.

The brain is subject to continual change even after its fundamental architecture and functional circuitry have been established, as evidenced by the capacity to learn new skills and establish new memories throughout life. Changes in brain structure in response to experience, learning, various physiological processes, and pharmacological or environmental agents are known as neural plasticity. Although the molecular mechanisms underlying neural plasticity are not fully understood, experience is known to induce anatomical changes across all levels of the nervous system, from molecular and cellular processes to entire neural pathways.

Such changes in brain structure begin with changes in the architecture of the synapse. Experience in the short term produces transient changes in the strength of communication across synaptic connections primarily by changing the availability of neurotransmitters and other signaling molecules. Experience in the longer term produces changes in synaptic activity, which can influence signaling pathways to regulate the function of receptors and other proteins or to change the number of receptors at the synapse. In addition, ongoing synaptic activity induces changes in gene expression that alter the production of proteins either to build up new synapses or to break down existing ones (Purves, Augustine, et al., 2000). The molecular pathways that alter gene expression and modify synaptic architecture have been studied most extensively in brain regions that subserve learning and memory, especially the hippocampus and the cerebellum. Whether and how these molecular pathways produce changes in the strength of synapses that encode other complex behaviors are not yet known.

In addition to these neuroplastic changes at the level of individual synapses, the brain is plastic at the level of cortical organization. Studies in monkeys have demonstrated that when a digit is amputated, the amount of tissue in the brain that controls movement and sensation changes over a period of weeks, so that the areas representing the remaining digits, which continue to receive sensory input, expand to take over the regions previously occupied by the missing digit (Merzenich, Nelson, et al., 1984; Purves, Augustine, et al., 2000). Similarly, if a monkey is trained to use a digit disproportionately to accomplish a task, the representation of that digit in the motor cortex expands to take over areas previously mapped to neighboring digits (Jenkins, Merzenich, et al., 1990; Purves, Augustine,

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