Brain Structures
Spear pointed out that the basic structures of the brain are relatively ancient from an evolutionary perspective. Thus, virtually all mammalian species share not only these structures, but also the timing of the structural changes that occur in the brain as the individual matures. Researchers have found, for example, a decrease of up to 50 percent in the number of synaptic connections among neurons in different regions of the brain during adolescence. In general, researchers think that an overproduction of synapses occurs early in life, which is then followed by gradual pruning. The pruning that occurs during adolescence is thought to be more selective than earlier pruning—based on a “use it or lose it” principle— and contributes to the fine-tuning of brain connections necessary for adult cognition. It is also possible that this stage of pruning provides an important opportunity for the brain to be molded by the individual’s environment.
Researchers have also documented an increase in the death of neurons and their support cells, which is likely to be associated with a decrease in gray matter and an increase in white matter.1 The white matter is important because it helps quickly connect distant regions of the brain and therefore also supports the emergence of adult-type thinking. This selective pruning of connections among neurons is accompanied by a decline in the brain’s need for energy. Spear noted that in general the brain is the “most expensive organ in the body, in terms of energy requirements.” The lower demand that comes with a reduced number of synapses and a larger proportion of white matter (which is more efficient than gray matter) is more comparable to an adult brain.
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The changes that take place in the adolescent brain are specific to particular regions—those that are most important for modulating behavioral responses to reward and affective behavior. Control over these behaviors is likely to influence risk-taking. The prefrontal cortex, which undergoes significant change during adolescence, is the site of executive control functions that start emerging early in life and continue to develop into adulthood. Spear described these cognitive controls as top-down systems that are critical in allowing the individual to exert control over a range of responses. They help modulate sensitivity to different kinds of rewards, identify the significance of stimuli, and exert control over impulses and emotional and social responses—the bottom-up brain systems.
Casey also highlighted the significance of the fact that development occurs at different rates in different parts of the brain. The development of the prefrontal cortex is gradual and is not complete until well into adulthood. This aspect of brain function has been a focus for many researchers but by itself does not completely explain the behavior patterns adolescents exhibit. The relationship between the prefrontal cortex and the limbic system—the area that supports emotion and many behavioral tendencies, as well as long-term memory—has received increasing attention. The limbic system develops on a steeper curve than the prefrontal cortex, as shown in Figure 3-1, so that the disparity between these two regions is greatest during adolescence. The result can be an imbalance that may favor behaviors driven by emotion and response to incentives over rational decision making. It is this imbalance—not just the protracted development of cognitive control alone—that contributes to the prevalence of risk-taking in adolescents (Casey et al., 2008).
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