Jetha / Segalowitz Adolescent Brain Development

Implications for Behavior
1. Auflage 2012
ISBN: 978-0-12-397917-9
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 100 Seiten

This comprehensive yet brief overview of the adolescent human brain discusses how the brain develops during this critical period of life and how that development impacts decision-making and risk-taking behavior in the adolescent. - This originated as a white paper requested by the Canadian government for a specific group looking to understand adolescent brain development in the context of adolescent behaviour - The paper was not made available to the Canadian government outside of the specific task force that requested it nor to the general public

Michelle Jetha is a research associate of the Brock University Centre for Lifespan Development Research, works in the Cognitive and Affective Neuroscience Lab, and teaches at McMaster and Brock Universities. She completed her PhD in 2007, examining electrophysiological responses to social and affective stimuli in individuals with schizophrenia or autism. She has published extensively on these topics, a major review of adolescent EEG/ERP development, and most recently on how shyness influences early brain responses to emotional face stimuli. Jetha completed postdoctoral work at Pennsylvania State University which focused on the identification of trait factors that predispose children to disruptive behavioral disorders. She is currently conducting research with adolescents in collaboration with a Niagara regional mental health agency. Her focus is translational neuroscience involving developmental psychopathology.

Autoren/Hrsg.

Jetha, Michelle K.

Segalowitz, Sidney

Leseproben

Chapter 2
Connectivity
Advances in imaging technology have led to new conceptualizations regarding the development of the brain. We now realize that brain maturation does not occur in stages mapped onto the development of specific regions, but rather it is dependent on the emergence of large-scale “networks.” Networks consist of brain regions that activate or work together through their interconnections. The gray and white matter changes discussed in Chapter 1, such as synaptic pruning and myelination, play important roles in their development. It is the development and formation of networks that provides the physiological basis for the sophisticated processing that occurs as the brain matures. Such changes serve to reorganize, rebalance, and fine-tune the “neural circuitry” of the brain in ways that lead to more efficient and controlled information processing and more complex cognitive functioning. With new technologies, neuroscientists are increasingly able to map the maturation of these networks. 2.1 Changes in Networks Over Childhood, Adolescence, and Young Adulthood
Over childhood, adolescence, and young adulthood, many of the connections between brain regions show dramatic changes.93,152,258,270 These changes lead to processing that is more specialized and efficient. Two main findings have consistently been reported when participants are at rest and/or when they are performing specific tasks: 1. Cortical connections change from being more nonspecific and spread out to being more focused. This is illustrated in Figure 2.1, which shows brain activity during a task that is designed to engage prefrontal areas. As can be seen, the activation of the prefrontal areas in childhood (7–12 years) covers a larger area (i.e., is more spread out) than in adolescents and adults. In childhood, brain activation during cognitive processing is relatively nonspecific, producing coactivation of regions that are anatomically close together. As the networks mature, this activation becomes more focused and specialized.92,93 Such changes in cortical activation over development coincide with the maturation of gray matter. Although the exact relation is not clear, there is general consensus in the literature that synaptic pruning plays a role in these developmental changes.93,170 Fig. 2.1 The cortical connections over childhood (8–12 years) through adolescence (13–17 years), and young adulthood (19–24 years) change from being more diffuse to more focused. Source: See Ref. 152, reprinted with permission from Oxford University Press 2. With development, we also see dramatic increases in long-range connections between brain regions. With the increase of long-range connections that is dependent on changes in white matter growth and maturation, processing becomes more integrative, involving multiple brain regions (see Figure 2.2). More efficient solutions to processing demands develop. These more efficient network solutions are utilized more frequently and thus increase in connection strength, whereas less efficient connections that are used infrequently decrease in connection strength. Fig. 2.2 From childhood (7–9 years) to adulthood (21–31 years), we see dramatic increases in long-range connections between regions. Source: Reprinted with permission from Fair et al., 2008. Copyright (2008) National Academy of Sciences, USA As networks develop, the workload of cognitive processing becomes more widely distributed, spanning multiple brain regions. A benefit to distributing the workload over multiple regions is that the more mature individual has more specialized functional resources to draw upon when performing a task, likely resulting in new and different strategies for problem solving. In addition, the demands on any specific region reduces to some degree. These “freed up” resources can then be allotted to other processing requirements, allowing for greater flexibility in meeting additional environmental demands. For example, as circuitry matures, tasks designed to engage prefrontal activity would require less effort from the prefrontal cortex freeing up its resources for other activities. Over adolescence and into young adulthood, maturational changes are most dramatic for prefrontal networks, which allow for more efficient “top-down” executive control over more “bottom-up” reactive processing.43,91,140 Our more refined understanding of network maturation has implications for understanding adolescent behavior. When the frontal networks are not fully mature, there is a greater chance adolescents will need to maximize prefrontal resources during tasks that engage prefrontal activity (i.e., organizing, strategizing, planning, or regulating emotion). Additional stressors in the environment, whether unexpected or ongoing (family, social, or illness-related), can lead to impaired behavior and choices for individuals who are already working at maximum prefrontal capacity. We must also keep in mind that a sleep-deprived adolescent starts the day with a reduction in prefrontal “load” capacity, and thus the likelihood for “overload” is increased over baseline. Box 2.1 Can We Measure Brain Age? One particular network is of special interest. This network—usually called the Default Mode Network or sometimes the Resting State Network—links central brain regions that consistently monitor our internal mental state when we are not dealing with externally presented challenges.214 It is now becoming apparent that this picture of brain activity—keeping in mind that the brain is never inactive, even in sleep—may hold clues to major developmental brain anomalies, including those in psychopathology (autism, schizophrenia, attention deficit hyperactivity disorder (ADHD), depression, and so on). The development of the Default Mode Network has now been clearly documented as slow and long, stretching from childhood to adulthood.92 Given the dramatic developmental changes in connectivity during the resting state, there is some hope that these measures, which require the person to lie still for only a few minutes, may eventually yield a measure of “brain maturity.” There is increasing discussion concerning using this type of measure for forensic issues in the United States.250,251,254 A recent prominent paper illustrated how well this might be eventually applied.80 The key to properly linking connectivity to brain age is to differentiate between those circuits that increase and those that decrease in strength over time. When only an overall measure of connectivity is taken into account, the overlap from 7 to 30 years is great, despite the obvious general increase. However, when the complexity of the connectivity is taken into account, with some circuits increasing and some decreasing with age, a fairly useful formula separating those under versus those over 18 years of age emerges, with accuracy at over 90% in three large cohorts.80 This formula makes use of six main connecting systems, but it is interesting that the strongest factor involves networks within the prefrontal cortex. Although much more normative data collection will be needed, especially across various demographic groups, before the data will be accepted as forensic evidence, the differences are apparently great enough to serve as a general indicator of maturation in the same way computerized axial tomography (CAT) scans have been used to indicate normal aging or psychopathology. 2.2 How are Changes in Connectivity Related to Development in the Cognitive Domain?
The mental processes involved in cognitive control (i.e., the ability to execute, guide, and monitor desired behaviors) steadily improve throughout childhood, adolescence, and into young adulthood.35,45,71,171,221 A review of brain development over this period shows that core networks necessary for cognitive control are present in childhood, but the ability to consistently and flexibly recruit these networks increases with age (particularly for the networks connecting frontal regions with parietal and with the striatal regions of the basal ganglia).169,170 The increased efficiency and specialization of frontal lobe areas is evident across a variety of tasks and research methods. The shifting of the balance towards more frontal lobe control (through its increased connectivity to other brain areas) is considered to be the biological basis for the maturation of behavior. For example, the development of effective self-regulation—the ability to delay gratification of immediate reward in order to follow the rules, make choices, and maintain goals—depends heavily on the development of the networks involved in cognitive control. When we examine changes in the cognitive domain with age, the implications for the healthy development of frontal networks supporting these functions becomes clear. For example, an important ability that facilitates problem solving, decision making, and the voluntary control of behavior is the capacity to keep relevant information available and “online,” a capacity referred to as working memory.10 Working memory involves the ability to maintain, attend to, update, and evaluate information. Although the ability to use working memory is in place in childhood, the more voluntary, flexible, and consistent use of working memory increases over adolescence...