Neuroscientists are changing how we think about the adolescent brain
With the onset of puberty, the beginning of adolescence is marked by pronounced hormonal changes in the human body. During this developmental period, increasing levels of growth hormones, gonadal steroids and adrenal androgens trigger a host of noticeable physiological changes that have become to be identified with pubertal maturation, such as the typical teenage growth spurts and amplified sexual dimorphism
. Among the changes our bodies undergo during adolescence are structural and functional changes of the human brain, accompanied by the emotional, behavioral and motivational changes that seem to frame our transition into adulthood.
Across disciplines ranging from anthropology and economics to behavioral genetics and developmental neuroscience, researchers continue to probe deeply into the maturation process of the adolescent brain, and how it relates to changes in mood, cognition and social behaviors that are experienced and expressed by young adults. Many of these research efforts have lead to popular accounts of adolescent development that are built on the premise of the adult brain being the matured, optimal, or ‘normal’ functioning brain, and which therefore have led many to view the marked differences between the adolescent brain and the adult brain primarily in terms of supposed deficits of the adolescent brain. For example, a prevailing model of brain development
points towards the different developmental trajectories of gray and white matter in the brain, as a reason for why humans engage in increasing amounts of dangerous, impulsive, and risky behaviors during adolescence.
However, in a recent article
for the journal Nature Reviews Neuroscience
, Evelin Crone and Richard Dahl synthesize many of the currently available studies on adolescent brain development into an emerging view that paints a more favorable picture of the adolescent period.
In particular, Crone and Dahl show that the numerous isolated studies on age related developmental differences in cognitive control, decision making, risk taking and social processing support a view of adolescence (and puberty in particular) as a designated developmental phase for learning during which our brains afford us with increased cognitive flexibility to adjust goal priorities and motivations, and during which our brains pay special attention to the social-affective cues that typically prove critical for acquiring the social competencies that define adulthood.
The review by Crone and Dahl presents an emerging view which departs decisively from previous accounts adolescent brain development. According to these previous models, developmental transitions during adolescence were thought of primarily in terms of what is often referred to as the “maturity gap”, a concept meant to capture the observation that during adolescence, development of those parts of the human brain which are typically involved in top-down cognitive control, attention and emotional regulation, lags behind the development of brain areas primarily involved in reward seeking, punishment avoidance and general processing of affective cues. As a consequence of this maturity gap, so the prevailing view argued, adolescents essentially experience a deficit in cognitive control, which leads to more impulsive, emotionally driven decisions, less regard for long-term consequences, and inadequate concern for risk.
Researchers are beginning to realize, however, that they may have overemphasized the effects of maturational differences in the brain. While the differential maturation of brain regions remains undisputed (From childhood through adolescence, cortical white matter in the brain increases in a more or less linear fashion, while cortical grey matter growth follows an inverted U-shape. This means that while grey matter development begins to slow down, white matter development is still ongoing at a steady pace, leading to a change in the relative proportion of the two, including the temporary imbalance referred to as the maturity gap. Similar changes occur in the brains’ reward and fear processing regions, which have often been linked to relative shifts in approach and avoidance behaviors that occur during beginning adulthood.), researchers have begun to reconsider the emphasis they place on the maturity gap for explaining adolescent behaviors, including those considered as dangerous. More importantly, they have also begun to take a more integrated approach to the question of adolescent development, to consider the changing social demands that mark adolescence, to pay additional attention to the nuanced context dependency of adolescent behavior, while also making use of improvements in brain scanning technology.
From doing so, they conclude that the simple account according to which adolescents are deficient in their ability to exhibit top-down control over their affective processes, misses important nuances of the available research.
For example, previous research comparing the performance of early and mid-adolescents (8-12 years and 13-17 years respectively) on a number of experiments requiring complex cognitive control, shows that early adolescents engage frontal brain regions (the regions used for cognitive control, reasoning and abstract thought) to a greater extent when evaluating positive performance feedback, while late adolescents engaged these regions more strongly during negative performance feedback. That is, the age-trajectory in the use of cognitive control areas is not one in which specific regions are used progressively more or progressively less as humans age, but one in which control regions are assigned different tasks as we age.
Similarly, when comparing subjects aged 11 to 30 while they performed a spatial reasoning task, researchers found that brain activation in frontal areas was highest for the mid-adolescent group aged 14-18, and reduced for the younger as well as the older participants, showing that certain tasks may even recruit frontal brain areas preferentiallly adolescence. [See main reference
for more details on these studies]
The findings from the above and other similar experiments, demonstrate that adolescents seem to use frontal brain regions for different types of problems and during different types of decision making than both young children, and mature adults, possibly indicating a shift in motivation as well as more complex changes in the brains’ hardwiring of automatic responses during adolescence.
For example, studies have shown that a mirage of contextual factors, such as the presence or absence of peers, or modifications in affective appraisal of the value or priority of an experimental task have especially strong effects on adolescents. In particular, adolescents seem to recruit cognitive control systems in a less automatic fashion, and instead respond more strongly to internally motivated changes in priority, as well as socially cued motivational contexts.
Considered as a whole, the emerging understanding of adolescence seems to describe this period as one during which the brain can more flexibly react to changing goals and shifts in motivation than during other periods of development, turning it into something akin to a “developmental window” especially suitable to learning.
Neurophysiologically, this developmental window seems to result from a particular pattern of connectivity within the brain’s control circuitry that emerges during adolescent development: A number of studies using more recent brain scanning technologies that can identify functional connectivity between different parts of the brain (i.e. technologies and statistical procedures that identify the extent to which different parts of the brain can communicate with each other) support the idea that, as the brain undergoes a general developmental process,during which long terms neural connections between 'automatically' communicating brain areas are strengthened, while neural connections for more independently functioning brain areas are weakened or extinguished. Adolescents, in this context, may find themselves in a special developmental moment of this reorganizing process, during which the brains connective pattern displays a special combination of widespread, yet only moderately strong (i.e. not to rigid, nor too weak) connections across brain circuits, which allows for particularly flexible communication of different control related brain networks, capable of beeing recruited more task-dependently.
It seems therefore, that during adolescence, a period which humans historically use to shape their social identities, and to acquire defining social competencies, and during which they typically transition more rapidly through different social environments than during any other phase in life, the brain appears to be particularly responsive to changes in the social environment and the presence of social affective-cues.
On the one hand, the above described view of the adolescent brain makes this developmental phase look especially adaptive for the purposes of acquiring social competency in adulthood, but on the other hand it also hints at potential vulnerabilities of this system.
One particularly salient vulnerability of the adolescent brain in this context, appears to be in how it responds to social-affective cues in the domain of peer relations: During the adolescent phase, when increased testosterone levels intensify social status motivations in young boys and girls, social acceptance can quickly become a primary reward, while social exclusion may loom as a particularly salient cognitive threat. Many of the high-impact health risks associated with adolescents (such as drug use, risky sexual behaviors, increased accidents), may at least partly follow from these shifts in motivation.
As Crone and Dahl write in their review, for example,
”[The] flexibility (and sensitivity to social and affective influences) may confer greater vulnerabilities for adolescents to act in ways that appear impulsive and immature, such as placing greater motivational value on gaining peer admiration for a daring action than considering the risks and long-term health consequences of that behaviour.”
But again, the above concern should not be viewed in terms of a developmental deficit, since on the other hand, as already mentioned,
“[the] capacity to quickly shift goal priorities may also enable adolescents to effectively engage cognitive systems in situations in which they are highly motivated to do so and in ways that facilitate learning, problem-solving and the use of divergent creative abilities."
“adolescence is an important period for developing cognitive control skills through training and experience. When adolescents are motivated, their capacity to engage can result in quick mastery of complex tasks. Consider, for example, a tedious and precision-demanding task such as using cell phone text messaging to communicate with peers — individuals who have learned these skills in adolescence typically reach a higher level of mastery than those who have learned as adults.“
Taking an evolutionary perspective, Crone and Dahl offer an interpretation of adolescent development in which the neural systems of the human prefrontal cortex exhibit several phases with so-called experience-expectant qualities (i.e. phases during which the brain is particularly configured for learning), adolescence marking one such phase. Noting that over the course of human history, the transition into adulthood has presented humans with fairly unique challenges and opportunities, the authors suggest that these
“social challenges and changes facing adolescents may have favored a slightly different cognitive style (more flexible, exploratory and sensitive to social– affective influences) compared with adults [for this phase].” This notion argues against the idea that the adult brain is the optimal or ‘normal’ functional system and that differences during adolescent development represent ‘deficits”.
Although Crone and Dahl provide a compelling synthesis of our current understanding of adolescent development, there remain many exciting open research avenues regarding the exact biochemistry that underlies many of the described neural changes, and many of the key assumptions of their proposed view are yet to be rigorously tested. But as the authors point out — irrespective of specific details — investigation into these open avenues promises new insights into
“early intervention and prevention for a wide range of adolescent-onset health problems, as well as [possess] broad implications for health, education, juvenile justice and social policies aimed at youths.
” Main Reference: Crone, Eveline A. (2012-8-20) Understanding adolescence as a period of social. Nature Reviews Neuroscience, 31(9), 852-650. DOI: 10.1038/nrn3313
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