William T. O’Donohue, Lorraine T. Benuto and Lauren Woodward Tolle (eds.)Handbook of Adolescent Health Psychology201310.1007/978-1-4614-6633-8_17© Springer Science+Business Media New York 2013
Brain Development and Health Implications in Adolescents
(1)
State of California Department of Corrections and Rehabilitation, California Correctional Health Care System, Susanville, CA 96127-0750, USA
Abstract
The development of the adolescent brain is a complex process, and as our technology improves, our interest in the trajectory of this development seems to grow. Our understanding as well as resulting theories continues to be reshaped by these new (and sometimes not so new) findings. The literature had previously discussed development as a process of spurts and stops where rapid growth occurred followed by a synaptic pruning. The traditional theories attempted to coordinate behavioral observations with volumetric changes in specific brain regions. Current research allows us to see development with greater resolution, including white matter tracts (those responsible for communication between brain regions). These recent studies describe a more heterogeneous development than previously considered. This means that the development is not as symmetrical as previously believed, and some of the differences (e.g., gender differences by region) help better explain some of the paradoxical adolescent behavior which has perplexed generations (e.g., risk-taking behavior, and increased accidental deaths when processing speed, and more rational thought should be developing). In this chapter we discuss the overall developmental processes, and the behavioral as well as health implications.
Introduction
The development of the human brain during adolescence is not insignificant. In fact, as a result of the developmental theories of Erickson and Piaget as well as our own anecdotal experiences, most of us are aware of the behavioral changes exhibited in adolescence. Given these obvious behavioral milestones, it would stand to reason that there are neurodevelopment and thus cognitive changes that underlie these behaviors. Understanding the normal genesis of development, as well as potential risk factors and limitations, is critical to helping us better understand the adolescent. An example of this is the relative paradox pointed out by Dahl and Spears (2004). This review identifies our knowledge of neurocognitive superiority of the adolescent in contrast to the seemingly unintelligent choices that result contemporaneously (e.g., high number of accidents occurring at a time of optimal reaction time and cognitive processing).
The consideration then for this chapter is, given what we know about adolescent brain development, what can we do to promote healthy behavior in exchange for preventing or limiting unhealthy behaviors. While we may not necessarily find definitive guidelines or protocols for such a goal, we can make some observations that help explain the resulting behaviors, both troubling and reassuring, with a biological explanation as an underpinning. Covered in this chapter is an explanation for the perplexing risk-taking behaviors of adolescents, the differences between males and females in neural and sociocognitive developmental trajectory as well as sex-specific risk and protective factors.
Neurological Changes
Neurological changes can be thought of in terms of being both structural and functional. Structurally we want to consider the aspects of white matter (association between cortices) myelination and growth, as well as the growth of gray matter areas, which can be thought of as the individual processing centers. To describe the process as one of growth might be a misnomer. In fact, much research has shown that the real change is not purely one of volume (which is to be expected in a maturing organism), but rather a change of function and efficiency of the brain. The adolescent growth spurt is much more a reorganization marked by shrinkage and recession in some areas co-occurring with growth in other areas. Thus it has been posited that this is a secondary process of synaptic pruning and dendritic expansion or reorganization not unlike the one seen in infancy, perhaps in preparation for new roles and expectations (e.g., preparation for relationships and work roles; Blakemore & Choudhury, 2006).
The white matter of the cortex is responsible for associations between cortical areas, and thus expansion in this area is what we deem responsible for the speed of processing between say the visual cortex responsible for perceiving information and the motor cortex responsible for the resulting behavioral response to the perceived stimuli. It has been largely shown that this myelination process may reach its peak at adolescence, and in fact much of the current fMRI research supports that this is the case (Giedd et al., 1999).
Gray matter on the other hand is responsible for the density of neural cells which process information. The growth pattern throughout the life span is one in which growth occurs from the top down, and the center out. Neurological development occurs in a pattern that develops from the brain stem forward, with the frontal cortex being the last to develop. Given that the frontal lobe has been considered largely responsible for impulse control, coordination (or executive processing), and higher-order thinking/abstract reasoning, it is no wonder that much of the importance of our focus on adolescent brain development focuses on the development of this area.
In general, while past research has suggested that there is an increase in white matter, while gray matter decreases, recent research has shown this to be untrue. In particular the areas of the prefrontal cortex as well as the temporal, parietal, and occipital lobes all show changes during the preadolescent, adolescent, and postadolescent stages. Further, these stages are not considered to be proportional. That is, the growth of the parietal lobe, for example, is not equivalent to that of the occipital or prefrontal cortices. This suggests that the maturation may be specific to a genetic “readiness,” and potentially indicates a critical stage of development in which environmental factors could influence the developmental trajectory and ultimately the functional outcome (Giedd et al., 1999; Perrin et al., 2008; Thompson, 2006).
Given that there are physical differences between genders it would stand to reason that we should explore potential differences in neurological development between the genders and as extension behavioral differences. There is no one-to-one correspondence however in these comparisons. For example while the female brain is, on average, smaller than that of her male counterpart, there is no corresponding difference in intellectual functioning or capacity. However, there are distinct hormonal differences, which we would expect are uniquely different (and research has shown are different in adults) throughout the developmental process.
While there has been much debate recently regarding the exact parameters of adolescence (largely due to the earlier onset of menarche for girls and some later prolonged puberty for males), the most traditional marking of adolescence has been from puberty to legal adulthood, and this would be the most reasonable parameters to consider when discussing neurological development. Additionally, we need to consider the function of the various lobes that demonstrate change in adolescence.
Parietal Lobe
The parietal is responsible for visuospatial processing and integration with the motor cortex. Thus, changes in this area would be relative to visuospatial acuity and resulting motor response. It is in fact during adolescence when most states allow for driving privileges in the form of temporary learning permits or restricted licenses. In fact, many states have recently considered and even passed legislation for graduated licenses related to the discordance between capacity and ability, which was all to evident in the disproportionate number of accidents for this age group, which should theoretically have the greatest ability to avoid them.
Temporal Lobe
The most prominent function of the temporal is language production, reception, and integration. It is no wonder that we begin to see this rapid acquisition of language capabilities not since seen in the childhood language explosion described by various developmental psychologists. This change in language abilities suggests that when we interact with adolescents, we can use a more sophisticated manner of communication than we would with children. However, it also means that we must assess who we are communicating to (an individual patient, a group of teens, or a broad-based preventative health campaign), in order to maximize the effectiveness of that communication.
Occipital Lobe
The occipital lobe is the only one whose growth pattern is linear in nature. That is, we see none of the spurts and stops observed in the other cortical regions previously discussed. Thus, while the implications for this nonlinear growth are not fully understood, one could posit that this is more of a normal developmental trajectory consistent with the overall growth of the human organism.
Prefrontal Cortex
The greatest area of interest for this discussion would be the development and functional processes associated with the prefrontal cortex. It is in this cortical region where we believe the true higher order functioning of human reason occurs. Here we believe the ability to control impulses and integrate various sensory information and the corresponding outputs of behavior occurs. An important aspect of this system is the dopamine feedback system. This system has been labeled as our basic reinforcement feedback loop and is responsible for those thoughts and behaviors we deem to be intrinsically motivating. Dysfunction in this area has been associated with several mental and physical health concerns such as schizophrenia, Parkinson’s, Alzheimer’s, and drug addiction. In fact, research (Iacono & McGue, 2006) has shown that it may be a decrease in activation of this area during adolescence that predicts externalized psychopathology (e.g., substance use disorders, risky sexual behaviors, and antisocial personality features).
The prefrontal cortex is also the area we associate with inhibition for socially inappropriate behavior as well as the basic constitution of our personality. We have some prominent examples of the role of the frontal cortex come from case studies of cortical injury such as that of Phineas Gage (Chap. 6 in Macmillan, 2000) where the loss of frontal cortical gray matter resulted in lasting personality changes and socially inappropriate disinhibition in individuals previously described as mild mannered and polite. Conversely, the decades of 1940s saw a period in the USA where intentional ablation of this area was utilized to reduce inappropriate behavior and deviance (Swayze, 1995).
Not all changes in the cortex can easily be seen as a result of pure structural change. In fact some of the more interesting research has discussed a differential pattern of activation for emotional discrimination and processing of such responses as fear and anger (Monk et al., 2003). Additionally and seemingly of critical importance are the changes seen in the reward system of our brain. This system is the dopaminergic system and is located here in the prefrontal cortex. Research has shown that adolescence is an extremely sensitive period for human development of sensation seeking and a period where we may attribute greater value to potentially rewarding behavior (Ernst & Spear, 2009). This inflation of rewarding value may explain the development of both adaptive and maladaptive patterns of behavior as well as addictions.
It is the development of the prefrontal cortex, which gives most who work with adolescents the most frustration. This area is the fastest and largest region of brain development in adolescence, and is responsible for rational thinking and impulse control, yet adolescents are often (and rightly so) portrayed as impulsive, irrational, and labile. In a review Schwartz (2008) discusses how the paradox is really not a paradox at all, but in fact just another misunderstood process of development. The review provides support for a rapidly developing yet immature prefrontal cortex. Thus, it is akin to gangly first steps we take as toddlers, where we have these rapidly growing and refining synaptic connections for bipedal locomotion, yet we stumble and fall with varying rates in development of true coordination. An additional caveat for this paradox suggests that it is a developmentally necessary process that actually guides the synaptic pruning process (Bessant, 2008). Thus suggesting that it is the experience of bad decisions and judgment which prevents us from making more serious errors in the future, and becoming more efficient at this improved (and informed) decision-making process.
Females
Gray matter in the parietal and frontal lobes peaks nearly a year earlier in females than in males. This peak corresponds to physiological developmental processes in puberty and thus may indicate differential periods of readiness to learn as well as corresponding behavioral patterns that would impact health and wellness.
Given that the parietal lobe is responsible for visuospatial acuity and the temporal lobe allows for more abstract and what we consider adult-type thinking, we could expect adolescent females to be more mature with better reaction times and coordination, as well as a greater level of insight and impulse control than their male counterparts. Thus, they may be more receptive to adult-like treatment paradigms and preventative interventions.
Males
In adolescence, males show a sharp increase in white matter as compared to females. To reiterate, white matter consists of the axons or communication pathways of the neurons within and between brain regions. It has been reported that this increase is mediated by an increase in the production of testosterone and number of androgen receptors (Perrin et al., 2008). This research goes on to suggest that interruptions or alterations (e.g., the use of androgen-based steroids) in testosterone production and androgen receptors could be implicated in adolescent and adult male depression.
In addition to volume of white and gray matter, we can also see a differentiation in activation. For males, one particular area of interest is the amygdala. Here research has shown a change in patterns of activation between males and females exposed to emotion-laden (read angry) faces, with males showing greater activation of the right hemisphere (and almost exclusive for the particular paradigm) of the amygdala than females (Schneider et al., 2011). This research goes on to suggest that given the greater prevalence rates for males of mental disorders related to emotional processing (e.g., conduct disorder, antisocial PD, schizophrenia), this may be an area of dysfunction and potential intervention/assessment. The problem currently lies in the inability to detect whether the dysfunction is a result of hyper- or hypoactivation of the amygdala or yet some other unexplained mechanism.
Related posts:
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
