1 Division of Child and Adolescent Clinical Neuropsychology at the Department of Child and Adolescent Psychiatry and Psychotherapy, Aachen University Hospital, and Institute of Cognitive Neuroscience, Institute of Neuroscience and Medicine (INM-3), Forschungszentrum Jülich
Find articles by Kerstin Konrad2 Department of Child and Adolescent Psychiatry and Psychotherapy, Aachen University Hospital
Find articles by Christine Firk3 Institute of Neuroscience and Psychology, University of Glasgow
Find articles by Peter J Uhlhaas1 Division of Child and Adolescent Clinical Neuropsychology at the Department of Child and Adolescent Psychiatry and Psychotherapy, Aachen University Hospital, and Institute of Cognitive Neuroscience, Institute of Neuroscience and Medicine (INM-3), Forschungszentrum Jülich
2 Department of Child and Adolescent Psychiatry and Psychotherapy, Aachen University Hospital 3 Institute of Neuroscience and Psychology, University of Glasgow*Lehr- und Forschungsgebiet Klinische Neuropsychologie, des Kindes- und Jugendalters, Klinik für Psychiatrie, Psychotherapie und Psychosomatik, des Kindes- und Jugendalters, Universitätsklinikum der RWTH Aachen, Neuenhofer Weg 21 52074 Aachen, Germany, ed.nehcaaku@darnokk
Received 2012 Aug 7; Accepted 2013 Mar 27. See letter "Correspondence (letter to the editor): Taking More Time" in volume 110 on page 732. See letter "Correspondence (reply): In Reply" in volume 110 on page 733. See "Mental Health and Psychological Illness in Adolescence" in volume 110 on page 423.Adolescence is the phase of life between late childhood and adulthood. Typically, adolescents seek diversion, new experiences, and strong emotions, sometimes putting their health at serious risk. In Germany, for example, 62% of all deaths among persons aged 15 to 20 are due to traumatic injuries. Neuroscientific explanations have been proposed for typical adolescent behavior; with these explanations in mind, one can derive appropriate ways of dealing with adolescents.
We selectively review pertinent articles retrieved from the PubMed database about the structural and functional development of the brain in adolescence.
New findings in developmental psychology and neuroscience reveal that a fundamental reorganization of the brain takes place in adolescence. In postnatal brain development, the maximum density of gray matter is reached first in the primary sensorimotor cortex, and the prefrontal cortex matures last. Subcortical brain areas, especially the limbic system and the reward system, develop earlier, so that there is an imbalance during adolescence between the more mature subcortical areas and less mature prefrontal areas. This may account for typical adolescent behavior patterns, including risk-taking.
The high plasticity of the adolescent brain permits environmental influences to exert particularly strong effects on cortical circuitry. While this makes intellectual and emotional development possible, it also opens the door to potentially harmful influences.
Adolescence is the phase of life between late childhood and adulthood. It is a time not only of physical maturation, but also of mental and emotional development into an independent, responsible adult. The major developmental tasks of adolescence include the establishment and nurturing of intimate relationships and the development of identity, future perspectives, independence, self-confidence, self-control, and social skills (1).
Many adolescents and young adults are prone to take risks and enjoy having extreme emotions (2, 3). This is reflected in statistics showing that risky behavior in adolescence is linked with an elevated risk to health (4). In Germany, for example, 62% of all deaths among persons aged 15 to 20 are due to traumatic injuries. The most common causes of death are motor vehicle accidents, other accidents, violence, and self-injury (5). The high mortality is attributable to drunk driving, driving without a seatbelt, carrying weapons, substance abuse, and unprotected sexual intercourse (4).
As can be seen in the Table , boys and girls engage in risky behavior at similar frequencies. In recent years, for example, the prevalence of smoking among boys and girls has become nearly equal, although some qualitative differences remain: Boys smoke more cigarettes, and they also more commonly smoke “harder” tobacco products such as cigars, black tobacco, and unfiltered cigarettes. Boys and girls also drink different alcoholic beverages: Boys tend to drink beer and hard liquor, while girls tend to drink wine, sparkling wine, etc. Boys drink alcohol more frequently and in larger amounts. They also consume illegal drugs more commonly than girls. Boys are more prone to accidents, and they take more risks when driving. Girls, on the other hand, are more likely to engage in health-endangering behavior in the area of nutrition (e.g., dieting, eating disorders).
| Behavioral domains | Boys | Girls | Age range |
|---|---|---|---|
| Substance abuse Current smoking Regular alcohol use Current cannabis use Curent use of other illegal drugs | 20.5 38.6 9.2 | 20.3 22.2 6.2 | KiGGS *1 : 11−17 years |
| Delinquency Aggressive and dissocial behavior | 7.9 | 7.2 | BELLA *2 : 7−17 years |
| Violence Commission of violent acts Being the victim of violent acts Both of the above | 19.6 5.2 7.6 | 9.9 3.9 3.6 | KiGGS *3 : 11−17 years |
| Sexual behavior Sexual intercourse at or before age 14 Too early sexual intercourse in own estimation No protection the first time Sexual intercourse or other sexual conduct against one’s will | 14 38 15 3 | 12 22 9 13 | BZgA *4 : 14−17 years |
| Mental abnormalities Depression Anxiety Attention deficit hyperactivity disorder (ADHD) | 5.4 10.1 2.9 | 5.3 10.0 1.4 | BELLA *3 : 7−17 years |
| Scholastic problems Ever at risk for repeating a grade Repeated a grade | 28 20 | 24 14 | Shell Youth Study *5 : 12−21 years |
| Physical inactivity Physically active less than once per week 3 or more hours of television watching per day | 10.1 22.1 | 21.5 23.6 | KiGGS *6 : 11−17 years KiGGS *7 : 11−17 years |
| Nutrition Overweight Obese Markedly underweight | 9.0 8.2 2.4 | 8.1 8.9 1.4 | KIGGS *8 : 14−17 years |
*1 Lampert and Thamm 2007 (e9);
*2 Ravens-Sieberer et al. 2007 (e11);
*3 Schlack and Hölling 2007 (e10);
*5 Shell Deutschland Holding (e13);
*6 Lampert et al. 2007 (e14);
*7 Lampert et al. 2007 (e15);
*8 Kurth and Schaffrath Rosario 2007 (e16). KiGGS, German Health Interview and Examination Survey for Children and Adolescents; BELLA, BELLA Study (mental health module within KiGGS), BZgA, German Federal Centre for Health Education. From (39) Bühler A: Risikoverhalten in der Jugend. In: Uhlhaas PJ. Konrad K (eds): Strukturelle Hirnentwicklung in der Adoleszenz. Stuttgart: Kohlhammer 2011; 189–205. Reprinted with the kind permission of Kohlhammer, Stuttgart
This review concerns new neurobiological insights into typical adolescent behavior and their implications for the best ways to deal with adolescents. We studied these issues with a selective search for relevant publications in German library catalogues, in the PubMed database using the search terms “adolescence/puberty,” “brain/neural,” and “development.” Cited publications were also considered. Special attention was paid to human neuro-imaging studies.
Until just a few years ago, there was a general assumption in developmental psychology and neuroscience that major changes in the architecture and functioning of the brain were limited to the prenatal period and the first five or six years of life. (For a historical overview, see [6].) In the meantime, however, new scientific discoveries have compelled a revision of this assumption.
Large-scale longitudinal studies have shown that a basic reorganization of the brain occurs during adolescence (7). Many synapses are eliminated (8) while, at the same time, there is an increase in white matter (9, 10), and there are changes in neurotransmitter systems as well (11, e1, e2). Thus, the anatomical and physiological maturation processes that take place in adolescence are much more dynamic than originally thought. It can be concluded that a reorganization of cortical circuits takes place in adolescence and is reflected in the changes in cognitive functioning and affect regulation that are typical of this period of life (12).
Interestingly, this pattern of human brain development differs from that of nonhuman primates. Although, for example, rhesus monkeys and chimpanzees (like human beings) are born with immature brains, all cortical brain areas in macaques mature at the same rate (13). In man, autopsy studies have shown that synaptogenesis reaches a maximum in the visual and auditory cortices a few months after birth, while synapses are formed much more slowly in the prefrontal cortex. Thus, over the course of human evolution, there was a switch from a synchronous to a heterochronous pattern of cortical development (8). This protracted developmental process presumably facilitates the development of specifically human skills, especially those acquired through embedding in a highly stimulating sociocultural environment, e.g., by schooling, music, verbal communication, and social interaction (14) ( Figure 1 ).
The development of the prefrontal cortex is protracted in man compared to other primates. The Figure shows the synaptic density per 100 µm 2 in the prefrontal cortex as a function of age in man (red), chimpanzees (blue), and rhesus macaques (olive green) (error bar = 95% confidence interval). From (40) Lui et al.: Extension of cortical synaptic development distinguishes humans from chimpanzees and macaques. Genome Research 2012; 22: 611–22. Reprinted with the kind permission of Cold Spring Harbor Laboratory Press, New York
The brain is fully grown relatively soon after birth, in the sense that the cerebral cortex soon reaches its maximal volume. Nonetheless, important structural maturation processes continue to occur in adolescence, as structural imaging studies have shown (15, e3– e5). In the brain, the gray matter matures from back to front, so to speak: The maximum density of gray matter is reached first in the primary sensorimotor cortex and last in higher association areas such as the dorsolateral prefrontal cortex, the inferior parietal gyrus, and the superior temporal gyrus. This means that, in particular, brain areas such as the prefrontal cortex—which subserves higher cognitive functions such as behavioral control, planning, and assessing the risk of decisions—mature later than the cortical areas associated with sensory and motor tasks (16) ( Figure 2 ).
Development of the white matter and gray matter of the frontal cortex over a human lifetime; separate curves for each sex. From (7) Giedd JN, et al.: Brain development during childhood and adolescence: a longitudinal MRI study. Nature Neuroscience 1999; 2: 861–3. Reprinted with the kind permission of Nature Publishing Group, London
Autopsy findings suggest that these gray matter changes are due to synaptic pruning (17). Many synapses are formed in childhood that are later removed in adolescence. This occurs in an experience-dependent way, i.e., the synapses that survive are the ones that are more often “in use.” There are also other cellular mechanisms that might account for gray matter changes in this phase of life, e.g., a reduction in the number of glial cells and an increase of myelination (18).
As the gray matter decreases in volume, the white matter increases in volume. The white matter is composed of myelinated axons that conduct neural information rapidly. The volume of white matter increases continually from childhood into early adulthood (19). This expansion is presumed to be due, in large part, to the progressive myelination of axons by oligodendrocytes (10). Myelination tends to proceed from inferior to superior brain areas, and from posterior to anterior.
The anatomical reorganization processes of the adolescent brain that are described above are associated with profound emotional and cognitive changes. In particular, there is progressive development of executive functions, i.e., cognitive processes that control thought and behavior and thereby allow the individual to adapt flexibly to new, complex situational tasks (20). In adolescence, at the same time that these basic cognitive skills are developing, there are also changes in social-affective abilities such as face recognition, the so-called theory of mind (i.e., the ability to put oneself mentally in another’s place), and empathy (21).
At the neural level, functional imaging studies of brain development have shown that children and adolescents often have a broader, less focal activation pattern than adults, and that the effective recruitment of neural resources increases with age, so that neural activity decreases in brain regions other than those that are relevant to the task at hand (22). It is not yet clear to what extent this pattern of neural development is due to experience-dependent or biologically determined influences. Imaging studies have also shown that adolescents have heightened activity in limbic areas in emotional situations: For example, Galvan et al. (23) found that the anticipation of a reward is associated with a more marked activation in the nucleus accumbens in adolescents than in children and adults. Interestingly, these researchers also found a positive correlation between activation in the nucleus accumbens and the adolescents’ individual risk-taking tendency (24).
Moreover, both structural and functional imaging studies have shown that the prefrontal cortex becomes more strongly linked to sensory and subcortical structures during adolescence (25, 26, e6). This implies a greater influence of frontal brain regions on cognitive and affective processes. The development of cognitive and affective neural circuits should not be regarded as the sole determinant of structural neurobiological maturation; rather, there appears to be a strong interaction of genetic factors with environmental demands. For example, affect regulation and the brain structures subserving it are influenced by the parent-child interaction (27).
Further findings showing that a profound reorganization of neural circuitry takes place in adolescence are derived from electrophysiological studies, including electroencephalographic (EEG) studies of changes in high-frequency and synchronous brain waves (28). Brain development in adolescence is associated with a decline of oscillatory activity at rest in the delta (0–3 Hz) and theta (4–7 Hz) bands, and an increase in the alpha (8–12 Hz) and beta bands (13–30 Hz). With task-dependent oscillations, the precision of synchronization of oscillatory activity in the theta, alpha, and beta bands increases. The late development of synchronized oscillations in adolescence is closely linked to structural (anatomical) maturation processes as well as to fundamental changes in neurotransmitter systems, which have been intensively researched in the past few years.
One of the more influential neurobiological models to explain typical adolescent behavior was developed by the group of Casey in New York (29, e7) ( Figure 3 ).
