Introduction

Adolescence can be seen as a strong contrasting period. In this period people start looking for sensation, but they are also extra sensitive to the development of depression. The question is how brain development is related to this.

Adolescence

Adolescence is the transition from childhood to adulthood. Adolescence starts around the start of puberty and ends when the individual is relatively independent of the parents. This means that the individual can live alone.

Looking at the past, adolescence is seen as a turbulent period. In this period there is a much larger amount of preventable deaths and there is a relatively large amount of psychopathology.

Understanding adolescence

There are two main approaches to understanding adolescents' behaviour and brain development:

• Translation. Behaviour that belongs to adolescence, such as increased time spent with friends, is investigated here. The age-related behaviour entails risks, but also provides important adjustment functions. The changed behavioural pattern associated with adolescence is not specific to adolescents but occurs in many mammals. Because adolescence is an important phase from an evolutionary point of view, animal studies can also contribute to our understanding.

• Transition. As mentioned, adolescence is a transition period between childhood and adulthood. Many different stages of development have been investigated by themselves, with professionals focusing on the young child, for example. However, it is difficult to understand whether a certain process belongs to adolescence when other developmental stages are not considered. The interaction between the various development stages is interesting here.

Various developmental steps are indicated in the article:

• Development non-specific for adolescence

• Development occurring in adolescence

• Development typical of adolescence

Note that the corresponding graphs can also be displayed the other way around: for example, development typically shows a peak during adolescence, but there may also be a trough. Non-specific development can increase linearly, as shown in the figure, but can also decrease linearly.

Neurobiological models

Some neurobiological models are shown in the article:

• Dual system model. The two systems model of willpower forms the basis for this model. According to this model, self-monitoring is the result of the balance between a cold and a warm system. The cool system is emotion-neutral, strategic and flexible, while the warm system is driven by fear, wishes and reflexes. Individual development and stress affect the balance between the two systems.

This model can contribute to our understanding of direct and deferred remuneration. In addition, the dual system model can be used to explain the difference between adolescents and adults in looking for sensation and making risky decisions.

• Triadic model of motivated behaviour. This model explains the differences between adolescents and adults. The limbic (emotional) system is subdivided into two parts:

  • reward (ventral striatum)

  • avoidance (amygdala)

According to this model, motivated behaviour arises from a balance of reward-driven and damage-avoiding behaviour. In adolescence, there is a stronger tendency towards reward-driven behaviour and there is, therefore, an imbalance.

• Imbalance model. Regional neurochemical, structural and functional changes in the brain cause an imbalance in brain circuits. Different brain regions mature earlier and faster than other brain regions. For example, the sensorimotor and subcortical areas are matured faster than the prefrontal cortex. In addition, the sensorimotor cortex reaches its peak in cortical volume in late childhood, while the association cortexes only reach their peak volume in adolescence. The dopamine system, involved in reward, also has a density peak of receptors in early adolescence in the striatum, while this peak in the prefrontal cortex is only reached in adulthood.

The above striking differences are not found in childhood since relatively many brain areas have not yet matured. In adulthood, however, a relatively large number of brain regions have matured, through which even such differences are no longer present.

Self-control

Definition

Self-control is being able to suppress inappropriate emotions, wishes and actions and to display appropriate behaviour instead. A classic example of this is resisting a direct reward to receive a larger reward later (delay of gratification or the Marshmallow task).

The underlying mechanism

The article identifies three areas of the brain that are important for the cognitive and motivational circuit that supports self-control:

• Amygdala. Important for associative learning and determining the value of emotional cues from the environment. This structure can additionally activate the striatum or inhibit it.

• Prefrontal cortex. Here lies the capacity for reasoning and behavioural regulation. This structure modulates the other two structures to suppress actions guided by emotion.

• Ventral striatum. This structure has to do with learning and predicting rewards.

These three structures are modulated by dopamine and the hippocampus.

The brain of the adolescent differs in many ways from that of children and adults. In adolescents, for example, there is reduced top-down regulation of increased emotional responses. In addition, motivated actions are supported by the reduced top-down modulation of the prefrontal cortex.

Incentive for self-monitoring

Self-control seems to increase linearly from childhood to adulthood. Nevertheless, something can be said about this: in the case of incitement, self-control changes. For example, a reward for performance may result in us doing better next time. There are person-specific differences in this area. Individuals can to a greater or lesser extent shift their focus from direct information to a focus on later pay.

Attractive cues

One of the first investigations into reward processing looked at differences in the level of financial remuneration. Both the ventral striatum and the orbitofrontal cortex were found to be sensitive to indications that predicted the greatest reward. This sensitivity was much higher in adolescents than in children and adults.

Adolescents appear to take more risk when they receive direct feedback than adults. Adolescents have a peak in reward sensitivity around the age of fifteen.

Adolescents are less good at suppressing a response to an unexpected positive cue compared to children and adults. This can be seen neurologically in increased activity in the ventral striatum in adolescents.

Incentives based on performance

In research, participants were told that for some achievements, if they were performed properly, they received a financial reward. The expectation of a reward led to a greater improvement in performance in adolescents than in adults. This is represented in the brain as increased activation in the ventral striatum in adolescents. Unfortunately, baseline measures were not taken into account in these studies. Perhaps adults were already performing so well that it was virtually impossible for them to improve their performance in anticipation of a reward. In addition, the reward for adolescents may be subjectively higher for adolescents than for adults.

Research with a point system instead of a financial reward had to circumvent this last problem. This research showed that adolescents with relatively high rewards are good at not responding impulsively.

Social environment

The social environment influences behaviour throughout life. It seems that the influence of the social environment during adolescence is by far the greatest. For example, a large increase in risky behaviour can be observed in adolescents, and not in adults, when peers are present. This can be indirectly associated with the dopamine-rich part of the ventral striatum. It seems that the presence of peers has a behavioural effect. A study into the extent to which peers ensure behavioural reinforcement has been investigated. Every form of positive reinforcement led to faster response time. In adolescents, there was an increased activation pattern in the premotor circuit with positive social feedback, regardless of the outcome. This was not observed in adults and children.

Self-Control in the event of a threat

Adolescents are not fearless: they overestimate the chance of a negative outcome after risky behaviour. Yet they cannot take this into account sufficiently at the moment, probably due to a combination of peers, the environment and their own emotions.

Indications of danger

The best indication of danger comes from the frightened facial expression of another person. Research shows that we learn fears at an early age. How we can express and suppress this depends on development. The first neuroimaging studies on anxiety focused on the amygdala, the structure that external cues must interpret on emotional value. When a fearful cue is found, the individual is prepared to fight or flee (or even freeze). The lateral nucleus projects onto the central nucleus. This then projects onto the brainstem, the hypothalamus, and the autonomic nervous system. This causes the expression of fear. When the danger cue is no longer present, the fear response is suppressed.

Research into the amygdala among adolescents and adults indicates that adolescents have a greater activation pattern with possible danger expressions. Follow-up research indicates that adolescents also have increased activation compared to children. Adolescents also respond more slowly to anxious cues than to neutral or cheerful cues.

Recent studies have shown that adolescents find it difficult to suppress responses to emotional stimuli. Adolescents, especially men, respond more impulsively to threatening cues than neutral cues compared to children and adults. This can also be seen from increased activity in the limbic cortical areas and the ventral striatum.

The increased level of impulsivity in adolescents could be the result of a not yet sufficiently mature prefrontal top-down regulation of the amygdala.

Conditioned fear

The traditional conditioning process involves the combination of a neutral cue with a negative stimulus. After repeated combining, the neutral cue receives a negative charge. Research into conditioned anxiety among children, adolescents and adults showed that adolescents appeared to be less able to extinguish conditioned fear than children and adults. Similar results have been found in research with mice.

Adolescence can be seen as a period in which anxiety associations are less well regulated, which can lead to reduced self-control when the anxiety is present.

Contextual fear

Contextual anxiety depends on being able to learn from threats in the environment. Research into this has been done among mice. Mice in the adolescence period seemed to suffer little from context-dependent anxiety, compared to younger and adult mice. What is special is that when the adolescent mice grow up, they do show the context-dependent fear (which they had already been taught in adolescence) as adult mice. An explanation may be that the amygdala activity has been dulled by changes in the hippocampus during context-dependent conditioning.

Conclusion

The ability to suppress inappropriate emotions, wishes and actions and replace them with appropriate behaviour diminished when striking environmental cues are present. Both in behaviour and in the brain this leads to an increased reactivity in adolescents compared to children and adults.

Why are the brains programmed that way?

It seems logical that brain mechanisms have evolved around socially relevant cues since social status in evolution is very important for survival.

The adolescence period is by no means unique to humans but is observed in many different mammals. During adolescence, it is important to acquire skills to survive independently of others in adulthood. The search for new things means that new sources can be explored and new (types of) relationships can be entered into. Everything leads to the ultimate goal of adolescence: becoming an adult.

The changes in adolescents' development are helped because adolescents are extra sensitive to socially important cues and are less troubled by potential dangers. Together this causes exploration in the world, beyond the safe environment of a home.

Mental health

One in five adolescents has a mental illness, for example, substance abuse. Early substance abuse is a good predictor of later addiction. In addition, narcotics affect the dopamine system. This reinforces the rewarding effect of the dopamine system, which is very active in adolescence.

In addition to substance abuse, there are also many adolescents (around 10 per cent) with an anxiety disorder. This is because being unable to suppress an emotional response when no danger threatens becomes pathological. Adolescents with an anxiety disorder have increased activity in the amygdala and there is a decreased connectivity in the front-amygdala circuit. Desensitization is the most common treatment for an anxiety disorder. This technique works with fear cancellation. Unfortunately, this form of treatment only works in half of the patients. The studies discussed earlier in this article seem to indicate that anxiety killing for adolescents is not the right way to reduce anxiety disorder.

Laws and policies

In recent years, an enormous number of laws and policy documents have been amended in juvenile criminal law. The studies aimed at the development of adolescents and the increased tendency towards risky behaviour and impulsiveness were used. This research indicates that young people should be held responsible for their behaviour, but that there should be a shared responsibility.

Further neuroimaging research into the brain-behavioural relationship in adolescents can make a major contribution to the legal system:

• Immature brain structures during adolescence affect judgment, decision-making, risk-taking, and criminal behaviour.

• Research can contribute to the treatment and rehabilitation of criminal youth.