Introduction

A great deal of development occurs during adolescence. The adolescence period begins with puberty and ends when the adolescent takes up adult roles and responsibilities. This paper explains how biological development interacts with social development during adolescence. There is an extra focus on the prefrontal cortex. Delay in the maturation of this brain structure compared to the maturation of the limbic areas seems to play an important role in the vulnerability to mental problems.

Depression in adolescence

From the start of puberty, there is a noticeable increase in the number of people who are depressed. The prevalence in childhood is 2 per cent, and 6 per cent at the end of adolescence. Around the age of twenty-five, around 25 per cent of adults has experienced depression at some point in their lives.

Various explanations have been devised for the prevalence of depression during adolescence:

• Hormonal changes in the brain

• Physical changes

• More interpersonal stress

• Changes in cognitive style and capacity, such as worry behaviour

Recent models

There are three models that try to explain the increased vulnerability to depression during adolescence in an interactive way:

• Social information processing model. Changes in social behaviour during adolescence are related to the development of social information processing. This includes affection and cognitive regulation. The brain mechanisms that go with this, develop at their own pace. This creates an imbalance, which can lead to extra vulnerability for mental problems. After all, violent responses to emotional stimuli are not inhibited by cognitive regulation.

• Triadic model. This model consists of three components: approach, evasion and regulation. These three components must explain the changes in adolescent behaviour. The first two components have matured faster than the regulatory component.

• Dysregulated positive affect. This model looks at the relationship between depression in adolescence and the development of the reward system in the brain. Depression is seen as a shortage of positive affect.

The above models can contribute well to the understanding of strange behaviours in adolescence, such as substance abuse and risky behaviour. However, none of the models explain how the changes in the regulatory and affective systems cause depression. Nor can they explain why part of depression with an onset in adolescence persists in adulthood. The brain systems are already fully matured in adulthood.

The authors of this paper have their own assumptions based on the three aforementioned models. Instead of a mistake in regulation, adolescents have to deal with rewards that are much more abstract in nature than the rewards they have experienced in childhood. This leads to frustration.

The first depressive period

Depression in adolescence is a good predictor of depression in adulthood. Since depression in adolescence resembles often a first depressive period, researchers assume that stress plays a greater role in the development of depression during this period than in later periods. Almost half of the depressed adolescents, for example, experienced a relationship breakup in the year prior to the depression. Rejection by peers also plays an important role.

Reward and depression

A reduced positive affect is often associated with depression in adolescence. This includes symptoms such as energy loss, reduced self-confidence and less appetite. Positive affect is achieved through a rewarding bond with the environment. Interpersonal rewards are particularly important. Connecting rewards show the value of deep interpersonal ties, creating warmth and affection. Agent benefits are related to leadership.

The social risk hypothesis of Allen and Baddock states that symptoms of depression are a reflection of a defensive psychobiological response mechanism when someone perceives interpersonal risks.

The neurobiology of reward

The dopaminergic system is the most important brain mechanism when talking about reward. This concerns the nucleus accumbens and the dopaminergic projections on the ventral tegmental region. Because dopamine is released into the nucleus accumbens, motivation is converted into actions and behaviour is initiated to achieve goals.

Dopamine leads to small changes in basal neuronal activity, but it also reinforces or suppresses responses from other neurotransmitters. For example, glutamate is suppressed.

Serotonin is an important neurotransmitter in the onset of depression. This neurotransmitter communicates with the dopaminergic system to create rewarding feelings. This happens because serotonin is projected on the dopaminergic system by the dorsal raphe nucleus. Serotonin has an inhibitory effect on this system.

Recognizing reward depends on learning. A certain stimulus is seen as rewarding because of the bond with something positive and fine. Reward-related learning is related to changes in the nucleus accumbens and orbitofrontal neurons. The hippocampus also plays a role in this, since this structure is related to the storage of memories.

Recent research has shown that the dopaminergic system is activated when one thinks of getting a reward and incentives that predict rewards (and not the reward itself). In addition, the dopaminergic system responds better to new and unexpected rewards. If an expected reward does not come, the dopaminergic system is suppressed.

Depression and reward-based learning

Research shows that depression can be linked to a reduced reward function. However, behavioural studies have been conducted in particular, and few studies with a neurological basis. There appears to be a reduced activity in the ventral striatum with positive stimuli. In addition, there appears to be a reduced activation in the medial prefrontal cortex and increased activation in the orbitofrontal and dorsal anterior cingulate cortex.

EEG studies have shown that activity in the left dorsolateral prefrontal cortex is related to a stronger bias to respond to reward directions. When less activity can be observed here, this is characteristic of depression.

The dopaminergic system changes as a result of depression; Amphetamines have an enhanced rewarding effect associated with the degree of anhedonia.

Also in the resting state, the reward systems for depressed patients are different. For example, in the amygdala, subgenital anterior cingulate cortex and the orbitofrontal cortex there is an increased activation pattern. The activation pattern in the dorsolateral prefrontal cortex, the dorsal anterior cingulate cortex and the hippocampus are lowered.

A few fMRI studies have also been done, but very little has happened.

What we know in any case is that there is a clear link between depression and the reward system. The depressive state is reached as a result of a reduced possibility of finding social reward when the social value of the person concerned is threatened.

Development during adolescence

The development of the adolescent has a major influence on that person in adulthood. For example, how well the adolescent is doing in school, influences the further course of life. Adolescence is an identity-forming period.

Social Experiences

More than a third of their time is spent by the adolescent with peers. Relationships with peers become more complex and hierarchical. In addition, adolescents increasingly deal with large groups with a similar identity. The adolescent's reputation is important for the groups that he or she can join.

Through new motivational systems, sexual development and culture, romantic relationships also develop during adolescence. A change in intercourse is also observed during adolescence. In early adolescence, people in particular deal with people of their own sex. The social groups are gradually becoming more mixed. Next, there is interaction with large groups, ultimately leading to interaction with couples.

Relationships during adolescence are particularly unstable for girls. This also applies to the reputation that adolescents have among their peers. This leads to a relatively high amount of interpersonal stress being experienced in adolescence.

Brain development

Biological changes are the hallmark of adolescence. For example, puberty starts in the brain with the release of large quantities of sex hormones. Recent research has also shown that brain development continues until the mid-twenties. This applies in particular to the prefrontal cortex. Synapse pruning takes place here, there is a reduction in grey matter and greater myelination (so an increase in white matter). The dorsolateral prefrontal cortex is the last part of the brain that matures.

Due to the increase in myelination, different neuronal networks can communicate better and faster with each other. This is reinforced because synapses are pruned simultaneously, eliminating weak connections.

Changes in the dopaminergic system also occur during adolescence. For example, more dopamine is released in the prefrontal cortex, and the density of the dopaminergic fibres is increased. In early adolescence, dopamine receptors proliferate, which are subsequently pruned, particularly in the subcortical areas (and to a lesser extent, in the prefrontal areas).

The development of the dopamine system during adolescence influences the pruning of compounds in the prefrontal cortical neurons.

Due to the above developments, the dopaminergic system becomes more integrated with the prefrontal cortex. This allows an adolescent to better deal with more complex forms of reward.

Cognitive development

Both the prefrontal cortex and the dopaminergic system are components of the learning process. Both parts of the brain are related to cognitive changes, which means that the adolescent can, for example, think in a more logical and abstract way.

Identity is formed during adolescence. This can be seen in cognitive changes, where adolescents can better apply self-reflection. The adolescent also has an increasingly better image of himself in the past and which direction he or she wants to go in the future.

During adolescence, people also get better at worrying, something that is associated with mental disorders such as depression and anxiety disorders.

The prefrontal cortex

The more complex rewards adolescents anticipate are made possible by the maturing of the prefrontal cortex. This structure is involved in many higher cognitive functions, such as reasoning, planning and abstract thinking.

The prefrontal cortex is relatively larger than that of most other mammals but differs little compared to primates. However, the Broadmann 10 area is significantly larger in humans compared to primates. This area is related to insight into events in the distant future and keeping an eye on long-term goals while completing secondary goals.

The prefrontal cortex is seen as the centre that guides other brain regions in the attainment of goals. This happens because the prefrontal cortex works with rules. These rules are used to activate the desired behaviour to achieve goals.

Another idea is that the prefrontal cortex uses representations of events to determine the desired behaviour in a given situation. This develops during adolescence. Activity in the Broadmann 10 area mediates this.

Suppression of the reward system and depression

The first depressive period during adolescence often arises from the frustration associated with an expected social reward. The expected reward leads to an increased state of excitement. The more abstract rewards can be seen as higher-order rewards, while the more straight-forward rewards are a representation of lower-order rewards. Motivation for the higher order rewards is needed to also be motivated for the lower order rewards. If one is disappointed in a higher order area, this can have an impact on the reward system and thus, in collaboration with other vulnerability factors, a depression can arise.

Time

Because abstract rewards involve an extended sense of time, problems with the reward system due to depression should also entail changes in time perception. People with depression often indicate that they do not feel like the future and that time passes slowly. Research shows that depressed patients overestimate the time it takes. In addition, people with depression are more focused on the past and less on the future.

Individual differences in development and vulnerability

When adolescents are extra sensitive to social situations and when they have difficulty dealing with social disappointments, why doesn't every adolescent develop a depression?

• Some people have a genetic vulnerability to the development of depression.

• The social environment plays a role. For example, when someone focuses on multiple long-term goals, disappointment in one goal is less serious than when someone has only one long-term goal.

• Women are more sensitive to developing depression than men. A possible explanation for this is that in women there are earlier changes in the amount of prefrontal grey matter.

• The nature of the reward changes in adolescence. The brain cannot always decode it properly.

• The first period of depression can cause changes that increase individual vulnerability for a subsequent period.

Hypotheses and future research

The prefrontal cortex undergoes major changes in adolescence. Due to the stronger bond with the dopaminergic system and the major social changes during adolescence, this is very important.

It is no coincidence that the increase in the incidence of depression occurs simultaneously with major changes in the prefrontal cortex and an increased sensitivity to social reward. It is not the delayed development of the prefrontal cortex in relation to the limbic areas that seems to be responsible for the development of depression, but the prefrontal cortex in itself.

Future research may focus on patients with a first depressive period and their response to social rewards. The activated areas of the brain should be central here. In addition, it is possible to look at the way in which depressed patients process information and which brain activation can be observed when they have to take various steps to receive a reward.