The teenage brain

Our brain reaches its full size in early adolescence. However, its wiring is still a work in progress and will continue – albeit to a lesser extent – even throughout later life.

Brain development during the first months and years of life is characterised by an overproduction of neurons and connecting synapses.

This provides the developmental plasticity needed to adapt to and learn from the avalanche of new information that infants and young children receive.

In contrast, brain development from late childhood through adolescence is dominated by processes that remove neurons and synapses. While this may seem unintuitive or even detrimental, it is an integral part of brain maturation and key to the behavioural, emotional, and cognitive changes that occur during the transition to adulthood.

While the death of excess neurons is pre-programmed, the reduction of synaptic connections – a process called synaptic pruning – is subject to experience.

Frequently used connections, say those used when playing a musical instrument, are reinforced and become hardwired. Unused connections are removed. Teenagers lose about 1% of their grey matter every year until their early twenties. Meanwhile, the amount of white matter increases.

But these changes do not affect all brain areas simultaneously. The sensory and motor areas mature first, followed by regions involved in language and spatial orientation, and then those involved in higher-order cognitive processes and executive functions, such as flexible thinking, reasoning, and self-control.

The last to mature is the dorsolateral prefrontal cortex at the very front of the frontal lobe. This area is involved in planning, judgement and decision making, and it also processes emotional information sent from the amygdala - the fight or flight centre of the brain.

This delay in maturation may help explain typical risky, short-sighted teenage behaviour and the often-apparent lack of control over impulses and emotions.

During this period, the adolescent brain remains highly malleable and still acts like a sponge for learning. Yet lacking impulse and emotional control makes teens vulnerable to stress and mental health problems.

Epidemiological studies have shown that about half of all lifetime mental disorders start by the mid-teens and three-quarters by the mid-twenties.

QBI researchers are exploring how the brain changes during these critical periods of growth.