What is really going on inside the heads of children?

Not all animals pass through adolescence. Some are born ready to be independent and begin fending for themselves immediately after hatching from an egg or leaving the womb - they do not require an extended period of nurturing and (often with a bit of luck) they can survive without adult help. Effectively, they are born with all of the instructions they will ever receive about how to survive outside of the egg or womb. They may learn some new tricks but if they have been born 'wired' to eat a specific food that isn't available, then they are unlikely to last for long. They will certainly not spend any time rationalising a change in survival strategy!

Humans by contrast, are adaptive beings. The skill-set we need to survive in different generations changes and we must adapt. The price for this though is that we are completely dependent when we are born, exploring the world using our families as a base. In order for the human race to survive another generation, eventually we need to venture away from the family and create families of our own - who, for the first years of their lives will be dependent on us.

Adolescence is the transition from childhood to adulthood, that period in which we acquire the skills necessary to survive on our own, away from parents. It can be a very confusing time - a time of conflicts, flux, uncertainty - emotionally, physically and behaviourally. The adolescent period generally occupies much of the second decade of an individual's life, which can be influenced to a greater of lesser extent by social expectations and family circumstances. Adolescents experience changes in sleep, diet, mood, weight, attitude, attention and an increasing sense of boredom. In many ways these changes are essential in order to step onto the next rung of the ladder to adulthood.

It's not just hormones

Generally attributed to hormones, and indeed these play a massive role, recent neuroscience research show that adolescent behavioural changes are underpinned by changes in brain functions. Puberty is a mass of hormonal and physical changes that prepare a child's body for procreation. Puberty and adolescence may occur during the same years, but they are not the same. Changes in brain function and hormonal activity brought about by puberty, help to facilitate the abandonment of childhood behaviours to a more independent state.

During adolescence, brain organisation and function enter a unique phase of flux - the teen years are not just a continuation of synaptic pruning and childhood plasticity. The circuits that coordinate our behaviours help us to make good decisions, respond appropriately, control our impulses, eating and sleeping habits are remodelled, paving the way for us to become adults. Often referred to as the imprint period by psychologists, much of this remodelling is influenced by our external experiences - and the adolescent brain is much more mouldable and adaptable than the adult brain. During this period we lay the foundations for who we become as adults.

Two main changes are now known to take place during adolescence - and there is a huge amount of on-going research to further validate existing findings and to learn more. The first involves the growth of fatty insulation around the electrically charged neurons. This extra insulation increases the speed of transmission a hundredfold. In the corpus callosum, the thick bundle of axons joining the 2 brain hemispheres this is evident. The corpus callosum itself increases in physical size during adolescence and interestingly, more so in females.

Adolescent synaptic pruning

The second change concerns the growth and then deliberate pruning back of the synapses that link neurons to one another. This re-shaping of the brain's connectors mirrors an earlier occurrence of synaptic pruning in the first few years of life and is considered a critical part of intellectual maturity.

Diagram courtesy of Time Magazine

A 1999 study has laid much of the groundwork for what we now know about adolescent synaptic pruning. Dr Judith Rapoport, a child psychologist at the US National Institute of Mental Health in Washington DC studied the brain development of 145 children using MRI scans. It was already known that an earlier growth of grey matter in the womb was followed by a pruning back in the first few years of life, now Dr Rapoport found a phase of overproduction of grey matter in the frontal cortex just prior to puberty and a second wave of continued growth and pruning during and after puberty.

Her discovery was focused mainly on the prefrontal cortex which provides one of the first clues that could explain the physical changes that may be behind some typical teenage behaviour. Some of the more immature aspects of adolescent behaviour may be due to the lack of maturity of some parts of the frontal lobes of the brain as the research showed a wave of loss of nerve connections sweeping from the back of the brain to the front.

We now know that despite the fact that neurons are dying at a rapid rate during childhood, those that survive are growing and branching out so quickly that there is a net gain in the amount of grey matter in the frontal lobes by the beginning of the second decade of life. During adolescence, a weeding out process begins leading to a reduction in overall grey matter. Sometimes it is helpful to think of the grey matter in a baby as like a lump of putty which grows during childhood and is shaped during adolescence into the structure that will take us through adult life. Unfortunately its malleability declines with age and it is becomes difficult to make changes. An example is that children and adolescents can move abroad and learn to speak the language without an accent but if an adult in his 20s or beyond was to make the move, most likely an accent would be noticeable - while we can certainly teach an old dog new tricks, it is much easier to learn new tricks when we are young!

Frontalisation

During adolescence, the amount of energy required by the frontal lobes begins to decrease and reaches adult levels by the ages of 16 - 18 (Dr Chugani "Biological Basis of Emotions", 1998). This doesn't reflect decreasing use, rather the frontal lobes take on an increasingly important role in guiding behaviour and controlling activity in other parts of the brain. This is known as "frontalisation" - at the same time as the frontal lobe grey matter volumes and metabolism (energy) decrease, the activity of the frontal lobes during certain tasks becomes more efficient and focused. It therefore appears that adolescent brain development represents a unique state of change in which the frontal lobes become more refined and efficient, using less energy but playing a more important role in guiding behaviour. Prior to this process of frontalisation, adolescents tend to perform more poorly than adults on tasks requiring impulse control and decision making.

Frontalisation continues long after the onset of adolescence - there is a long gap between the time when adolescents begin to feel compelled to seek novelty and take risks and the time when the frontal lobes which control impulses and decision making become completely operational. With adolescents, their drive to seek novelty often comes before the frontal lobes are fully developed and they often fail to consider the consequences of their actions before it is too late.

Dr Sarah-Jayne Blakemore, a cognitive neuroscientist at the University of London, has compared decision making with groups of adolescents (11 - 17) and adults (21 - 37). In responding to questions about their own actions, both groups used the same neural pathways, but different parts of the pathway were most active in the 2 groups. Adults used the pre-frontal cortex whereas adolescents showed more brain activity in an area towards the rear - the superior temporal sulcus. This area is used in making simple actions, whereas in order to think further about consequences of actions, the frontal lobes would need to be activated as in the adult group.

It seems then that teenagers go through a period when their pre-frontal lobes are "learning" to work more efficiently during the process of frontalisation. This is important because this is the part of the brain that distinguishes man from the rest of the animal kingdom. It's what makes us - and teenagers - human.

So, it isn't just the hormones, it is the natural drive to seek out new experiences and take risks and the way the adolescent brain deals with that, that leads to their behaviour. It is the process all adolescents need to go through in order to build confidence and gain the skills they need to survive independently in the world.

Published July 2009

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