Brooding, bad-tempered adolescents have often been forgiven because their bodies are going through the dramatic shift from immature child to sexually potent adult. But can this physical change brought on by a surge in sex hormones really explain, and perhaps forgive, the aberrant behaviour of teenagers?
Maybe it can. "In looking at teenager behaviour, you not only have to deal with hormonal changes and changes in social life, you also have to take into account changes within the brain," says Sarah-Jayne Blakemore, a cognitive neuroscientist at University College London, who studies teenage brain development.
It was once accepted currency among neuroscientists that, after the initial reorganisation of the brain's neural circuits in very early childhood, the organ remained much the same for the rest of a person's life - save for a gradual loss of brain cells. In simple terms, it was thought that the physical development of the brain stopped in childhood.
However, in recent years all that has changed. Scientists now realise that the teenage brain undergoes the same sort of radical re-development seen in the rest of the body. The appearance of the secondary sexual characteristics during puberty, such as facial hair in boys and breasts in girls, appears to be paralleled inside the head with some equally dramatic changes in the physical structure and layout of the brain. It was not until the advent of brain scans - in particular magnetic resonance imaging technology, which allowed scientists to study the living brain in real time - that the true scale of the changes in teenage brain circuitry became fully apparent.
Two main changes are now known to take place during adolescence. The first involves the growth of fatty insulation around the electrically charged neurons - the message-transmitting cells of the brain. This extra insulation increases the speed of transmission a hundredfold. The second change concerns the growth and then deliberate pruning back of the critical connections, or synapses, that link neurons to one another. This re-shaping of the brain's connections mirrors an earlier occurrence of "synaptic pruning" in the first few years of life and is considered a critical part of intellectual maturity.
Of course teenage angst is not the only feature of adolescent brain development. Tests show that there is a marked change in the way the adolescent brain handles information and deals with problems - a sign of growing maturity. "There seems to be a qualitative shift in the nature of thinking such that adolescents are more self-aware and self-reflective than prepubescent children," says Dr Blakemore. "Adolescents develop a capacity to hold in mind more multi-dimensional concepts and are thus able to think in a more strategic manner."
It was a 1999 study that laid much of the groundwork for the recent reappraisal of the teenage brain. Judith Rapoport, a child psychiatrist at the US National Institute of Mental Health in Washington DC, studied the brain development of 145 children using magnetic resonance imaging.
Dr Rapoport found a phase of overproduction of grey matter in the frontal cortex - the "thinking" part of the brain - just prior to puberty. The brain's grey matter is made of nerve cells, or neurons, and their connections. 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 it became apparent that there was a second wave of growth and pruning during and after puberty.
Even more interesting was the discovery that this was focussed primarily on the brain's pre-frontal cortex, the part of the outer cortex responsible for "higher" functions such as decision-making, planning, the control of emotions, empathy and the understanding of other people's facial expressions.
This provided one of the first clues that could explain the physical changes that might be behind some typical teenage behaviour. "One could speculate that 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," says Dr Rapoport. "What you see is a wave of loss of nerve connections sweeping from the back of the brain to the front."
It may, at first glance, seem rather odd that cutting back on nerve connections is a critical part of achieving intellectual maturity. But in fact it is not a good thing to have too much grey matter. "We need to prune it back, to get rid of the excess," Dr Blakemore explains.
It seems then that teenagers go through a period when their pre-frontal lobes are "learning" to work more efficiently, which 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.
One of the key roles of the pre-frontal cortex, for instance, is understanding and interpreting the facial expressions of others. Teenagers are not very good at this, or at least they seem to be not very good. Take the expression of fear. When taking part in psychological tests young teenagers are notoriously bad at detecting fear in the faces of others. Brain scans show why. In early adolescence, teenagers use a part of the brain called the amygdala to interpret fear in a facial expression. This is an evolutionary ancient part of the brain and it forms part of the primitive "gut reactions" - instincts that do not involve much thought.
During adolescence, possibly because of the reshaping of the brain, adolescents shift from relying on the amygdala to using the pre-frontal cortex. Adults continue to use the amygdala - the "seat of fear" - but they rely more on the frontal cortex to interpret faces. This leads to more reasoned perceptions and improved performance - they get better at understanding the facial emotions of others.
So then next time a Kevin or a Vicky gives you lip or blanks you with a defiant stare, spare a thought for the "work in progress" still taking place between their ears.
Steve Connor is the science editor of 'The Independent'
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