Teenage kicks in the hormones
During adolescence the volume of white matter, which contains the fibres connecting different brain regions, gets larger. Meanwhile, the volume of grey matter, which contains the “bodies” of brain cells and their connections to each other, gets smaller, explains Dr Iroise Dumontheil, formerly at UCL and now a lecturer at the Department of Psychological Sciences, Birkbeck, University of London.
“Grey-matter volume decreases during adolescence and this is thought to reflect the elimination of unnecessary connections between neurons, to adapt to experiences and the environment and only maintain the connections that are useful and thus obtain a more efficient, less noisy system,” she says. “So these [imaging] studies suggest that adolescence is an important period where the brain continues to be shaped by our experience and environment.”
Functional MRI studies have also compared which parts of the adult and adolescent brains become active during the same task. “In some cases, there is greater activity in adults, for example, in working memory tasks, which require participants to remember something for a few seconds. In other cases, there is reduced activity in adults, for example, in response inhibition,” says Dr Dumontheil. “We are still trying to understand what these changes in activation reflect, whether they reflect the structural changes or how different regions of the brain collaborate together, or different strategies used by the participants during the tasks.”
Brain imaging may also offer some possible clues about the potential for risk-taking during the teenage years. During adolescence there are changes in an area of the brain called the prefrontal cortex, which is an important region for “executive functions” involved in making judgments, planning, inhibiting actions and making decisions, says Goddings.
“We know that the structure of the prefrontal cortex is continuing to mature during adolescence, and that some of the most dramatic changes that happen in adolescence affect the prefrontal cortex,” she says.
“The connections between brain cells – the synapses – which increase in number throughout childhood, start to decrease in adolescence. This is thought to be because some of the synapses are being pruned, to make the brain more efficient.”
As well as the structural changes, the prefrontal cortex also seems to become activated differently during a number of tasks in adolescence, compared with adults and children, she adds. So can we make a link with behaviour?
“We think that this prolonged development of the prefrontal cortex might mean that adolescents’ abilities to make judgments and decisions isn’t yet fully mature,” says Goddings.
“In contrast, other brain regions may be more functionally developed in adolescence, meaning that teenagers may be particularly sensitive to the environment they are in but less able to make adult-like decisions within it.”