Cambridge scientists have found that the adolescent brain undergoes a “disruptive” form of remodelling – during which new networks come online – allowing teenagers to develop more complex adult social skills.

But this disruptive phase, which sees a somewhat radical shift in the strengthening and weakening of certain neural connections, may explain why young people have an increased risk of mental illness.

This discovery was part of a big-picture finding that the functional connectivity of the human brain – how different regions of the brain communicate with each other – changes in two main ways during adolescence.

According to a statement from Cambridge, the study collected functional magnetic resonance imaging (fMRI) data on brain activity from 298 healthy young people, aged 14 to 25 years, each scanned on one to three occasions about six to 12 months apart.

As the participants were bid to lie still and as quietly as possible, the researchers analysed the pattern of connections between different brain regions while the brain was in a resting state.

Please don’t move, don’t talk

Measuring functional connectivity in the brain presented particular challenges, said Dr František Váša, who led the study.

“Studying brain functional connectivity with fMRI is tricky as even the slightest head movement can corrupt the data – this is especially problematic when studying adolescent development as younger people find it harder to keep still during the scan,” said Dr Vasa, in a prepared statement.

“Here, we used three different approaches for removing signatures of head movement from the data, and obtained consistent results, which made us confident that our conclusions are not related to head movement, but to developmental changes in the adolescent brain.”

What the scientists found

The scans revealed that “the brain regions that are important for vision, movement, and other basic faculties were strongly connected at the age of 14 and became even more strongly connected by the age of 25”.

The scientists called this a “conservative” pattern of change, as areas of the brain that were rich in connections at the start of adolescence become even richer during the transition to adulthood.

In other words, the changes were of a consolidating character – and perhaps not so surprising.

However, the scans showed “the brain regions that are important for more advanced social skills, such as being able to imagine how someone else is thinking or feeling (so-called theory of mind), underwent a more “disruptive” pattern of change.

In these regions – mainly in what’s known as the association cortex – connections were redistributed over the course of adolescence: Connections that were initially weak became stronger, and connections that were initially strong became weaker.

By comparing the fMRI results to other data on the brain, the researchers found the network of regions that showed the disruptive pattern of change during adolescence had high levels of metabolic activity typically associated with active remodelling of connections between nerve cells.

The mystery of adolescent mental illness

Professor Ed Bullmore, joint senior author of the paper and head of the Department of Psychiatry at Cambridge, said: “We know that depression, anxiety and other mental health disorders often occur for the first time in adolescence – but we don’t know why”.

“These results show us that active remodelling of brain networks is ongoing during the teenage years and deeper understanding of brain development could lead to deeper understanding of the causes of mental illness in young people.”

The new study appears to build on a 2016 Cambridge experiment that used magnetic resonance imaging (MRI) to study the brain structure of almost 300 individuals aged 14 to 24 years old.

By comparing the brain structure of teenagers of different ages, they found that during adolescence, the outer regions of the brain, the cortex, shrink in size, becoming thinner.

However as this happens, levels of myelin – the sheath that insulates nerve fibres, allowing them to communicate efficiently – increase within the cortex.

According to a statement from the university, myelin was thought mainly to reside in the so-called “white matter”, the brain tissue that connects areas of the brain and allows for information to be communicated between brain regions.

Levels of myelin in nerve fibres change during adolescence. Photo: Getty

However, the researchers show that it can also be found within the cortex, the “grey matter” of the brain, and that levels increase during teenage years.

In particular, the myelin increase occurs in the association cortical areas – the areas of the brain shown to undergo disruptive changes in the more recent study.

These are regions of the brain that act as hubs, the major connection points between different regions of the brain network.

The researchers compared their MRI results with Allen Brain Atlas, which maps regions of the brain by gene expression – the genes that are switched on in particular regions.

They found that “those brain regions that exhibited the greatest MRI changes during the teenage years were those in which genes linked to schizophrenia risk were most strongly expressed”.