By Emine Saner, Illustration: Fran Pulido/The Guardian, The Guardian, March 12, 2023
When I was studying as an undergraduate, almost 30 years ago, I was taught that the human brain stops developing in childhood. But that is wrong. Now that we have the technology to look inside the living human brain and track changes in its structure and function across a lifespan, we know that the brain continues to develop substantially throughout adolescence and into early adulthood.
We define adolescence as the period of life between 10 and 24. This is a relatively new definition, which is partly based on this new knowledge about brain development. The brain changes, both in terms of its function – how it is activated when you do things like make decisions, plan actions, think about other people, or process emotions – and also in its structure. Although the brain is almost adult size by about the age of eight or nine, its internal composition changes radically during adolescence.
Specifically, the amount of grey matter and white matter the brain contains undergoes substantial transition during adolescence. White matter contains long fibres called axons, which allow different regions of the brain to communicate with each other, and white matter increases steadily from very early development until the thirties. This reflects a process called myelination, which speeds up signalling in the brain, making it become more efficient. Meanwhile, from late childhood, in many brain regions, grey matter – which contains cell bodies and connections between neurons, called synapses – undergoes a steady, slow, protracted decline.
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