By Penny Carmichael, on 19 March 2013
– Article by Jack Humphrey
The human brain is the most complicated and enigmatic structure that we know of and neuroscience hopes to understand how it is wired together, how it functions and (importantly for this talk), how it changes over time. You can approach the problems of the brain from any level you like; from individual neurons (the constitutive cells of the brain), to small neuronal networks and farther out still to observing the behaviour that emerges. Sarah Jayne Blakemore is a Professor of Cognitive Neuroscience at UCL. After working on schizophrenia, a mental illness that appears during puberty, Professor Blakemore moved her interests towards the study of how the brain changes during adolescence. Adolescence can clearly be defined as starting with the onset of puberty but it is harder to define when it ends and this differs between cultures. Thanks to modern technology we can investigate the changing biology of the teenage brain whilst developing sophisticated tests to examine behaviour.
There are fundamental changes that occur in the brain during this adolescence. Looking at the brain as whole the ratio of grey matter (the cell bodies of neurons and synapses – the local connections between them) to white matter (the axons – long wires connecting different brain areas) shifts dramatically in favour of the former during childhood before dropping off after puberty. This is not due to increased birth or death of neurons themselves, in fact our number of brain cells stays relatively constant throughout life. The changes are due to the increase in synapses (synaptogenesis) following birth as our brains wire themselves up. Following puberty, synapses start to reduce (synaptic pruning) as useful connections are maintained whereas underused ones are cut off. This raising and lowering of synaptic strengths is known as plasticity and is believed to be the mechanism of long term memory and learning. This is greatly shaped by our environment. Professor Blakemore used the example of Japanese speakers. Because the Japanese language does not differentiate between “R” and “L” sounds, Japanese children not exposed to languages that do lose this ability after nine months of age. However, Professor Blakemore stressed that plasticity is a constant feature of the brain all throughout life and the idea of “critical periods” for learning is falling out of favour.
Social cognition is the brain’s ability to emphasise with other brains. The “theory of mind” idea is that social cognition develops in childhood. This can be observed from specific tests of toddlers’ ability to understand other people’s viewpoints. A ceiling effect is observed where after four years of age nearly all children can grasp this whereas autistic children take much longer. This is not set in stone as it appears that different tasks have different ages of onset. But does social cognition change during adolescence? Professor Blakemore’s lab have applied theory of mind experiments to adolescents to investigate changes in social cognition. Their task involves picking objects either from a different person’s perspective or simply by recognising a pattern. They showed that even adults still make mistakes on the shifted perspective task 50% of the time. Adolescents were shown to be the same at the pattern recognition version but significantly worse at the perspective shift. It is not surprising that adolescence ushers in an improvement in social cognition. People change schools, make new friends and widen their social environments.
Both small and large-scale approaches can be combined in investigating the prefrontal cortex (PFC). Located just behind the forehead, the PFC is believed to be the seat of complicated decision making and self control. In line with previous findings, the PFC increases in size during adolescence and decreases afterwards. Brain imaging studies have shown the PFC is activated during theory of mind tasks along with several other brain regions in a possible social brain matrix, but this activation decreases after puberty. Is this because the brain possibly adopts new, more efficient strategies? As always in neuroscience there is much more work to do, which makes it so exciting.
PC: If you weren’t able to make it to the talk, you can see Professor Sarah-Jayne Blake’s ‘The mysterious workings of the adolescent brain” on the TED talks series here.