Scientists have identified five major “epochs” of human brain development in one of the most comprehensive studies to date of how neural wiring changes from infancy to old age.
The study, based on the brain scans of nearly 4,000 people aged under one to 90, mapped neural connections and how they evolve during our lives. This revealed five broad phases, split up by four pivotal “turning points” in which brain organisation moves on to a different trajectory, at around the ages of nine, 32, 66 and 83 years.
“Looking back, many of us feel our lives have been characterised by different phases. It turns out that brains also go through these eras,” said Prof Duncan Astle, a researcher in neuroinformatics at Cambridge University and senior author of the study.
“Understanding that the brain’s structural journey is not a question of steady progression, but rather one of a few major turning points, will help us identify when and how its wiring is vulnerable to disruption.”
The childhood period of development was found to occur between birth until the age of nine, when it transitions to the adolescent phase – an era that lasts up to the age of 32, on average.
In a person’s early 30s the brain’s neural wiring shifts into adult mode – the longest era, lasting more than three decades. A third turning point around the age of 66 marks the start of an “early ageing” phase of brain architecture. Finally, the “late ageing” brain takes shape at around 83 years old.
The scientists quantified brain organisation using 12 different measures, including the efficiency of the wiring, how compartmentalised it is and whether the brain relies heavily on central hubs or has a more diffuse connectivity network.
From infancy through childhood, our brains are defined by “network consolidation”, as the wealth of synapses – the connectors between neurons – in a baby’s brain are whittled down, with the more active ones surviving. During this period, the study found, the efficiency of the brain’s wiring decreases.
Meanwhile, grey and white matter grow rapidly in volume, so that cortical thickness – the distance between outer grey matter and inner white matter – reaches a peak, and cortical folding, the characteristic ridges on the outer brain, stabilises.
In the second “epoch” of the brain, the adolescence era, white matter continues to grow in volume, so organisation of the brain’s communications networks is increasingly refined. This era is defined by steadily increasing efficiency of connections across the whole brain, which is related to enhanced cognitive performance. The epochs were defined by the brain remaining on a constant trend of development over a sustained period, rather than staying in a fixed state throughout.
“We’re definitely not saying that people in their late 20s are going to be acting like teenagers, or even that their brain looks like that of a teenager,” said Alexa Mousley, who led the research. “It’s really the pattern of change.”
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She added that the findings could give insights into risk factors for mental health disorders, which most frequently emerge during the adolescent period.
At around the age of 32 the strongest overall shift in trajectory is seen. Life events such as parenthood may play a role in some of the changes seen, although the research did not explicitly test this. “We know that women who give birth, their brain changes afterwards,” said Mousley. “It’s reasonable to assume that there could be a relationship between these milestones and what’s happening in the brain.”
From 32 years, the brain architecture appears to stabilise compared with previous phases, corresponding with a “plateau in intelligence and personality” based on other studies. Brain regions also become more compartmentalised.
The final two turning points were defined by decreases in brain connectivity, which were believed to be related to ageing and degeneration of white matter in the brain.
