Neuroscientists at the University of Cambridge have identified five "major epochs" of brain structure over the course of a human life, as our brains rewire to support different ways of thinking while we grow, mature, and ultimately decline.
A study led by Cambridge's MRC Cognition and Brain Sciences Unit compared the brains of 3,802 people between zero and ninety years old using datasets of MRI diffusion scans, which map neural connections by tracking how water molecules move through brain tissue.
In a study published in Nature Communications, scientists say they detected five broad phases of brain structure in the average human life, split up by four pivotal "turning points" between birth and death when our brains reconfigure.
Childhood brain "topology" runs from birth until a turning point at the age of nine, when it transitions to the adolescent phase – an era that lasts right up to the age of 32, on average.
Our early thirties see the brain's neural wiring shift into adult mode. This is the longest era, which lasts over three decades. A third turning point around age 66 marks the start of an "early aging" phase of brain architecture. Finally, the "late aging" brain takes shape at around 83 years old.
"We know the brain's wiring is crucial to our development, but we lack a big picture of how it changes across our lives and why," said Dr Alexa Mousley, a Gates Cambridge Scholar who led the research. "This study is the first to identify major phases of brain wiring across a human lifespan."
"These eras provide important context for what our brains might be best at, or more vulnerable to, at different stages of our lives. It could help us understand why some brains develop differently at key points in life, whether it be learning difficulties in childhood, or dementia in our later years."
From infancy through childhood, our brains are defined by "network consolidation", as the wealth of synapses – the connectors between neurons – overproduced in a baby's brain are whittled down, with the more active ones surviving.
Across the whole brain, connections rewire in the same pattern from birth until about nine years old.
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.
By the first turning point at nine years old, the brain is experiencing a step-change in cognitive capacity, as well as an increased risk of mental health disorders.
The second "epoch" of the brain, the adolescence era, sees white matter continue to grow in volume, so organisation of the brain's communications networks is increasingly refined, as measured by the diffusion of water in the scans.
This era is defined by the efficiency of connections both within specific regions as well as rapid communication right across the whole brain, which is related to enhanced cognitive performance.
"Neural efficiency is as you might imagine, well connected by short paths, and the adolescent era is the only one in which this efficiency is increasing," said Mousley.
These developments peak in the early thirties, on average, which is the "strongest topological turning point" of the entire lifespan say researchers.
"Around the age of 32, we see the most directional changes in wiring and largest overall shift in trajectory, compared to all the other turning points," said Mousley.
"While puberty offers a clear start, the end of adolescence is much harder to pin down scientifically. Based purely on neural architecture, we found that adolescent-like changes in brain structure end around the early thirties."
At age 32, the longest era, that of adulthood, begins. Brain architecture stabilises compared to previous phases – with no major turning points for thirty years. This corresponds with a "plateau in intelligence and personality" based on other studies, say researchers.
They also found "segregation" is more noticeable during this epoch, as regions slowly start to become more compartmentalised.
The turning point at age 66 is far milder, and not defined by any major structural shifts, although researchers still found meaningful changes to the pattern of brain networks on average at around this age.
"The data suggest that a gradual reorganisation of brain networks culminates in the mid-sixties," said Mousley. "This is probably related to aging, with further reduced connectivity as white matter starts to degenerate.
"This is an age when people face increased risk for a variety of health conditions that can affect the brain, such as hypertension."
The last turning point comes around age 83, and the final brain structure epoch is entered. While data is limited for this era, the defining feature is a shift from global to local, as whole brain connectivity declines even further, with increased reliance on certain regions.
"Looking back, many of us feel our lives have been characterised by different phases. It turns out that brains also go through these eras," added senior author Prof Duncan Astle, Professor of Neuroinformatics at Cambridge.
"Many neurodevelopmental, mental health and neurological conditions are linked to the way the brain is wired. Indeed, differences in brain wiring predict difficulties with attention, language, memory, and a whole host of different behaviours"
"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 research was supported by the Medical Research Council, Gates Foundation and Templeton World Charitable Foundation.
