The human brain has five major “epochs” of development, according to one of the most comprehensive studies to date of how neural connections change from childhood to old age. The work, published in the scientific journal Nature Communications, is based on brain scans of nearly 4,000 people aged from less than a year to 90 years old.
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Researchers have mapped neuronal connections and their evolution throughout life. The results revealed five major phases, divided by four crucial “turning points”, during which brain organization follows a different trajectory, around ages 9, 32, 66 and 83.
“Looking back, many of us feel that our lives have been characterized by different phases. It turns out that the brain also goes through these eras,” says Professor Duncan Astle, a neuroinformatics researcher at the University of Cambridge and lead author of the study, in a statement. “Understanding that the brain’s structural journey is not a matter of constant progression, but rather a few key turning points, will help us identify when and how its connections are vulnerable to disruption.”
The period of child development extends from birth until the age of nine, and then moves into the adolescence phase – a period that lasts on average until the age of 32. In your early 30s, the brain’s neural structure shifts into adult mode – the longest phase, lasting more than three decades.
A third turning point, around age 66, marks the beginning of a phase of “premature aging” of the cerebral architecture. Finally, the “late aging” brain consolidates around age 83.
The scientists quantified the brain’s organization using 12 different measures, including the efficiency of connections, the degree of compartmentalization, and whether the brain relies heavily on connectivity centers or has a more diffuse connectivity network.
From childhood to adolescence, our brains are defined by “network consolidation,” as the number of synapses – the connections between neurons – in a baby’s brain decreases, leaving only the most active. During this period, the study found that the efficiency of brain connections decreases.
During this time, gray matter and white matter rapidly increase in volume, so that cortical thickness (the distance between the outer gray matter and the inner white matter) reaches a maximum and the cortical folds, the characteristic ridges on the outer surface of the brain, stabilize.
In the second “phase” of the brain, adolescence, the white matter continues to grow in volume, so that the organization of the brain’s communication networks becomes more and more refined. This phase is defined by the steady increase in the efficiency of connections throughout the brain, linked to better cognitive performance.
Epochs were defined by the brain maintaining a constant trend of development over an extended period of time, rather than remaining in a fixed state throughout.
“We’re certainly not saying that people in their late 30s will behave like teenagers, or even that their brains will look like teenagers,” said Alexa Mousley, who led the study. “It’s really about the model for change.”
She added that the findings could provide information on risk factors for mental health disorders, which appear more frequently in adolescence. It is around age 32 that we observe the most pronounced general change in trajectory.
Life events, such as motherhood/fatherhood, may play a role in some of the observed changes, although research has not explicitly tested this.
“We know that the brains of women giving birth go through changes,” says Mousley. “It is reasonable to assume that there may be a relationship between these stages and what is happening in the brain.”
From the age of 32, brain architecture seems to stabilize compared to previous phases, corresponding to a “plateau of intelligence and personality”, according to other studies. Brain regions also become more compartmentalized.
The last two inflection points were defined by a decrease in brain connectivity, which is believed to be linked to aging and degeneration of white matter in the brain.