As we age, the human brain rewires itself.
This process unfolds in different phases or “epochs,” as the structure of our neural networks changes and our brain reconfigures the way we think and process information, according to new research.
For the first time, scientists say they have identified five “epochs” with four distinct turning points in the average brain: at 9 years old, at 32, at 66 and at 83. During the periods between those years, our brains show strikingly different characteristics in their architecture, according to scientists.
The findings, published Tuesday in the journal Nature Communications, suggest that human cognition does not simply increase with age until it reaches a peak and then declines. In fact, according to research, the phase between 9 and 32 years old is the only time in life in which our neural networks become increasingly efficient.
During the adult phase, from ages 32 to 66, the average person’s brain architecture stabilizes essentially without major changes, at a time when researchers believe that people They usually reach a plateau in intelligence and personality.
And in the years after the last turning point, starting at age 83, the brain becomes increasingly dependent on individual regions, as the connections between them begin to disappear.
“It’s not a linear progression,” said Alexa Mousley, a postdoctoral research associate at the University of Cambridge and lead author of the study. “This is the first step in understanding how the brain changes with age.”
The findings could help identify why mental and neurological disorders develop during certain phases of rewiring.
Rick Betzel, a neuroscience professor at the University of Minnesota who was not involved in the research, said the findings are interesting, but more data are needed to support the conclusions. Theories may not stand up to close scrutiny over time, Betzel insisted.
“They have carried out a really ambitious project,” Betzel added of the study. “Let’s see where it is in a few years.”
For their research, Mousley and his colleagues analyzed diffusion MRIs—which are essentially images of how water molecules move within the brain—of about 3,800 people, from newborns to people in their 90s. The goal was to map the neural connections in the brain of an average person, at different stages of life.
In the brain, bundles of nerve fibers that transfer signals are encapsulated in fatty tissue called myelin, which resembles wiring or pipes. Water molecules diffused in the brain tend to move in the direction of these fibers, rather than through them, meaning researchers can infer where neural pathways are located.
“We can’t open skulls, so we depend on non-invasive methods,” Betzel said of this type of neuroscientific research. “What we’re trying to figure out is where these fiber bundles are located.”
Based on MRI scans, the new study maps the average person’s neural network over the course of their life, determining where connections strengthen or weaken. The five “epochs” he describes are based on the neural connections observed by researchers.
The first phase covers up to 9 years, according to what the study suggests. The brain rapidly increases its gray and white matter, prunes excess synapses and restructures itself.
Between the ages of 9 and 32, a long period of rewiring occurs. The brain is characterized by rapid communication as a whole and by efficient connections between different regions.
Most mental health disorders are diagnosed during this period, according to Mousely.
“Is there anything in this second stage of life, as we know it, that can make people more vulnerable to the onset of mental health disorders?” said the researcher.
From 32 to 66 years old, the brain stabilizes. It continues to reconfigure, but less drastically and more slowly.
Then, from ages 66 to 83, the brain trends toward “modularity,” in which the neural network divides into highly connected subnetworks with less central integration. At 83, connectivity declines even further.
Betzel added that the theory described in the study likely aligns with people’s lived experiences of aging and cognition.
“It’s something we’re intuitively drawn to. I have two kids and they’re very young. I think all the time, ‘I’m coming out of the toddler stage,'” Betzel said. “Maybe science will end up confirming it. But are those the exact ages? I don’t know.”
In the ideal version of a study like this, Betzel added, researchers would have MRI diffusion data from a large group of people, each of whom would have had a scan every year of their life, from birth to death. But that was not possible because decades ago these technologies were not available.
Instead, the scientists combined nine different data sets containing neuroimaging from previous studies and attempted to harmonize them.
Betzel said each of those data sets varies in quality and focus, and the effort to match them to each other could eliminate important variability, ultimately leading to bias in the results.
However, he asserted that the authors of the article are “thoughtful” and competent scientists who did everything possible to control that possibility.
“Brain networks change throughout life, without a doubt. Is it so discrete that there are exactly five change points? I would say you have to pay attention. It’s an interesting idea.”
