As we age, the human brain rewires itself.
According to new research, this process occurs in distinct phases or “epochs” as the structure of our neural networks changes and our brains rewire the way we think and process information.
For the first time, scientists say they have identified four distinct turning points between these phases in the midbrain: at ages 9, 32, 66 and 83. In each period between these years, they say, our brains exhibit markedly different characteristics in brain architecture.
Conclusions, published Tuesday in the journal Nature Communicationssuggest that human cognition does not simply increase with age, reaching a peak and then declining. In fact, according to research, the period from age 9 to 32 is the only period in life when our neural networks become increasingly efficient.
During adulthood, from 32 to 66 years of age, the average person's brain architecture essentially stabilizes without major changes, while researchers believe that people's intelligence and personality tend to plateau.
And in the years after the last turning point—age 83 and beyond—the brain becomes increasingly dependent on individual regions as connections between them begin to fade.
“It's not a linear progression,” said Alexa Moseley, a research fellow at the University of Cambridge and lead author of the study. “This is the first step toward understanding how brain changes vary with age.”
The results may help determine why mental health and neurological conditions develop at certain stages of rewiring.
Rick Betzel, a professor of neurobiology at the University of Minnesota who was not involved in the study, said the results are intriguing, but more data is needed to confirm the findings. According to him, over time, these theories may not stand up to criticism.
“They did something really ambitious,” Betzel said of the study. “Let's see what happens in a few years.”
For the study, Moseley and her colleagues analyzed diffusion MRI scans—which are essentially images of how water molecules move in the brain—of approximately 3,800 people ages 0 to 90. The goal was to map the neural connections in the average person's brain at different stages of life.
In the brain, bundles of nerve fibers that carry signals are encased in fatty tissue called myelin. Think of it like wiring or plumbing. Water molecules dispersed in the brain tend to move in the direction of these fibers rather than across them, meaning researchers can infer where neural pathways are located.
“We can't open skulls…we rely on non-invasive approaches,” Betzel said of this type of neuroscience research. “We're trying to figure out where these fiber bundles are.”
Using MRI scans, a new study maps the average person's neural network throughout a lifetime, identifying where connections are strengthened and where they are weakened. The five “epochs” he describes are based on the neural connections researchers have observed.
In their opinion, the first phase is from 0 to 9 years. Gray and white matter rapidly increases in the brain; it removes unnecessary synapses and restructures itself.
From 9 to 32 years there is a long period of restructuring. The brain is characterized by rapid communication throughout the brain and efficient connections between different brain regions.
Most mental disorders are diagnosed during this time period, Mousley said: “Is there something about this second age of life as we see it that might make people more vulnerable to developing mental disorders?”
From 32 to 66 years old, the brain plateaus. It is still rebuilding, but less dramatically and more slowly.
Then, from age 66 to 83, the brain tends toward “modularity,” where the neural network divides into highly connected subnetworks with less central integration. At age 83, the connection continues to deteriorate.
Betzel said the theory described in the study is likely consistent with people's life experiences related to aging and cognition.
“It's something we intuitively gravitate toward. I have two kids and they're very young. I keep thinking, 'I'm coming out of childhood,'” Betzel said. “Maybe science does exist after all. But is this really the right age? I don't know”.
In an ideal version of such a study, he added, researchers would have diffusion MRI data from a large group of people, each of whom was scanned during each year of life from birth to death. But this was not possible because the technology was not available decades ago.
Instead, the researchers combined nine different datasets containing neuroimaging from previous studies and attempted to harmonize them.
Betzel said each of these data sets differs in quality and approach, and trying to make them consistent with each other could erase important variability, ultimately biasing the results.
However, he said the paper's authors were “thoughtful” and experienced scientists who had done their best to rule out that possibility.
“Brain networks change throughout life—absolutely. Are they really discrete and have five precise points of change? I'd say stay tuned. It's an interesting idea.”
