The human brain is home to approximately 86 billion neurons. They are the “mysterious butterflies of the soul”, as Nobel Prize winner Santiago Ramón y Cajal called them, the main cells of the nervous system, responsible for transporting all the information that allows us to think, laugh, remember or breathe. These butterflies communicate, Cajal said, through “kisses,” synapses, weaving sophisticated connections to transmit the nerve impulses that build life.
But this network of neural pathways that populate the brain is not static; it changes and reconfigures itself throughout life. Research published Tuesday in the journal Nature communications delved into how these structures are organized over time and identified five ages of the human brain, or five distinct periods of neural development. The authors, a group of scientists from the University of Cambridge (UK), concluded that crucial changes in the arrangement of neural networks occur around the ages of nine, 32, 66 and 83.
After comparing the brains of more than 3,800 people aged between zero and 90 using MRI scans that map neural connections, scientists have identified four key turning points that mark the beginning and end of “brain ages.” This finding is significant, especially considering that the way the brain is wired is linked to neurological, mental health, and neurodevelopmental disorders. “By understanding these key turning points, we can better understand what the brain is most vulnerable to at different ages. The more we learn about expected changes in brain connections throughout life, the better we can distinguish what is considered healthy, typical changes from signs of something related to a disease or disorder,” explains Alexa Mousley, the study’s lead author.
The first turning point identified by researchers occurs around age nine. Until then, they argue, children’s brains undergo “neural network consolidation,” where the most active synapses survive and there is an increase in both gray matter (containing neurons) and white matter (made up of connections). But at the end of this first phase of childhood – which coincides with the start of puberty – the brain experiences a radical change in its cognitive capacity and in its socio-emotional and behavioral development.
The second identified phase, which the authors call “adolescence”, ranges from nine to 32 years of age. During this period, the organization of all neural wiring remains more or less constant: the entire network becomes more and more refined, and the connections become more and more efficient.
However, the fact that this second phase lasts until the age of 30 does not mean that the brain is adolescent until then, clarifies Sandra Doval, research professor at the International University of La Rioja. Speaking to the SMC Spain website, the researcher, who was not involved in this study, explains that “the study identifies when patterns of reorganization of brain wiring change, not when the brain matures, ages or declines in functional terms.” The same authors, in fact, underline that “the transition to adulthood is influenced by cultural, historical and social factors”, making it a change more dependent on the context than on biology.
The “strongest” change.
At age 32, Cambridge researchers identify another turning point, the “strongest change in the organization of neural networks in life,” they say. This coincides with the peak maturation of the white matter – other studies had already pointed out that brain connectivity peaks in the early 30s – and with changes in the architecture of the neural network, which until then had occurred rapidly and are starting to slow down. This brain age is the longest phase, ranging from 32 to 66 years. “This period of network stability also corresponds to a plateau in intelligence and personality,” the authors agree.
There is another turning point at age 66, which coincides with a significant change in health and cognition in high-income countries, the scientists note. In fact, it is from this age onwards that dementia or hypertension can begin to appear, which also means cognitive decline and accelerated aging. This initial phase of aging lasts until age 83.
Around that age the last of the inflection points identified by the Cambridge researchers occurs, marking the beginning of the final stage of the brain. While they admit that data on this stage is limited, they note that different areas of the brain have more difficulty communicating.
Mousley says that how the brain changes its connections over the course of life could help “better understand related changes in cognition and behavior.” “Understanding these fluctuations could help us understand how people change over the course of their lives and why they are vulnerable to different disorders at different ages,” the researcher explained in an email response.
Rafael Romero García, director of the Laboratory of Neuroimaging and Brain Networks at the University of Seville, told SMC that this is “a rigorous study” and, although it has limitations acknowledged by the authors – for example, the analysis was not separated by gender and men and women may have different developmental rates – he stresses: “It is a great contribution that has allowed us to identify turning points in development and that could help us better understand brain alterations associated with neurodevelopmental disorders and dementia.”
“Strict borders”
Romero García, who also did not participate in the study, clarifies however that these phases of brain maturation should not be interpreted as “rigid limits”. “The distinction between maturation and aging is relatively arbitrary. Furthermore, it is important to keep in mind that the study focuses only on brain connectivity; it does not analyze how cognitive aspects such as learning, memory, problem-solving skills, etc., change during these phases,” he points out.
According to Sandra Doval, “the results fit very well with known milestones in neurological development and aging,” but she also points out the limitations of the research – another example: those over 60 are probably healthier than average for their age, and this may not accurately represent typical aging – and urges caution in interpreting the results, although she recognizes their “scientific relevance.” “These findings do not generate immediate and direct clinical recommendations, but rather establish a valuable scientific context for future research on critical windows for preventive or therapeutic interventions at different stages of life.”
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