Right Now | Modern Myelination
The Brain at Midlife
Those of you approaching the golden years, take heart: though scientists once thought brain development was finished by age five, professor of psychiatry Francine Benes has found that it actually continues well past kindergarten. In fact, her research shows that the human brain may continue to develop physically until approximately age 55.
When Benes began delving into brains in the Yakovlev-Haleem Collection at the National Museum of Health and Medicine in Washington, D.C., she had no intention of overturning a truism that had existed since the beginning of neuroscience. In fact, Benes, who is based at McLean Hospital, wasn't even investigating late-life brain development. "My principal interest lies in schizophrenia," she says. Benes believed that a normal developmental process of the brain gone awry might cause schizophrenia, because onset of the disease typically occurs between 16 and 25 years of age. While exploring this process, she discovered something too revolutionary to ignore.
Myelin is the covering of nerve fibers; it both insulates them and speeds nerve conduction. The accumulation of myelin is called myelination. During child development, myelination correlates with maturing patterns of behavior. Infants, for example, lack the fine motor coordination to move an index finger independently, since their nerves are insufficiently myelinated. "As a child is beginning to walk and talk," Benes explains, "the brain is actively myelinating."
Because the size, shape, and proportions of the brain don't change much after age five, it was easy for neuroscientists to assume that development ended then. But the brain "is a very big place," says Benes. "If you don't know what you're looking for, it's easy to miss things."
Even so, in the part of the brain where Benes was looking, the evidence was hard to overlook. Long after age five, significant increases in myelination occurred in the medial temporal lobe, a brain region responsible for emotion, memory, and the integration of the two. "I scratched my head and thought, 'That's kind of strange,'" she says. "It was an important finding that needed to be replicated."
So Benes put aside her schizophrenia research to do a more exhaustive study, using a collection of 162 brains from Massachusetts General Hospital and Children's Hospital in Boston. With this brainy array, which included specimens from birth through age 76 and sufficient numbers from each age group, Benes was able to use statistical methods to quantify the changes she'd seen in the lab.
Myelination levels kept rising into the early twenties, she found, and then flattened out after having doubled in the second decade of life. But myelination took off again in the forties and continued into the mid fifties, accumulating, on average, another increase of 50 percent before leveling off again.
Although cognitive development is generally believed to end by age 16 or sooner, emotional development may be ongoing even into the sixth decade of life. Might the spurts of myelination correspond to stages of increased emotional maturation?
"It may be suggesting that the changes we all go through during the midlife period have a biological basis," says Benes. Behavioral analyses of midlife have shown "very predictable changes," she asserts: "People generally become less anxious, more isolative, less apt to socialize. There's a greater calmness that comes during midlife." In contrast, in the other period of high myelination, "Adolescents are very emotive, very responsive to external stimuli, very excitable. As we mature, we become more focused and more in command of our emotions."
The topic of brain development recalls the work of Jean Piaget, the Swiss psychologist who, in the 1920s, formulated a theory of how cognitive development proceeds in stages that is still taught in every introductory psychology class. Though Piaget's model does not extend past adolescence, it does go beyond age five: his final stage, in which children become capable of abstract thought, begins at 12. But because psychologists and neuroscientists have historically operated independently, declining to integrate and capitalize on each other's ideas, no one investigated the discrepancy between Piaget's theory and the neuroscientific conviction that brain development was complete at five. "It was like a blind spot in the neuroscience community," says Benes. "No one really thought about what it meant that a child matured into an adolescent, and an adolescent into an adult."
Now that the age-five myth has been disproved, Benes says there are limitless possibilities for other types (and consequences) of brain development later in life. "Myelination is a fairly crude change," she says. "If we can see changes in myelination, then it is likely that there are more important changes taking place."