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If your mind does indeed grow more agile as you age, one of the things that may help it do so is the amount of glue you carry around in your brain--glia (Greek for glue) being what the 19th century German anatomists called it. Only about half the mass of the brain is composed of gray matter, or nerve cells; the rest is white matter, the connecting tissue that, in a sense, glues it all together. Much of that white matter is made of conductive nerve strands, and covering each fine wire is a fatty sheath of myelin that keeps nerve signals from sputtering out or cross firing during transmission. "Myelin is what makes us human," says Bartzokis. "We have 20% to 30% more than other primates do."
Throughout our lives, fresh layers of myelin sheathing are laid down in the brain. In infants and children, who grow increasingly coordinated as they mature, the bulk of that takes place in the motor and sensory lobes. If we acquire better reasoning skills in middle age, Bartzokis long suspected, it would follow that most of the myelin added in those years would appear around the signal-transmitting axons in the higher brain regions that are the seat of sophisticated thought. Essentially, the brain spends decades upgrading itself from a dial-up Internet to a high-speed version, not fully completing the job until age 45 or so.
To test that idea, Bartzokis used magnetic resonance imaging to study the volume and distribution of white matter in 300 healthy subjects from 18 to 75 years old as well as in hundreds of older people suffering from such brain-related ills as Alzheimer's and Parkinson's diseases. As he suspected, the healthy adults had the most myelin in the frontal and temporal lobes--where big thoughts live. The quantity of sheathing reached its peak around 45 or 50, exceeding the amount in unhealthy older subjects and healthy younger ones.
"This last little bit of myelination essentially puts us online," Bartzokis says. "You may not have the same amount of information you had when you were 20, but you can use it better in everyday life."
It's not just the wiring that charges up the brain as we age, it's the way different regions start pulling together to make the whole organ work better than the sum of its parts. For all its plasticity, the brain is a specialized machine, with specific regions handling specific operations. The greatest divergence comes between the left and the right hemispheres, which often work almost independently of each other. That is not such a bad thing because one hemisphere can be busy writing a grocery list or solving an equation while the other scans the environment and tends to other basic chores. As we age, however, the walls between the hemispheres seem to fall, with the two halves working increasingly in tandem. Neuroscientist Roberto Cabeza of Duke University dubs that the HAROLD (hemispheric asymmetry reduction in older adults) model, and judging by his work, the phenomenon is a powerful one.