Turns out you can teach an old dog new tricks—the dog just needs to use a different kind of nerve cell to learn them. That's the thrust of a study out today that presents perhaps the first clear evidence that aging people's brains still undergo physical changes as they learn, just not the way their youthful counterparts do.
At issue is the idea that brains don't change much in adulthood. A host of magnetic resonance imaging (MRI) studies haveprovedthatwrong, but those same studies seemed to confirm another intuition that, as we age, we lose the capacity to grow, repair, and modify connections between nerve cells, a capacity called neural plasticity.
At issue is the idea that brains don't change much in adulthood. A host of magnetic resonance imaging (MRI) studies have proved that wrong, but those same studies seemed to confirm another intuition that, as we age, we lose the capacity to grow, repair, and modify connections between nerve cells.
"It has been said that old people are less plastic, meaning the effect of learning is much less," says Takeo Watanabe, a psychologist at Brown University and one of the authors of the new study. But, he says, behavioral experiments "have shown that is not necessarily the case." In visual learning experiments in which participants must remember sets of images or look for minute changes in an image, older experimental subjects can learn at about the same rate as younger people, Watanabe says. But if older brains are less plastic, how are older people still able to learn so well?
The key turns out to be which kinds of brain tissue are plastic in younger and older people. Standard MRI techniques are really only designed to study the brain's gray matter, which mainly comprises neuron cell bodies. White matter, on the other hand, is made up of long fibers called axons that can stretch far across the brain. White matter is just as essential as gray matter, but it doesn't show up in much detail on a standard MRI. For that, the team needed a relatively new technique called diffusion tensor imaging.*
After using standard MRI and DTI to scan the brains of 18 adults aged 65 to 80 and 21 others aged 19 to 32 before and after several days practicing a visual learning task, the researchers found that both age groups learned at similar rates—but their brains responded differently. Gray matter changed only in younger adults, while white matter changed only in older ones—there, DTI results suggested that axons had grown thicker and developed more robust myelin shells, which can help prevent crosstalk between the brain's electrical connections.
That sort of "double dissociation" between changes in younger and older people's brains is a clear sign that something is changing, despite similar learning abilities in the young and old alike. Perhaps, Watanabe says, increased axon plasticity in older people helps compensate for a degradation in synapse efficiency—synapses are where nerve cells take in information from others—though he is quick to point out that's just one guess. Our brains may have to age a bit more before we actually figure it out.
*UPDATE — December 09, 2014: The original version of this post described synapses and axons as different kinds of nerve cells in the brain. In fact, they are different parts of a neuron.