"The brain was constructed to change," asserts Michael M. Merzenich as he sits in a small conference room at the University of California at San Francisco Medical Center. The large windows to his left look out onto a hill thick with eucalyptus trees, their branches moving now this way, now that, in the morning's wind. Merzenich's observation--no longer so radical as it was when he and a handful of others put it forth in the 1980s--is that the brain does the same: it moves this way, then that, depending on how experience pushes it. This may seem an obvious idea: of course our brains revise themselves--we learn, after all. But Merzenich is talking about something bigger: this ability of the brain to reconfigure itself has more dramatic implications.

It is as if the brain is a vast floodplain. One year the water might run eastward in a series of small channels; the next it might cut a river deep through the center. A year later, and a map of the floodplain looks completely different: streams are meandering to the west. It is the same with a brain, the argument goes. Change the input--be it a behavior, a mental exercise, such as calculating a tip or playing a new board game, or a physical skill--and the brain changes accordingly. Magnetic resonance imaging machines reveal the new map: different regions light up. And Merzenich and others who work in this field of neuroplasticity are not just talking about young brains, about the still developing infant or child brain, able to learn a first language and then a second in a single bound. These researchers are describing old brains, adult brains, your brain.

This article was originally published with the title The Mutable Brain.

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