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Thanks to those early discoveries, Schlaggar says, scientists understand that certain areas of the brain control certain aspects of human function.
Nobody has studied a group of kids with hemispherectomies long term. But researchers have been able to draw some broad conclusions about what Olivia can expect.
The right hemisphere controls the left side of the body. Remove that area of the brain, and a patient's left arm and leg will weaken. Such patients also lose half their field of vision in both eyes. Because she had the right side of her brain removed, Olivia can't see the world to the left.
A child who has had her right hemisphere removed can quickly regain her ability to speak or understand language, but she loses the emotional aspects of language. Subtleties in humor or sarcasm or change in tone may never register with Olivia.
The right hemisphere is also more involved in understanding spatial relationships, says Jordan Grafman, Ph.D., chief of cognitive neuroscience at the National Institute of Neurological Disorders and Stroke in Bethesda, Maryland. Take out the right side of the brain, and that patient is less likely to be a champion chess player or a point guard who is able to anticipate a player slicing to the basket.
Without the right side of the brain, patients may be less at ease in the world because they can't predict what will logically come next in a sequence of events.
"If you're in an office with a door, you already know what's outside that door. It's stored in your brain," Grafman says. "But sometimes, for people who've had surgery on the right side of their brain, they have no access to that knowledge. What's outside the door is of some concern because they don't know what's on the other side until they open it. That elevates anxiety."
But the brain isn't like other organs, such as the heart or the lungs, that serve one function in a predictable way. It has ways of getting around chinks in its system.
And fortunately for kids, their brains are more adaptive.
As strange as it sounds, Olivia was lucky that the stroke damaged 90 percent of her right hemisphere. That allowed her brain to evolve in a different way, cashing in on its plasticity. Schlaggar describes the functions of the right hemisphere as likely able to migrate meaning that some of Olivia's right-brain functions could have moved to her left hemisphere before her surgery.
But Grafman cautions that plasticity can stretch only so far. Something has to give.
"It's like real estate," he says. "Basically, you've got a certain amount, and you have to devote it to something. And when you do, it leaves less room for something else."
He says that Olivia could surprise her family with the skills she'll regain.
"But she's not going to look and be like a child that never had a hemisphere removed. Anybody who suggests otherwise is misleading," he says. Retraining the brain takes years of therapy and countless hours of physical strengthening and repetitive exercises. Olivia took those first steps toward recovery at the Rehabilitation Institute, a brown-brick building at 30th Street and Baltimore in Kansas City, Missouri. More than a year later, kids are following Olivia's example as they recover from hemispherectomies. On a frigid January morning, the tiny lights on Zachary Meier's black tennis shoes blink as he walks with a slight limp into the therapy room. The 6-year-old has a buzz cut, and a soft pink scar etches a letter T through his short blond hair. The incision is still flecked with black stitches.