Eric Courchesne managed to find a positive thing about getting polio: It gave him a clear idea of what he would do when he grew up. Courchesne was stricken in 1953, when he was 4. The infection left his legs so wasted that he couldn’t stand or walk. “My mother had to carry me everywhere,” he says. His parents helped him learn how to move his toes again. They took him to a pool to learn to swim. When he was 6, they took him to a doctor who gave him metal braces, and then they helped him learn to hobble around on them. Doctors performed half a dozen surgeries on his legs, grafting muscles to give him more strength.

Courchesne was 11 when the braces finally came off, and his parents patiently helped him practice walking on his own. “Through their encouragement, I went on to have dreams beyond what you’d expect,” he says. He went to college at the University of California, Berkeley. One day he stopped to watch the gymnastics team practicing, and the coach asked him to try out. Before long Courchesne was on the team, where he won the western U.S. championship in still rings.

When Courchesne wasn’t competing at gymnastics, he was studying neuroscience. “I understood a neurological disorder firsthand, and I wanted to help other children,” he says. Fortunately, the polio outbreak that snared him in 1953 was the last major one in the United States; a vaccine largely eliminated the disease in this country. But in the mid-1980s, as a newly minted assistant professor of neuroscience at the University of California, San Diego, Courchesne encountered a 15-year-old with another kind of devastating neurological disorder: autism.

At the time, Courchesne was investigating how children’s brains respond to new pieces of information. “I encountered a clinical psychologist who studied children with autism,” he says. “She told me, ‘Autistic children aren’t interested in novelty. They’re interested in routine.’ ” Yet the young man Courchesne met showed more range. At first he responded to Courchesne’s questions only with short answers, “but when I talked with him further, I discovered he had a tremendous wealth of knowledge,” the neuroscientist recalls. “He had calendar memory. He just wasn’t interested in being sociable.”

Autism had cut the boy off from the social world, Courchesne realized. “I could see his loneliness, and I could see his parents’ heartache,” he says. He could also see that the boy’s parents refused to give up on him, in the same way his parents had refused. “As they say, that was it,” he says. He swung his entire career toward autism.

In the three decades since, autism has gone from obscurity to painful familiarity. The Centers for Disease Control and Prevention estimates that about 1 in 110 children in the United States are autistic. Yet the disorder remains enigmatic. “Every turn of my research has been about figuring out how this thing began,” Courchesne says. Gradually he built up a picture of the autistic brain from infancy to adulthood, zeroing in on a crucial distinction between those who have autism and those who don’t.

As they develop, autistic brains bloom with an overabundance of neurons, Courchesne finds. It might sound like bad news, implying that autism is rooted in such a fundamental change to the structure of the brain that there’s no hope of undoing it. But Courchesne says his findings could lead to key treatments in years to come.

Back when Courchesne began his work, the notion of a neuroscientist studying autism seemed a bit odd. Many researchers considered the disorder a psychological problem, perhaps the result of bad mothering. “It was a medieval way of thinking,” Courchesne says. As time went on, he became convinced that autism was not only a neurological disorder but more specifically a developmental disease that altered the structure of the nervous system as it matured.

Scientists had done a few anatomical studies on the autistic brain, but the results were ambiguous. Even normal brains can vary enormously in size and structure, so it was hard to see what, if anything, set autistic brains apart. To push past this confusion, Courchesne needed to look at a much larger sample of brains.

In 1988 he sought out parents of autistic children and got their permission to have the children lie in MRI scanners so he could take high-resolution anatomical pictures of their brains. Then he used computers to mark the boundaries of different brain regions and estimate their volume. The subjects spanned a wide range of ages, from adults down to toddlers as young as 2. Courchesne did not scan infants, but he went back through medical records to look at the circumference of the heads of his volunteers since birth.

Courchesne hoped to find something, anything, that set the autistic subjects apart. “We didn’t know what it might be or where it might be found,” he says. “We didn’t know if it would come on in the youngest stages or older. It was wide open.”