At birth, children with autism had normal-size brains. But by the time they were a year old, the brains of most autistic children had grown far beyond average. The average adult human brain weighs 1,375 grams, but Courchesne encountered one 3-year-old autistic boy whose brain weight was estimated at 1,876 grams.Which would mean, of course, that autism could not have anything to do with vaccines, bad mothering, or any other childhood event.
The MRI scans further revealed that only certain parts of the brain became larger. The growth was striking in the prefrontal cortex, the region just behind the eyes that is responsible for language, decisions, and other sophisticated thinking. Courchesne also saw an increase in both the gray matter (consisting of dense clusters of neurons) and the white matter linking different regions of the brain. This explosive neural expansion continued in many autistic children until the age of 5, and then it stopped. Past that age, Courchesne found, the rate of brain growth slowed in autistic children, falling behind that of ordinary children. By the teen years, some brain regions actually started to shrink.
Over the past two decades, Courchesne has replicated these results in three additional sets of brain scans. And he has moved beyond MRI, working with tissue banks at institutions like the National Institutes of Health, which stores donated brains. Working with the brains of six normal children and seven autistic children ages 2 to 16, most of whom died of drowning, Courchesne has studied neurons under the microscope and even counted the number of neural cells in different tissue samples. Last November he reported the first results: On average, autistic brains had many more neurons in some regions than normal brains. In the prefrontal cortex, autistic children had 67 percent more neurons than average. . . .
These results provide insight into the origin of autism. During the second trimester of pregnancy, the precursors to neurons in the brain divide furiously. Then they almost all stop, well before birth. When the brain gets bigger after delivery, all that is happening is that the individual neurons are growing and sprouting branches. The only time autistic children can get their extra neurons, in other words, is while they are in the womb. “We established a time zone,” Courchesne says.
Zimmer makes optimistic noises about how this knowledge could be used to treat autistic infants, but I think it looks pretty bad. If autism is such a fundamental problem of brain wiring, what could anyone do about it?