This increased quantity of nerve cells must result from something that occurred prenatally, like an infection or genetic abnormality


A small study looking at the brains of children with autistic spectrum disorder who have died has shown that the autistic children have more neurons in a certain region of the brain than their normally-developing peers. This region, called the prefrontal cortex, is known to be involved in social, emotional, and thinking skills.

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Since the number of cortical neurons is determined before birth, this increased quantity of nerve cells, the researchers maintain that it must have resulted from something that occurred prenatally. An infection, a toxic exposure, a genetic abnormality, a dysregulation of cell growth, or other mechanism may be the agent responsible.

In a painstaking process, the researchers studied the brains of seven autistic children and six non-autistic children who had died. They measured the weights of the brains, examined the brain tissue, and counted the neurons in the brain regions of interest. The diagnosis of autism in the deceased children was established by a rigorous questionnaire administered to a parent or guardian of the child.

The brains of the autistic children were on average 17 percent heavier than brains of typically-developing, same-aged peers. Looking at the two main areas of the prefrontal cortex, the researchers also found there were 79 percent more neurons to the sides in the back (dorsolateral area) of the prefrontal cortex of autistic children compared to the controls. There were 29 percent more neurons in the middle (medial) area of the prefrontal cortex. These are substantial differences. These same brain regions are larger in living autistic children when their brains are examined by MRI scans.

During fetal brain development, brain cells proliferate and then die off in a programmed way, in a process called apoptosis. If too many brain cells develop, or if not enough die off, the result is an excess of neurons at birth. The location, number, and type of extra cells will determine the child's clinical appearance and developmental outcome.

This new research sheds light on a prenatal brain growth abnormality in the prefrontal cortex that seems to be associated with autism. This information supports previous investigations that have found differences in head size and brain weight in autistic children and may help focus future studies on the causes of autism. The study was published in the November 9 issue of the Journal of the American Medical Association (JAMA).

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