In the brain, nerve cells transmit important messages that regulate body functions -- everything from social behavior to movement. Imaging studies have revealed that autistic children have too many nerve fibers, but that they're not working well enough to facilitate communication between the various parts of the brain. Scientists think that all of this extra circuitry may affect brain size. Although autistic children are born with normal or smaller-than-normal brains, they undergo a period of rapid growth between ages 6 and 14 months, so that by about age four, their brain tends to be unusually large for their age. Genetic defects in brain growth factors may lead to this abnormal brain development.

Scientists also have discovered irregularities in the brain structures themselves, such as in the corpus callosum (which facilitates communication between the two hemispheres of the brain), amygdala (which affects emotion and social behavior), and cerebellum (which is involved with motor activity, balance, and coordination). They believe these abnormalities occur during prenatal development.

In addition, scientists have noted imbalances in neurotransmitters -- chemicals that help nerve cells communicate with one another. Two of the neurotransmitters that appear to be affected are serotonin(which affects emotion and behavior) and glutamate (which plays a role in neuron activity). Together, these brain differences may account for autistic behaviors.

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