A Biologically Based Disease
Whatever the actual causes of autism may turn out to be, it appears beyond question at this time that autism itself is a fundamentally biologically based disease. This finding has been borne out by many independent research results, each identifying distinctive biological features that differentiate people with autism from others. For example, people with autism show a distinctive pattern of neurological development compared to typical people when considered as a population. Early in the developmental process, children with autism's head circumference is unusually small in comparison to typical children. Later in the developmental process, however, their head circumference turns out to be significantly larger than children without autism. Brain differences are not simply superficial either. Neuroimaging studies of male brains with autism show that particular brain areas, including the temporal, parietal and occipital lobes, are significantly larger than corresponding areas in the brains of those without autism. Other brain areas, such as the frontal lobes, are not distinct, however, and none of these findings held up for female patients with autism.
More fine-grained analysis of autistic brains revealed structural abnormalities of the frontal lobe and the corpus callosum in some patients. The structural abnormalities found in the frontal lobe involve the area of the brain known as the amygdala. The amygdala is associated with the individual's ability to process emotions and social cues. Lesions on the amygdala interfere with the individual's ability to put emotional meaning to sensory input. A baby who has damage to this area of the frontal lobe may not find being held by his mother to be comforting. In some studies, monkeys who have damage in the amygdala region of the brain engage in self-stimulatory behaviors and demonstrate social disturbances similar to those found in humans with autism. Similarly, functional neuroimaging (which reveals how hard different parts of the brain work as opposed to how large they are) has revealed abnormalities in blood flow in the prefrontal cortex and the right medial temporal lobe of brains with autism.
There even appear to be differences in how brains with autism and without metabolize (digest) various proteins. Apparently, individuals with autism tend to have elevated levels of glutamate (a hormone and neurotransmitter) in their blood. Studies have found that they have reduced levels of glutamic acid decarboxylase 65 and 67 kDa, which may account for this abnormally high levels of glutamate.