Autism is a neurodevelopmental disorder characterized by impairments in reciprocal social interaction, deficits in verbal and nonverbal communication, and a restricted repertoire of activities or interests. We performed a magnetic resonance imaging study to better define the neuropathology of autistic spectrum disorders. Here we report findings on the amygdala and the hippocampal formation. Borders of the amygdala, hippocampus, and cerebrum were defined, and their volumes were measured in male children (7.5-18.5 years of age) in four diagnostic groups: autism with mental retardation, autism without mental retardation, Asperger syndrome, and age-matched typically developing controls. Although there were no differences between groups in terms of total cerebral volume, children with autism (7.5-12.5 years of age) had larger right and left amygdala volumes than control children. There were no differences in amygdala volume between the adolescent groups (12.75-18.5 years of age). Interestingly, the amygdala in typically developing children increases substantially in volume from 7.5 to 18.5 years of age. Thus, the amygdala in children with autism is initially larger, but does not undergo the age-related increase observed in typically developing children. Children with autism, with and without mental retardation, also had a larger right hippocampal volume than typically developing controls, even after controlling for total cerebral volume. Children with autism but without mental retardation also had a larger left hippocampal volume relative to controls. These cross-sectional findings indicate an abnormal program of early amygdala development in autism and an abnormal pattern of hippocampal development that persists through adolescence. The cause of amygdala and hippocampal abnormalities in autism is currently unknown.

Historically, there have been numerous proposals that the size of the brain correlates with its capacity to process information. Little is known, however, about which specific brain regions contribute to this correlation in children and adolescents. This study evaluated the relationship between intelligence and the size of various brain structures in typically developing male children 8-18 yrs of age. Magnetic resonance imaging (MRI) scans were used to measure the volume of the cerebrum, cerebral gray and white matter, cerebellum, amygdala, and hippocampus. Gray matter and hippocampal volume significantly correlated with full scale and verbal IQ. Since the hippocampus strongly correlated with verbal but not performance IQ, our findings reinforce the hypothesis that the hippocampus is involved in declarative and semantic learning, which contributes more notably to verbal IQ, than to performance IQ. Given the substantial evidence for environmentally induced changes in hippocampal structure, an unresolved issue is whether this relationship reflects genetically determined individual variation or learning induced plasticity.