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What does brain imaging show in people with autism?

Alexander Kolevzon, MD
Adolescent Medicine
Over the last 20 years, neuroimaging studies have made great contributions to our understanding of the neurobiological underpinnings of autism spectrum disorders (ASD). Using structural magnetic resonance imaging (MRI), the most replicated finding so far is increased brain volume in early childhood. Most studies suggest that the brain is enlarged by two years of age, but normal or reduced in size by adolescence and adulthood. Several specific brain regions may also be disproportionately affected in ASD, including the amygdala, the emotional center of the brain. Another type of brain imaging study uses diffusion tensor imaging (DTI), a technique that examines the integrity of white matter fibers in the brain which reflects the extent of myelinization. Myelin fibers surround neurons and help with nerve cell communication; most DTI studies in children and adults with ASD have found evidence of lower coherence and organization of fiber tracts in a variety of regions. Finally, functional magnetic resonance imaging (fMRI) examines brain activation patterns by studying blood oxygen level and blood flow. Abnormalities in the function of regions considered to be part of the “social brain,” such as the medial prefrontal cortex, orbitofrontal cortex, amygdala, fusiform gyrus, and superior temporal sulcus, are most consistent findings in the literature. These studies suggest that ASD is associated with a lack of specialization in systems evolved for social attunement that are normally automatically engaged.
Brain imaging, including functional scans, and techniques such as diffusion tensor imaging (for examining longer myelinated tracts) paint a picture of brains of individuals with autism spectrum disorder (ASD) who have overdeveloped local connections but deficits in the pathways that connect and integrate cortical systems. Individuals with ASD focus on details (such as looking at mouth movements rather than eyes and facial expressions). They often are overwhelmed by specific sensory inputs and are unable to regulate and integrate information to make social judgments (focusing on the mechanical elements of a toy is much easier for them than figuring out social signals emanating from a school cafeteria).
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