This research project will pinpoint the structural factors that contribute to the enlargement of white matter in the prefrontal cortex in autism. Dr. Barbas' fellow will examine post-mortem tissue from autistic and control individuals for differences in axon density, axon size, myelination, and number of non-neuronal cells that surround axons ("glia"). A second goal of this project is to develop the use of non-invasive imaging of white matter so that future studies will not have to rely on post-mortem tissue. As a first step, the prefrontal pathways in rhesus monkeys will be mapped. These pathways will initially be scanned using non-invasive diffusion tensor imaging (DTI); subsequently, these pathways will be labeled via injections of tracers into the same brains. Information from both DTI and tracer injections will then be compared to help optimize non-invasive scanning and image analysis protocols for future use in humans.