Near Infrared Spectroscopy in Children With Autism and ADHD

Background: \- Near-infrared spectroscopy (NIRS) is a functional imaging technique that can be uses light to study brain function while allowing for movement. To look at blood flow in the brain, NIRS uses a low-power light source with detectors that see how the light changes as it passes through brain tissue. Brain blood flow can indicate which parts of the brain are active during different tasks. Researchers want to study children with attention deficit hyperactivity disorder (ADHD) and autism spectrum disorders (ASD) and will use NIRS to compare the blood flow in the brain of children with ADHD and ASD with that of typically developing children. Objectives: * To see how well NIRS can detect changes in brain blood flow during tests of thinking and memory in children. * To compare blood flow in the brains of typically developing children and those with ADHD or ASD. Eligibility: \- Children between 4 and 8 years of age with ASD, ADHD, or children with no psychiatric diagnoses. Design: * Participants will be screened for eligibility. Those who are taking stimulant medication for ADHD or ASD will need to stop taking it for 3 days before the study visit. * After participating in a screening assessment, all participants will have one study visit. At this visit, they will have be asked to complete two tasks during a NIRS scan. For both tasks, they will react to images on a computer screen. This visit will last about 2 hours. * This is a testing study only. No blood or other samples will be needed for this study.