An estimated 8 to 15 percent of younger siblings of children with autism are anticipated to be diagnosed with the disorder themselves by about age two. Yet little is known about early developmental processes that differentiate "at-risk" infants and toddlers who go on to develop autism from those who do not. The researchers hypothesize that early indicators of autism may be found in distinct but interrelated features of infants' developing brain systems during their first three years of development. Compared to typically developing infants and toddlers, the researchers suggest, infants who eventually develop autism show early differences in visual pathways which, in turn, lead to later-developing differences in their socially-relevant cognitive skills and their abilities to process emotions conveyed by facial expressions. This junior investigator in autism research will test her hypothesis in studies involving 75 "at-risk" younger siblings of children with autism, and 100 siblings of typically developing children. Participants will be enrolled at age six months, and followed up to age three. Initially, infants' visual pathways will be studied as they respond to luminance (black/white) versus chromatic (red/green) stimuli, and as they track random motion versus socially important motion. At ten months, the infants' visual pathways will be studied as they process objects compared to socially relevant faces. At 18 months, the toddlers' responses to emotional signals from others will be measured, as they are confronted with new toys that their parents respond to either positively or negatively. In these three studies of visual responses to ordinary and emotion-laden stimuli, the researchers will record the brain's electrical responses to events (called "event-related potentials"). These measures will be made using electroencephalography (EEG), which consists of placing small sensors on the scalp to record brain networks of electrical activity generated by responses to specific actions. By age three, participants who develop autism will be identified. The researchers then will, retrospectively, identify visual patterns that differed between those who developed autism compared to those who did not. Significance: If the study reveals that infants and toddlers who eventually develop autism have different patterns of visual system development, these differences could become diagnostic "markers" that could provide for early diagnosis. This could facilitate development and use of therapies that could begin early in the course of the disease.