This study investigates the role that atypical attentional disengagement may have on the emergence of the autism spectrum disorder (ASD) phenotype. Impaired disengagement is the earliest attentional deficit reported in infants at-risk for ASD and may be associated with a later diagnosis of the disorder. Efficient disengagement of attention plays a significant role in arousal regulation and joint attention. Therefore, early aberrant attentional disengagement may result in the atypical development of both of these processes and thus represent a domain-relevant mechanism for the development of higher-level sociocommunicative deficits in ASD. The objective of the research is to determine if abnormal attentional disengagement is a marker for ASD diagnosis and to assess the role that attentional disengagement plays in the development of novelty processing, arousal modulation, and joint attention. The research will continue to track a cohort of 3-5 year old at-risk and typically developing children that have been assessed at 6, 9, 12, 18, 24, and 36 months. Children will complete EEG, eye-tracking, and behavioral/observational measures designed to assess novelty processing, arousal, and joint attention and orienting abilities. The research team will then examine the relationship between the development of disengagement abilities (assessed at earlier visits) and these measures to determine the impact of early attentional function on the development these processes. The training plan will provide the fellow with a unique skill set, including expertise in 1) administering diagnostic assessments and measures of early sociocommunicative and cognitive functioning, 2) use of electrophysiological methods and experimental design used to study infant cognition, and 3) novel analytic approaches to electrophysiological and eye-tracking techniques. The mentorship of Drs. Nelson and Spence and experience obtained from the research will enable the candidate to gain a comprehensive understanding of the emergence of the ASD phenotype and of typical and aberrant brain and behavioral development. These experiences will provide the fellow with r an in-depth understanding of ASD from a dynamic developmental perspective and to translate the knowledge that will support future research on novel diagnostic tools and more efficacious early interventions.