Project Summary/Abstract Many studies utilize resting-state functional magnetic resonance imaging (rsfMRI) metrics, such as functional connectivity (FC), to study the neural underpinnings of autism and identify functional brain networks related to autistic behaviors. However, these findings remain inconsistent, with reports of underconnectivity and overconnectivity in multiple resting-state networks. FC measures the temporal correlation between the blood oxygen level dependent (BOLD) signal of distinct brain regions. In fMRI, the BOLD signal indirectly measures neuronal activity by detecting changes in oxygenated blood flow, which occurs through local blood vessel dilation. In addition to neuronal activity, changes in brain vascular function can affect local oxygenated blood flow, and thus the BOLD signal within a region and FC between regions. Characterizing these cerebrovascular effects on FC is critical to understanding neuronal and vascular sources of connectivity in populations with potential cerebrovascular heterogeneity, such autism or early development. In this study, we utilize a novel, non- invasive method to characterize cerebrovascular reactivity (CVR), the vasodilatory capacity of cerebral blood vessels, in autistic and non-autistic children, based solely on rsfMRI BOLD signals (Aim 1). We then use this novel information to investigate the mechanism linking vascular function to FC, using the amplitude of low frequency fluctuations (ALFF), a resting-state measure of local BOLD amplitude that has been associated with both neuronal and vascular physiology (Aim 2). Using data from the Autism Phenome Project (APP), a longitudinal neuroimaging program that specializes in early age scanning (2-12 years old) in autistic and non- autistic children, we will compare age-trajectories of CVR and its effect on FC in autistic and non-autistic children starting from 2 years old. In acquiring and analyzing data from young autistic and non-autistic participants, I will receive unique clinical and neuroimaging training, specifically in autism, to support my career as an independent investigator, developing novel neuroimaging biomarkers to characterize cerebral physiology in autism. Ultimately, providing insight into cerebrovascular differences and enhancing FC interpretations in autism, starting from early age, will help disentangle its heterogeneity to inform meaningful, early interventions.