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Abstract The current study used functional near‐infrared spectroscopy (fNIRS) to investigate whether and how individual differences in positive social engagement among 5‐month‐old (N= 109;N= 35 final sample) infants relate to variability in functional connectivity in the human brain's Default‐Mode Network (DMN). Neuroimaging results showed that on average infants displayed greater functional connectivity in the right than in the left hemisphere of the DMN, adding to prior work indicating faster connectivity development in the right hemisphere. Results did not show any positive associations between our preregistered measures of positive social engagement and functional connectivity in the DMN. However, an additional analysis revealed that higher levels of infants’ smiling and laughter during daily social interactions with their caregivers positively predicted DMN functional connectivity in the left hemisphere. This suggests that individual differences in connectivity in a long‐range brain network implicated in a host of social and cognitive functions are associated with some aspects of infants’ positive social‐interactive behaviors. 

Functional magnetic resonance imaging (fMRI) studies with adults provide evidence that functional brain networks, including the default mode network and frontoparietal network, underlie executive functioning (EF). However, given the challenges of using fMRI with infants and young children, little work has assessed the developmental trajectories of these networks or their associations with EF at key developmental stages. More recently, functional near‐infrared spectroscopy (fNIRS) has emerged as a promising neuroimaging tool which can provide information on cortical functional networks and can be more easily implemented with young children. Children (N= 207;n= 116 male;n= 167 White) had fNIRS data recorded at infancy, 3, 5, and 7 years of age while watching a 2‐min nonsocial video. At 3, 5, and 7 years, children completed behavioral assessments and parents completed questionnaires to assess child EF abilities. Results showed that, although early functional brain network connectivity was not associated with later functional brain connectivity, EF was concurrently and longitudinally associated with functional connectivity levels in both networks. Overall, these results inform the understanding of early emerging neural underpinnings of regulatory abilities and point to considerable change in the composition of functional brain networks and a conservation of function across development. 

Variability in functional brain network connectivity has been linked to individual differences in cognitive, affective, and behavioral traits in adults. However, little is known about the developmental origins of such brain-behavior correlations. The current study examined functional brain network connectivity and its link to behavioral temperament in typically developing newborn and 1-month-old infants ( M [age] = 25 days; N = 75) using functional near-infrared spectroscopy (fNIRS). Specifically, we measured long-range connectivity between cortical regions approximating fronto-parietal, default mode, and homologous-interhemispheric networks. Our results show that connectivity in these functional brain networks varies across infants and maps onto individual differences in behavioral temperament. Specifically, connectivity in the fronto-parietal network was positively associated with regulation and orienting behaviors, whereas connectivity in the default mode network showed the opposite effect on these behaviors. Our analysis also revealed a significant positive association between the homologous-interhemispheric network and infants' negative affect. The current results suggest that variability in long-range intra-hemispheric and cross-hemispheric functional connectivity between frontal, parietal, and temporal cortex is associated with individual differences in affect and behavior. These findings shed new light on the brain origins of individual differences in early-emerging behavioral traits and thus represent a viable novel approach for investigating developmental trajectories in typical and atypical neurodevelopment.