Social Smiling and Laughter Are Linked to Enhanced Functional Brain Connectivity in Young Infants' Default Mode Network.

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.