Examining Variability in Positive Social Engagement and Functional Brain Connectivity in the Default Mode Network in Young Infants

Positive social engagement relies upon the motivation to attend to, take pleasure in, and
maintain social interactions. The default mode network (DMN) comprises superior temporal and
medial prefrontal brain regions previously shown to be involved in infants’ processing of
emotionally expressive and smiling faces in the first year, and longitudinally predicting increased
sociability in the second year. It is currently unknown whether and how positive social
engagement in young infants relates to variability in functional connectivity in the DMN. The
current study tested the pre-registered hypothesis that variability in positive social engagement
predicts functional connectivity in the DMN among 5-month-old infants using functional near
infrared spectroscopy (fNIRS). Neuroimaging results showed that infants displayed greater
functional connectivity in the right than in the left hemisphere of the DMN, indicating faster
long-range connectivity development in the right hemisphere. Contrary to our prediction, there
were no significant associations between positive social engagement and DMN functional
connectivity in 5-month-old infants. However, an exploratory analysis showed that higher levels
of infants’ smiling and laughter during daily interactions with their caregivers positively
predicted DMN functional connectivity in the left hemisphere. This suggests that, already by 5
months of age, infants’ functional connectivity in a brain network implicated in a host of social
and cognitive functions, is associated with variability in infants’ positive affect displayed during
social interactions in close relationships.