Neural selectivity for social interactions in the infant brain

Social cognition abilities develop rapidly in the second year of life, yet even in the first months, infants show sophistication in representing social events, such as dyadic interactions. The neural structures supporting these early capacities remain unknown. Human neuroimaging has identified a posterior occipitotemporal pathway, originating in the visual cortex, that responds selectively to interacting (vs. non-interacting) people by representing features (e.g., spatial relations) relevant to social event categorization. We asked whether such neural selectivity is present in preverbal infants, as a potential neural mechanism for early social perception. Using functional near-infrared spectroscopy (fNIRS), we recorded brain activity in 6- and 10-month-old infants, as well as adults, while they viewed point-light displays of interacting (face-to-face) or non-interacting (back-to-back) dyads. In adults, interacting dyads yielded increased activity in posterior superior temporal sulcus (pSTS) and adjacent occipitotemporal and temporoparietal regions, replicating established neuroimaging findings. In infants, the same contrast engaged a similar network. No reliable difference emerged between 6- and 10-month-olds, though the effect was less widespread in the younger group, with a peak around pSTS. This social-interaction effect was distinct from that of body motion perception, suggesting separate mechanisms for perceiving general biological agents versus interacting agents. These findings demonstrate that, early in life, the human brain is tuned to visual cues of social engagement. We propose that early processing of human social relationships relies on a perceptual cue-based mechanism for recognizing interactions from relational properties in the stimulus, such as face-to-face interpersonal orientation.