Attachment Stimuli Trigger Widespread Synchrony across Multiple Brains

Infant stimuli elicit widespread neural and behavioral response in human adults and such massive allocation of resources attests to the evolutionary significance of the primary attachment. Here, we examined whether attachment-related cues also trigger cross-brain concordance, generating greater neural uniformity among individuals. Post-partum mothers were imaged twice in oxytocin/placebo administration design and stimuli included four ecological videos; two of infant/mother alone ( Alone ) and two mother-infant dyadic contexts ( Social ). Theory-driven analysis measured cross-brain synchrony in preregistered nodes of the parental caregiving network (PCN), which integrates subcortical structures underpinning mammalian mothering with cortical areas implicated in simulation, mentalization, and emotion regulation, and data--driven analysis assessed brain-wide concordance using whole brain parcellation. Results demonstrated widespread cross-brain synchrony in both the PCN and across the neuroaxis, from primary sensory and somatosensory areas, through insular-cingulate regions, to temporal and prefrontal cortices. The Social context yielded significantly more cross-brain concordance, with PCN’s striatum, parahipporcampal gyrus, superior temporal sulcus, anterior cingulate cortex (ACC), and prefrontal cortex displaying cross-brain synchrony only to mother-infant social cues. Moment-by-moment fluctuations in mother-infant social synchrony, ranging from episodes of gaze aversion to rhythmically-coordinated positive bouts, were tracked online by cross-brain concordance in the pre-registered ACC. Findings indicate that social attachment stimuli, representing evolutionary-salient universal cues that require no verbal narrative for immediate comprehension, trigger substantial inter-brain concordance and suggest that the mother-infant bond, an icon standing at the heart of human civilization, may function to glue brains into a unified experience and bind humans into social groups. Significance Statement Infant stimuli elicit widespread neural response in human adults, attesting to their evolutionary significance, but do they also trigger cross-brain concordance and induce neural uniformity among perceivers? We measured cross-brain synchrony to ecological mother-infant videos. We employed theory-driven analysis, measuring cross-brain concordance in the parenting network, and data-driven analysis, assessing brain-wide concordance using whole-brain parcellation. Attachment cues triggered widespread cross-brain concordance in both the parenting network and across the neuroaxis. Moment-by-moment fluctuations in behavioral synchrony were tracked online by cross-brain variability in ACC. Attachment reminders bind humans’ brains into a unitary experience and stimuli characterized by social synchrony enhance neural similarity among participants, describing one mechanism by which attachment bonds provide the neural template for the consolidation of social groups. ### Competing Interest Statement The authors have declared no competing interest.