Similar cognitive processing synchronizes brains, hearts, and eyes

Neural, physiological and behavioral signals synchronize between human subjects in a variety of settings. Multiple hypotheses have been proposed to explain this interpersonal synchrony, but there is no clarity under which conditions it arises, for which signals, or whether there is a common underlying mechanism. We hypothesized that similar cognitive processing of a shared stimulus is the source of interpersonal synchrony, measured here as inter-subject correlation. To test this we presented informative videos to participants in an attentive and distracted condition and subsequently measured information recall. Inter-subject correlation was observed for electro-encephalography, gaze position, pupil size and heart rate, but not respiration and head movements. The strength of correlation was co-modulated in the different signals, changed with attentional state, and predicted subsequent recall of information presented in the videos. There was robust within-subject coupling between brain, heart and eyes, but not respiration or head movements. The results suggest that inter-subject correlation is the result of similar cognitive processing and thus emerges only for those signals that exhibit a robust brain-body connection. While physiological and behavioral fluctuations may be driven by multiple features of the stimulus, correlation with other individuals is co-modulated by the level of attentional engagement with the stimulus.The synchronization of human neural, physiological and behavioral signals happens in a variety of settings. In this work we show that physiological synchrony requires only two things, namely, effective cognitive processing of a common stimulus, and a robust coupling between brain activity and the physiological signal in question. We confirm this theory for heart rate, pupil size, gaze position and saccade rate, as positive examples, and respiration and head movements as negative examples. We show that the strength of this correlation is co-modulated, i.e. correlation is modulated in unison for all signals in which correlation is detected. We propose that this common modulation is the result of attentional engagement with the stimulus.