“…The co-modulation observed here is surprising given the diversity of factors that have been previously proposed to underlie interpersonal physiological synchrony. Leaving out brain signals, this includes emotions (HR [5][6][7]11 ), arousal (HR 4,24 , head velocity 36 ), empathy (HR 6,8,23 , skin conductance 22 ), attention (eye movement 3 , HR 24,25 ), auditory features (HR and respiration 39,40 ) visual features (gaze position 41,42 ), physical movement (respiration 43,44 , HR 21 ), social interaction (HR 45 ), and more. We argue more generally, that cognitive processing of the stimulus is a minimum requirement to evoke any of these constructs in a viewer's mind, and indeed, it suffices to explain the attention and memory effect we observed, namely, without processing the information imparted in the stimulus it is not possible to recall this information, and distracting the viewer will disrupt processing and thus reduce inter-subject correlation.…”