Sick and detached: Does experimental inflammation impact on movement synchrony in humans?

Interpersonal connectedness is a central feature of human interaction that can be compromised during illness. Nonverbal signals play a crucial role in this context, and humans, like other animals, have evolved a behavioral immune system that enables individuals to detect subtle cues of sickness in others. Conversely, sick individuals often tend to avoid social interaction, a key component of sickness behavior. The coordination of body movements between two individuals (movement synchrony) is a measure of the quality of relationships that could provide insights into an interlocutor's sickness state. In the present study, we explored the effect of lipopolysaccharide (LPS) administration, a naturalistic stimulus for inflammation-induced sickness, on movement synchrony in healthy volunteers randomly assigned to a double-blind interview with a non-treated interviewer conducted 2.5 h after intravenous injection of either LPS (N = 26) or placebo (N = 25). Movement synchrony was assessed by automated video analysis of subject's and interviewer's head movements. Lagged cross-correlations were used to objectively quantify coordination in dyads and to assess patterns of temporal movement synchronization. Data analysis revealed that dyads with subjects under placebo displayed a pattern of movement coordination comparable to that seen in previous studies. However, dyads with subjects under LPS showed a loss of simultaneous movement (i.e. moving at the same time) with the interview partner, which is normally the temporal domain providing the highest level of synchrony. Together, the findings suggest that immediate social interaction is attenuated when one interlocutor is exposed to systemic inflammation, while the other is unaffected. This effect can be attributed to both sickness behavior on one hand and correlates of the behavioral immune system on the other hand.