Mirroring multiple agents: motor resonance during action observation is modulated by the number of agents

Abstract

Although social situations regularly involve multiple persons acting together, research on the mirror neuron system has focused on situations in which a single agent is observed. Therefore, the goal of the current study was to explore the role of the mirror mechanism in situations involving multiple agents. Specifically, we used transcranial magnetic stimulation (TMS) to investigate whether mirror activation is modulated by the number of observed agents. Based on group contagion research, we hypothesized that multiple agents would provide a stronger trigger to the motor system and would therefore produce a stronger mirror response than a single agent. Participants observed movements performed by a single hand or by two hands while TMS was applied to the primary motor cortex. The results confirmed that activation in the motor system was stronger for two hands. This suggests that input to the motor system increases as the number of agents grows. Relating back to group contagion, our study suggests that groups may be more contagious simply because their actions resonate louder. Given that the mirror mechanism has been linked to a variety of social skills, our findings additionally have important implications for the understanding of social interaction at the group level.

Keywords: TMS; action; imtation; mirror neuron; observation.

Fig. 1.

Design of the experiment. TMS was applied over the left primary motor cortex while participants observed two hands. Either a single hand made a movement or both hands made an identical movement. Two static hands were used as a baseline condition.

Fig. 2.

TMS results of the experiment. The two lines depict the MEP amplitude relative to the static hands condition in the action relevant muscle and in the action irrelevant muscle. The action relevant muscle was defined as the muscle involved in executing the observed movement and the action irrelevant muscle as the muscle not involved in executing the observed movement. Error bars represent standard errors of the mean corrected for within-subject designs according to Morey (2008). The TMS results for the two separate muscles are available in supplementary material (Supplementary Figure S1).