When emulation becomes reciprocity.

It is well known that perceiving another's body movements activates corresponding motor representations in an observer's brain. It is nevertheless true that in many situations simply imitating another's actions would not be an effective or appropriate response, as successful interaction often requires complementary rather than emulative movements. At what point does the automatic tendency to mirror another's actions become the inclination to carry out appropriate, complementary movements? In the present study, single-pulse transcranial magnetic stimulation (TMS) was used to explore corticospinal excitability in participants observing action sequences evoking imitative or complementary movements. TMS was delivered at five time points corresponding to different moments in time when key kinematic landmarks characterizing an observed action occurred. A variation in motor evoked potentials (MEPs) confirmed that the motor system flexibly shifts from imitative to complementary action tendencies. That shift appears to take place very precociously in time. Observers are attuned to advance movement information and can use it to anticipate a future course of action and to prepare for an appropriate, complementary action. Altogether, these findings represent a step forward in research concerning social action-perception coupling mechanisms providing important data to better understand the role of predictive simulation in social contexts.