Fair play doesn't matter: MEP modulation as a neurophysiological signature of status quo bias in economic interactions

Transcranial magnetic stimulation (TMS) studies show that watching others' movements enhances motor evoked potential (MEPs) amplitude of the muscles involved in the observed action (motor facilitation, MF). MF has been attributed to a mirror neuron system mediated mechanism, causing an excitability increment of primary motor cortex. It is still unclear whether the meaning an action assumes when performed in an interpersonal exchange context could affect MF.This study aims at exploring this issue by measuring MF induced by the observation of the same action coupled with opposite reward values (gain vs loss) in an economic game. Moreover, the interaction frame was manipulated by showing the same actions within different economic games, the Dictator Game (DG) and the Theft Game (TG). Both games involved two players: a Dictator/Thief and a receiver. Experimental participants played the game always as receivers whereas the Dictator/Thief roles were played by our confederates. In each game Dictator/Thief's choices were expressed by showing a grasping action of one of two cylinders, previously associated with fair/unfair choices. In the DG the dictator decides whether to share (gain condition) or not (no-gain condition) a sum of money with the receiver, while in TGs the thief decides whether to steal (loss condition) or not to steal (no-loss condition) it from the participants. While the experimental subjects watched the videos showing these movements, a single TMS pulse was delivered to their motor hand area and a MEP was recorded from the right FDI muscle.Results show that, in the DG, MF was enhanced by the status quo modification, i.e. MEP amplitude increased when the dictator decided to change the receivers' status quo and share his/her money, and this was true when the status quo was more salient. The same was true for the TG, where the reverse happened: MF was higher for trials in which the thief decided to steal the participants' money, thus changing the status quo, in the block in which the status quo maintenance occurred more often. Data support the hypothesis that the economic meaning of the observed actions differently modulates MEP amplitude, pointing at an influence on MF exerted by a peculiar interaction between economic outcomes and variation of the subjects' initial status quo. © 2014 Elsevier Inc.