Neuromodulation of the Left Inferior Frontal Cortex Affects Social Monitoring during Motor Interactions

Motor interactions require observing and monitoring a partner’s performance as the interaction unfolds. Studies i n monkeys suggest that thi s form of soci al moni tori ng might be mediated by the activity of the ventral premotor cortex (vPMc), a critical brain region in action observation and motor planning. Our previous fMRI studies in humans showed that the left vPMc is indeed recruited during social monitoring, but its causal role is unexplored. In three experiments, we applied online anodal or cathodal transcra-nial direct current stimulation over the left lateral frontal cortex during a music-like interactive task to test the hypothesis that neuromodulation of the left vPMc affects participants’ performance when a partner violates the agent’s expectations. Participants pl ayed short musi cal sequences together with a virtual partner by playing one note each in turn-taking. In 50% of the trials, the partner violated the participant’s expectations by generating the correct note through an unexpected movement. During sham stimulation, the partner’s unexpected behavior led to a slowdown in the participant’s performance (observation-induced posterror slowing). A significant interaction with the stimulation type showed that cathodal and anodal transcranial direct current stimulation induced modulation of the observation-induced posterror slowing in opposite directions by reducing or enhancing it, respectively. Cath-odal stimulation significantly reduced the effect compared to sham stimulation. No effect of neuromodul ati on was found when the partner behaved as expected or when the observed violation occurred within a context that was perceptual l y matched but noni nt eracti ve i n nature. These results provide evidence for the critical causal role that the left vPMc might play in social monitoring during motor interactions, possibly through the interplay with other brain regions in the posterior medial frontal cortex.