TPJ-M1 interaction in the control of shared representations: New insights from tDCS and TMS combined

There is extensive evidence that perceived and internally planned actions have a common representational basis: action observation can induce an automatic tendency to imitate others. If perceived and executed action, however, are based on shared representations, the question arises how we can distinguish self-related and other-related representations. It has been suggested that the control of shared representations involves a neural network centered on the temporo-parietal junction (TPJ). However, the specific role of the TPJ in controlling shared representations is still not clear. In a conflict situation where participants have to execute action A while observing action B, the TPJ might either facilitate the relevant action A or inhibit the irrelevant action B (mirror response). In the present study, we used transcranial Direct Current Stimulation (tDCS) to condition neural activity in the right temporo-parietal junction (TPJ). We then analyzed the corticospinal output as indexed by motor-evoked potentials (MEPs) induced by single-pulse TMS (spTMS) of the left primary motor cortex (M1) during action observation in the context of a conflict task. Results showed that tDCS-mediated increased control did not entail the attenuation of the task-irrelevant response activation: the effect of motor mirroring was not suppressed or reduced. Rather, facilitating TPJ activity via anodal tDCS selectively enhanced the instructed motor plan (self-related representation). This outcome supports the idea that TPJ plays a critical role in detecting the mismatch between self-related and other-related representations and is at work to enhance task-relevant representations.