Frontal and Parietal Influences on the Corticospinal Mapping of Observed Actions: A 1-Hz rTMS Investigation

Transcranial Magnetic Stimulation (TMS) studies in humans indicate that the mere observation of an action can increase amplitude of motor-evoked potentials (MEPs) recorded from the muscles that would be recruited during the actual execution of the very same action1. This phenomenon may be functionally akin to the “mirror” activation of premotor neurons in the monkey and be related to mental simulation of actions2. Previous TMS3 and neuroimaging4 studies indicate that comparable motor “mirror” activations – primary motor cortex (M1), inferior frontal gyrus (IFG) – are observed during the view of both bio-mechanically possible and impossible actions. The observation of impossible body movement seems to be linked to the activation of sensorimotor parietal regions and may even include primary somatosensory cortex (S1). Although mirror activations may be inherently linked to mental simulation of motor and/or somatosensory components of action, the causative role of motor and somatic areas in modulating the corticospinal facilitation during observation of others’ actions has not hitherto been explored. In the present study, we used low-frequency repetitive-TMS (1 Hz rTMS) with the aim of inhibiting M1, IFG, and S1. MEPs to focal single-pulse TMS over M1 during the observation of bio-mechanically possible and impossible finger movements were recorded in a baseline condition and after 1 Hz rTMS (lasting 15 minutes) over S1, M1 and IFG. In the baseline condition, we found a significant MEP facilitation that was specific for the muscle involved in the observed movement and comparable for possible and impossible movements3. A similar pattern was observed after rTMS over M1. By contrast, MEPs facilitation was significantly disrupted by rTMS over the S1 and IFG. rTMS over IFG mostly reduced MEP facilitation contingent upon the observation of biologically possible action. rTMS over S1 mainly reduced MEP facilitation induced by the observation of impossible action. These results indicate that IFG and S1 inhibition reduces “mirror” responses to observed body movements. Moreover they strongly suggest that IFG and S1 may be involved in different aspects of the “mirror” corticospinal mapping of bio-mechanicallly possible and impossible body movements