Stimulus-driven modulation of motor-evoked potentials during observation of others' pain.

Empathy may allow interindividual sharing not only of emotions (e.g., joy, sadness, disgust) but also of sensations (e.g., touch, itching, pain). Although empathy for pain may rely upon both sensory and affective components of the pain experience, neuroimaging studies indicate that only the affective component of the pain matrix is involved in empathy for pain. By using transcranial magnetic stimulation (TMS), we highlighted the sensorimotor side of empathy for pain by showing a clear motor inhibition during the mere observation of needles penetrating body parts of a human model. Here, we explored stimulus-specific and instruction-specific influences on this inhibition by manipulating task instructions (request to adopt first- or third-person perspective vs. passive observation) and painfulness of the experimental stimuli (presentation of videos of needles deeply penetrating or simply pinpricking a hand). We found a significant reduction in amplitudes of motor-evoked potentials (MEPs) specific to the muscle the subjects observed being penetrated that correlated with the intensity of the pain attributed to the model. Crucially, this motor inhibition was present during observation of penetrating but not of pinpricking needles. Moreover, no MEPs modulation contingent upon different task instructions was found. Results suggest that the motor inhibition elicited by the observation of 'flesh and bone' pain stimuli is more stimulus-driven than instruction-driven.