Action observation : does grasping of emotional objects modulate motor resonance?

In the present study we investigate whether and how the emotional valence of objects to be grasped modulates motor resonance, the subliminal reproduction of a motor program induced by action observation. Utilizing emotionally priming stimuli, other studies have shown that motor responses during action execution or observation are amplified more effectively by negative stimuli, suggesting an arousal effect rather than the modulation of a specific motor program. Subjects (n=11) observed videos showing the same action, performed by an avatar’s hand, grasping either a ball (neutral object), or chocolate muffin (pleasant), or a large spider (unpleasant). Stimuli were previously selected based on ratings from an anonymous survey. Motor-evoked potentials (MEPs) were elicited, at different delays during the observed grasping (start, open-hand, close-hand), by a single transcranial magnetic stimulation of the hand area in the left primary motor cortex and recorded from the right Opponens Pollicis (OP). Neutral and pleasant stimuli produced in the OP a typical pattern of activation consistent with its functional role during grasping, i.e. larger MEP amplitudes during observation of the hand-closing phase. The unpleasant stimulus did not evoke a motor resonant response. However, when subjects were divided in two groups (“fight” or “flight”) based on a request to list actions evoked by the sight of the spider, differences appeared. In “fight” subjects (n=3), MEPs were larger at all delays, while in “flight” subjects (n=8), MEP amplitude was not modulated. We conclude that object emotional valence modulates motor resonance differentially: observation of grasping of pleasant and neutral objects evoked true motor resonant responses, reflecting an appropriate subliminal motor program; responses to unpleasant objects were influenced by subjective emotional reactions; the overall activation of hand motor circuits during observation of grasping of unpleasant objects in “fight” subjects revealed a state of action readiness, rather than a specific resonant response.