P 146. Motor resonance requires conscious perception but is modulated by unseen actions

Abstract

Evidence for automatic mirror-like activation of the motor cortex (motor resonance) during observation of others’ actions has been found by several studies. Motor resonance has also been found during observation of implied actions, i.e. static pictures depicting ongoing actions. Here we explored whether motor resonance with implied actions requires the perceptual awareness of the observer or is also triggered in conditions of reduced awareness. In order to test our hypothesis, we analyzed motor response to consciously and unconsciously perceived implied actions using a masked priming paradigm. We presented a sequence of 3 static stimuli depicting right hands: sample, masked prime and target stimulus. While the sample stimuli were always static hands, both prime and target stimuli could display a static hand, an index finger or a little finger abduction. The prime stimuli were presented for 53 ms and were forward and backward masked by a rotating masking stimulus, a procedure that allowed a relatively long lasting presentation of the prime while at the same time disrupting conscious perception: indeed, 17 out of 22 subjects reported they did not see the prime when the experimenter revealed the actual presentation structure after the experiment. A transcranial magnetic stimulation (TMS) pulse was delivered over the motor cortex 133 ms (early) or 307 ms (late) after the onset of the probe. Electromyography was used to record TMS motor evoked potentials (MEPs) from the muscles that corresponded to the driving muscle of the observed implied actions, first digital interosseus (FDI) and abductor digiti minimi (ADM). Analysis of the MEPs recorded in the 17 subjects reporting unawareness of the prime replicated the finding of a mirror-like increase in corticospinal excitability following observation of consciously perceived target hands (i.e., greater MEP amplitude during observation of the movement involving the recorded muscle as compared to observation of the static hand or of a movement not involving that muscle). Such facilitation was present only in the late TMS delay. Furthermore, no change in corticospinal excitability was found in response to implied action prime, suggesting that masked actions are not sufficient to evoke motor resonance. Importantly, however, we found a significant interaction between prime and target stimuli. In particular, the amplitude of MEPs recorded from a muscle during observation of a target showing the action driven by that muscle was lower when preceded by a prime showing an incongruent action with respect to when it was preceded by a prime showing a congruent action or a still hand. Crucially, no difference was found between MEPs recorded from the muscle during conscious observation of actions driven by that muscle but primed with an unconsciously perceived incongruent action and MEPs recorded during conscious observation of incongruent or neutral movements regardless of the prime. This finding suggests that the masked presentation of a movement that does not involve a certain muscle yields a somatotopic inhibition of the motor resonance in that muscle. Our results provide first time evidence that conscious perception of implied actions is not required for modulation of motor resonance, yet it is required for a direct mirror motor activation. Moreover, they shed additional light on how the cognitive system copes with a crowded social world and the needs for a fast, accurate and dynamic representation of others’ movements.