| It has been suggested that physical similarity with an observed model facilitates action perception and understanding. For example, increased corticospinal excitability is found in participants observing actors of their own ethnicity performing culture-specific hand movements (Molnar-Szakacs et al., 2007). Tactile perception is also said to be increased when individuals watch a model of the same race being touched (Serino et al., 2009). Moreover, imaging data suggest that stronger activations are observed in the cingulate cortex when a subject observes a person of their own race feeling pain (Xu et al., 2009). Some studies have linked these findings with a motor resonance mechanism, possibly associated with the mirror neuron system (MNS), suggesting that action representation in motor areas is facilitated by physical similarity. However, most of the observed stimuli in those studies include emotional or cultural components, which may blur the link between physical similarity and motor resonance per se.;The present thesis is comprised of three studies that aimed at evaluating the effect of physical similarity on motor resonance using stimuli that are purely motor in nature. The effect of physical similarity on motor responses during action observation was assessed with behavioral and electrophysiological measures. To this end, imitation of simple finger movements, as well as corticospinal excitability and mu rhythm desynchronization during passive observation of simple finger movements was evaluated, using stimuli that were similar or dissimilar to the participant in terms of skin color. In line with previous results, observation of biological movement resulted in faster reaction times and greater mu desynchronization compared to non-biological movement. Physical similarity with the imitated or observed hand did not affect imitation speed or mu desynchronization. It did, however, have a surprising effect on corticospinal excitability, where the amplitude of transcranial magnetic stimulation-induced motor evoked potentials was greater in the right hemisphere of female participants observing hand movement executed by hands of a different color.;These data suggest that physical similarity with an observed action in terms of skin color does not modulate MNS activity in sensorimotor cortex when cultural and emotional components are absent. The present results also strengthen the notion that the motor cortex node of the MNS is tuned to the biological nature of an observed action.;Keywords : Action observation, mirror neurons, physical similarity, biological movement, transcranial magnetic stimulation, imitation, mu rhythm. |
