The shape of motor resonance: right- or left-handed?

The human mirror neuron system is a fronto-parietal neural pathway which, when activated by action observation, gives rise to an internal simulation of the observed action (motor resonance). Here we demonstrate how handedness shapes the resonant response, by engaging right-handed (RH) and left-handed (LH) subjects in observation and execution of actions preferentially performed by the dominant hand. We hypothesize that since motor resonance reproduces subliminally the specific motor program for the observed action, it should be subject to motor constraints, such as handedness. A conjunction analysis for observed and executed actions revealed that handedness determines a lateralized activation of the areas engaged in motor resonance. Premotor-BA6 and parietal-BA40 are strongly left lateralized in RH subjects observing or moving their right hand, and to a lesser degree their left hand. Extremely LH subjects show a similar pattern of lateralization on the right, while more ambidextrous LH subjects show a more bilateral activation. The activation of a cortical network outside the mirror neuron system is also discussed.