Can motor resonance be evoked by both static and dynamic components of observed movements?

The observation of movements done by others induces the subliminal activation of motor circuits (motor resonance, MR), reproducing the specific motor program encoding the same movements. Here we investigate whether both static and dynamic components of an observed action are effective in evoking a MR response. Single pulse transcranial magnetic stimulation was applied to the left primary motor cortex of 15 subjects, eliciting motor evoked potentials (MEPs) in the right flexor and extensor carpi radialis (FCR, ECR) muscles. MR-induced MEP amplitude modulation was sampled at different intervals during the observation of a single oscillation of a prone hand, starting in mid-flexion and continuing through a protracted full extension of the wrist. In the FCR, motor resonant MEP modulation showed the expected significant (p<0.004) positive peak during the observation of flexion and dis-facilitation during extension, while no significant modulation was observed in the ECR during either dynamic or static wrist extensions. Subjects were also asked to execute the identical movement: in this condition a single EMG burst was measured in the FCR during flexion, while constant activity was recorded in the ECR during both dynamic and static movement phases, with a short silent period occurring during flexion. The present data suggest that MR, failing to reproduce the constant motor command necessary to organize the entire observed movement, is selectively sensitive to its dynamic parameters.