Activation of human primary motor cortex during action observation: A neuromagnetic study

The monkey premotor cortex contains neurons that discharge during action ex ecution and during observation of actions made by others. Transcranial magn etic stimulation experiments suggest that a similar observation/execution m atching system also is present in humans. We recorded neuromagnetic oscilla tory activity of the human precentral cortex from 10 healthy volunteers whi le (i) they had no task to perform, (ii) they were manipulating a smalt obj ect, and (iii) they were observing another individual performing the, same task. The left and right median nerves were stimulated alternately (interst imulus interval, 1.5 s) at intensities exceeding motor threshold, and the p oststimulus rebound of the rolandic 15- to 25-Hz activity was quantified,In agreement with previous studies, the rebound was strongly suppressed bilat erally during object manipulation. Most interestingly, the rebound also was significantly diminished during action observation (31-46% of the suppress ion during object manipulation). Control experiments, in which subjects wer e instructed to observe stationary or moving stimuli, confirmed the specifi city of the suppression effect. Because the recorded 15- to 25-Hz activity is known to originate mainly in the precentral motor cortex, we concluded t hat the human primary motor cortex is activated during observation as well as execution of motor tasks. These findings have implications for a better understanding of the machinery underlying action recognition in humans.