Reactivity of the human primary motor cortex during observation of action

“Mirror neurons” are visuomotor neurons which are activated both when the individual is observing and performing an action. These cells were originally discovered in the ventral premotor cortex (area F5) of monkeys by Rizzolatti and collegues. Mirror neurons directly match observed actions and their motor counterparts and probably have a role in action understanding, as well as learning and imitation of action. Brain imaging and electrophysiological studies have recently shown that a similar system exist also in the human brain. Areas comprising the human mirror-neuron system (MNS) include at least the inferior frontal gyrus and the adjacent premotor area, the primary motor cortex (MI), and possibly the superior temporal sulcus and the inferior parietal lobule, as well. Motor cortex involvement was first shown in our laboratory using magnetoencephalography (MEG). In this thesis I studied with my colleagues the reactivity of the human primary motor and somatosensory areas during observation of action using magnetoencephalography. A neuromagnetic ~20-Hz rhythm was used as an indicator of the functional state of MI. In Study I, we showed the ~20-Hz rhythm was dampened more when the subjects observed live than videotaped hand movements, indicating stronger activation of the primary motor cortex and this result suggests that the human mirror neuron system is sensitive to differentiating natural and artificially presented movements. In Study III we found that the human MI is activated during observation of tool use, in contrast to monkeys. MI was activated more strongly during observation of goal-directed than nongoal-directed tool use. This difference could be related to the observer’s ability to understand and imitate the perceived goal-directed actions. Study II showed that ~20-Hz reacts differently to electrical stimulation of thumb and middle finger. Post-stimulus rebounds were stronger after thumb stimuli, suggesting that the sensorimotor processing differs for thumb and middle finger in the human primary motor and somatosensory cortices. This result might be due to larger and more complex cortical representation of thumb. The somatosensory cortices are considered to contribute to the MNS . In Study IV we showed that somatosensory evoked fields after tactile lip stimulation are somatotopically enhanced in primary somatosensory cortex during silent lip-reading. This enhancement could reflect simulation of other person’s action-related sensations and hence could enable the observer to experience what the other person feels while performing motor acts. In Study V we investigated the reactivity of MI in schizophrenic patients and their healthy co-twins during action observation. We found that the reactivity of the motor-cortex rhythm