Primary motor area as main direct target of somatosensory projections

Objectives: To show that neuronal activity originating within a section of M1 that is devoted to hand control (M1-h) can be extracted from the general response of the cortex to an external galvanic stimulation of a nerve supplying hand districts.Methods: Left rolandic activity was recorded with magnetoencephalography (MEG) in 8 right-handed healthy volunteers during stimulation of the right median nerve and during non-fatiguing weak isometric gripping under visual feedback. M1 and S1 sources (FS_M1 and FS_S1) were identified by the Functional Source Separation procedure (FSS). FS_M1fS1 was the FS obtained by maximizing the responsiveness to the median nerve stimulation at around 30 ms in the channel space subtracted by the contribution of FS_S1. Results: FS_M1 and FS_M1fS1 did not differ from each other, while both differed with respect to FS_S1, for position, direction, responsiveness to median nerve stimulation (higher at 30 ms than 20, while the opposite was the case for FS_S1) and coherence with thumb opponent electromyography during the isometric gripping. Conclusion: The new FS_M1fS1 identifies the activity of M1's region devoted to hand control by simply acquiring the response to contralateral median nerve stimulation. Remarkably, once provided with the source activity and weights exploiting the functional constraint active in a short time interval (median nerve stimulation in this case), the source can be investigated in other conditions of interest. Here, although we identified M1 only exploiting its responsiveness to the galvanic median nerve stimulation, we can study its behavior during a voluntary movement.