Gating of somatosensory evoked magnetic fields during the preparatory period of self-initiated finger movement.

The temporal change in somatosensory evoked magnetic fields (SEFs) in the preparatory period of self-initiated voluntary movement was investigated. The SEF following stimulation of the right median nerve was recorded, using a 204-channel whole-head MEG system, in nine healthy subjects during a self-initiated extension of the right index finger every 5 to 7 s. The preparatory period before finger movement was divided into six subperiods, and the MEG signals following the stimulation in each subperiod were averaged separately. SEFs were also recorded in the resting state. The ECD strengths for N20m and P60m were not significantly changed in any subperiod before movement compared with those in the resting state. The ECD strength for P30m was significantly smaller 500 ms or less before movement than during the resting state and 1,500 ms or less before movement compared to that during the period from 3,000 to 4,000 ms before movement. Thus, we confirmed that the SEF components were attenuated even during a period of self-initiated voluntary movement. The modulation started at least 1,500 ms before movement and was greater for the P30m than the N20m component. These findings suggested that motor-associated cortices attenuated SEF components by a centrifugal gating process.