A whole-head 306-channel MEG, whichconsisted of 204 planar gradiometers, was utilized to record the corticalmagnetic responses. The planar gradiometers can detect the largest corticalresponse directly above the cerebral regions with activation. The MEG signalsdetected from the planar gradiometers were analyzed.Fourhead position indicators were used to determine the exact location of the headin respect of the sensors. The position relationship between the coil locationsand the anatomical landmarks (including right pre-auricular point, leftpre-auricular point, and nasion) were calculated via a 3D digitizer.
TheMEG signals were digitized and the sampling rate was 1000 Hz, with an onlinebandpass of 0.1, 120 Hz. The interval of analysis was 1100 milliseconds, includinga pre-stimulus baseline of 100 milliseconds.Ineach motor condition, there were at least 90 artifact-free evoked corticalresponses acquired after excluding the contaminated epochs due to MEG artifactsand prominent electro-oculogramAnelectrical stimulator (KonstantstromStimulator, Schwind, Erlangen, Germany) wasused to stimulate right median nerve. For obtaining obvious twitch of the abductorpollicis brevis and better cortical responses, the settings of the stimulatorincluded inter-stimulus interval of 1.5 seconds and supramaximal stimulation witha stimulus intensity set at 20% above the motor threshold. All subjects were askedto ignore the stimulation in each experimental condition (Cheng & Lin,2013).
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Surface electromyogram (EMG) was utilizedin every subject to record the twitching impulse from the flexor digitorumsuperficialis for control purpose. These muscle twitching signals were filteredwithin 20-200 Hz bandpass off-line. The absolute magnitude of signals was thencalculated via rectification. In order to quantifying muscle activity overtime, the EMG signals were averaged from artifact-free cortical responses ineach motor condition (Chenget al., 2017).