A detect the largest cortical response directly above

A whole-head 306-channel MEG, which
consisted of 204 planar gradiometers, was utilized to record the cortical
magnetic responses. The planar gradiometers can detect the largest cortical
response directly above the cerebral regions with activation. The MEG signals
detected from the planar gradiometers were analyzed.Four
head position indicators were used to determine the exact location of the head
in respect of the sensors. The position relationship between the coil locations
and the anatomical landmarks (including right pre-auricular point, left
pre-auricular point, and nasion) were calculated via a 3D digitizer.The
MEG signals were digitized and the sampling rate was 1000 Hz, with an online
bandpass of 0.1, 120 Hz. The interval of analysis was 1100 milliseconds, including
a pre-stimulus baseline of 100 milliseconds.In
each motor condition, there were at least 90 artifact-free evoked cortical
responses acquired after excluding the contaminated epochs due to MEG artifacts
and prominent electro-oculogramAn
electrical stimulator (KonstantstromStimulator, Schwind, Erlangen, Germany) was
used to stimulate right median nerve. For obtaining obvious twitch of the abductor
pollicis brevis and better cortical responses, the settings of the stimulator
included inter-stimulus interval of 1.5 seconds and supramaximal stimulation with
a stimulus intensity set at 20% above the motor threshold. All subjects were asked
to ignore the stimulation in each experimental condition (Cheng & Lin,
2013).Surface electromyogram (EMG) was utilized
in every subject to record the twitching impulse from the flexor digitorum
superficialis for control purpose. These muscle twitching signals were filtered
within 20-200 Hz bandpass off-line. The absolute magnitude of signals was then
calculated via rectification. In order to quantifying muscle activity over
time, the EMG signals were averaged from artifact-free cortical responses in
each motor condition (Cheng
et al., 2017).


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