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FIG 2. Magnetoencephalography waveforms and source localization.
A, Representative magnetoencephalography waveforms for one, two, three, and four digits, for a single participant, single session. Waveforms are shown from the 37 sensor channels, collapsed onto a single horizontal time axis, ranging from 150 ms before stimulation to 150 ms after stimulation, capturing the initial evoked response. The y axis of these plots is a measure of evoked magnetic field amplitude, and all plots are shown to scale (50 fT/vertical division). All have recognizable peaks occurring approximately 30 to 40 ms after stimulus onset (arrow), with subsequent activity that is not the focus of this study. The amplitude of this peak seems to increase, somewhat monotonically, with an increasing number of stimulated digits.
B, Source localization (single equivalent dipole modeling) of this magnetoencephalography-detected peak onto high resolution MR images (SPGR; 38/6/1; flip angle, 35°), to form magnetic source images (MSI) for one-, two-, three-, and four-digit stimulation shows the source estimate (colored point) to lie in the postcentral gyrus, which is expected to contain primary somatosensory representations. The ring surrounding the modeled current source indicates the 95% confidence limit for model/data fit. The 95% confidence volumes (from 3D ellipsoids of confidence) for the example in the figure are 0.17, 0.06, 0.08, and 0.02 cm3, respectively, for one-, two-, three-, and four-digit stimulation.
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