The effect of stimulation type, head modeling, and combined EEG and MEG on the source reconstruction of the somatosensory P20/N20 component

Modeling and experimental parameters influence the Electro- (EEG) and Magnetoencephalography (MEG) source analysis of the somatosensory P20/N20 component. In a sensitivity group study, we compare P20/N20 source analysis due to different stimulation type (Electric-Wrist [EW], Braille-Tactile [BT], or Pneumato-Tactile [PT]), measurement modality (combined EEG/MEG - EMEG, EEG, or MEG) and head model (standard or individually skull-conductivity calibrated including brain anisotropic conductivity). Considerable differences between pairs of stimulation types occurred (EW-BT: 8.7 +/- 3.3 mm/27.1 degrees +/- 16.4 degrees, BT-PT: 9 +/- 5 mm/29.9 degrees +/- 17.3 degrees, and EW-PT: 9.8 +/- 7.4 mm/15.9 degrees +/- 16.5 degrees and 75% strength reduction of BT or PT when compared to EW) regardless of the head model used. EMEG has nearly no localization differences to MEG, but large ones to EEG (16.1 +/- 4.9 mm), while source orientation differences are non-negligible to both EEG (14 degrees +/- 3.7 degrees) and MEG (12.5 degrees +/- 10.9 degrees). Our calibration results show a considerable inter-subject variability (3.1-14 mS/m) for skull conductivity. The comparison due to different head model show localization differences smaller for EMEG (EW: 3.4 +/- 2.4 mm, BT: 3.7 +/- 3.4 mm, and PT: 5.9 +/- 6.8 mm) than for EEG (EW: 8.6 +/- 8.3 mm, BT: 11.8 +/- 6.2 mm, and PT: 10.5 +/- 5.3 mm), while source orientation differences for EMEG (EW: 15.4 degrees +/- 6.3 degrees, BT: 25.7 degrees +/- 15.2 degrees and PT: 14 degrees +/- 11.5 degrees) and EEG (EW: 14.6 degrees +/- 9.5 degrees, BT: 16.3 degrees +/- 11.1 degrees and PT: 12.9 degrees +/- 8.9 degrees) are in the same range. Our results show that stimulation type, modality and head modeling all have a non-negligible influence on the source reconstruction of the P20/N20 component. The complementary information of both modalities in EMEG can be exploited on the basis of detailed and individualized head models.