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

Abstract

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° ± 16.4°, BT-PT: 9 ± 5 mm/29.9° ± 17.3°, and EW-PT: 9.8 ± 7.4 mm/15.9° ± 16.5° 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° ± 3.7°) and MEG (12.5° ± 10.9°). 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° ± 6.3°, BT: 25.7° ± 15.2° and PT: 14° ± 11.5°) and EEG (EW: 14.6° ± 9.5°, BT: 16.3° ± 11.1° and PT: 12.9° ± 8.9°) 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.

Keywords: EEG; MEG; finite element method; multimodal imaging; somatosensory cortex; somatosensory evoked fields; somatosensory evoked potentials.

Figure 1

Six‐compartment anisotropic realistic head model and source space: The left subfigure shows sagittal (left column), coronal (middle column), and axial (right column) slices of (a) T1w (upper row) and T2w (middle row) MRIs and (b) a color‐coded (red for left–right; green for anterior–posterior; blue for superior–inferior) fractional anisotropy map computed from the registered DTI and plotted on the T1w‐MRI (lower row). (c) the six‐compartment segmentation result with scalp (light blue), SC (dark blue), skull spongiosa (gray), CSF (red), GM (orange), and WM (yellow; in order to avoid an unnecessary amount of computational work and without losing accuracy, the model in (c) is cut along an axial plane 40 mm below the skull (in average across all subjects), following the recommendations of Lanfer et al. (2012); (d) the geometry‐adapted hexahedral FEM mesh for each of the six tissue compartments (same coloring as in (c)); (e) the source space nodes (green dots) on T1w MRI [Color figure can be viewed at http://wileyonlinelibrary.com]

Figure 2

Summary of our data analysis pipeline: The data acquisition block shows the MRI datasets (T1w, T2w, DTI) and the combined SEP/SEF data elicited by electric‐wrist (EW), braille‐tactile (BT), and pneumato‐tactile (PT) somatosensory stimulation. After preprocessing of the structural and functional data, calibrated realistic head models are generated that are then used for source analysis of the somatosensory P20/N20 component. The corresponding software tools used in each step are also indicated [Color figure can be viewed at http://wileyonlinelibrary.com]

Figure 3

Calibrated head models: (a) Six compartment anisotropic calibrated (6CA_Cal) head model and b) three‐compartment isotropic calibrated (3CI_Cal) head model. Both models are color‐coded (logarithmic scale) according to the conductivity range for subject 1 using a single color‐bar. The spread of the maximum norm of the conductivity tensors, visualized in the brain compartments in model 6CA_Cal, is due to the procedure as described in section 2.4 [Color figure can be viewed at http://wileyonlinelibrary.com]

Figure 4

EW (electric‐wrist) stimulation: Effect of modality and head model on P20/N20 reconstruction: Single dipole deviation scan differences for all subjects with regard to source location (left plot, in mm), source orientation (middle plot, in degrees) and source strength (right plot, in %). In (a, upper row), the reconstruction differences are shown when using as a reference the combined EMEG data versus the data of a single (EEG or MEG) modality for the most detailed head model 6CA_Cal (left) and the more homogenized version 3CI_Cal (right) (blue for EMEG vs. EEG and yellow for EMEG vs. MEG). Panel (b) (lower row) depicts the reconstruction differences when using as a reference the most detailed head model 6CA_Cal versus the more homogenized 3CI_Cal for EEG, MEG, and combined EMEG. In each boxplot, the central mark is the median, the edges of the box are the 25th and 75th percentiles. The circles indicate the values for each subject [Color figure can be viewed at http://wileyonlinelibrary.com]

Figure 5

Somatosensory evoked fields (SEFs) and potentials (SEPs) for the three types of stimulation: For the averaged SEFs (upper row) and SEPs (lower row), butterfly plots with global mean field power (GMFP) (left plot, MEG in green, EEG in blue, GMFP in red) and P20/N20 topographies (right plot) are shown from one subject for each type of stimulation: (a) electric‐wrist (EW), (b) braille‐tactile (BT), and (c) pneumato‐tactile (PT). The vertical black line at 20 ms in the left plot represents the highest peak of the P20/N20 component for each stimulation type and the EEG and MEG topographies are then shown in the right plot. Note the different amplitudes for the three stimulation types [Color figure can be viewed at http://wileyonlinelibrary.com]

Figure 6

Effect of individual head modeling with focus on skull conductivity calibration on P20/N20 reconstruction: Single dipole deviation scan differences for all subjects with regard to source location (left plot, in mm), source orientation (middle plot, in degrees), and source strength (right plot, in %). Upper row: Most detailed six‐compartment anisotropic head model with individually calibrated skull compartment (6CA_Cal) in comparison to the standard isotropic three‐compartment model (3CI_41). Lower row: 6CA_Cal versus 6CA_41. Results are shown for EEG (blue) and EMEG (orange) and all three stimulation types electric‐wrist (EW), braille‐tactile (BT), and pneumato‐tactile (PT) on the x‐axes. In each boxplot, the central mark is the median, the edges of the box are the 25th and 75th percentiles. Notice that the data from the volunteers are overlapping in case that less than five circles are depicted [Color figure can be viewed at http://wileyonlinelibrary.com]

Figure 7

Effect of stimulation type on P20/N20 reconstruction: EMEG single dipole deviation scan differences for all subjects and for the standard head models (3CI_41, in blue), the more detailed head models (6CA_41, in green), and the individually calibrated head models (6CA_Cal, in red) with regard to source location (left plot, in mm), source orientation (middle plot, in degrees), and source strength (right plot, in %). Reconstructions for the stimulation type electric‐wrist (EW) were compared to braille‐tactile (BT), as well as EW to pneumato‐tactile (PT) and BT to PT (x‐axes). In each boxplot, the central mark is the median, the edges of the box are the 25th and 75th percentiles [Color figure can be viewed at http://wileyonlinelibrary.com]