Similarities Between Simulated Spatial Spectra of Scalp EEG, MEG and Structural MRI

Abstract

Electrical dipoles oriented perpendicular to the cortical surface are the primary source of the scalp EEGs and MEGs. Thus, in particular, gyri and sulci structures on the cortical surface have a definite possibility to influence the EEGs and MEGs. This was examined by comparing the spatial power spectral density (PSD) of the upper portion of the human cortex in MRI slices to that of simulated scalp EEGs and MEGs. The electrical activity was modeled with 2,650 dipolar sources oriented normal to the local cortical surface. The resulting scalp potentials were calculated with a finite element model of the head constructed from 51 segmented sagittal MR images. The PSD was computed after taking the fast Fourier transform of scalp potentials. The PSD of the cortical contour in each slice was also computed. The PSD was then averaged over all the slices. This was done for sagittal and coronal view both. The PSD of EEG and MEG showed two broad peaks, one from 0.05 to 0.22 cycles/cm (wavelength 20–4.545 cm) and the other from 0.22 to 1.2 cycles/cm (wavelength 4.545–0.834 cm). The PSD of the cortex showed a broad peak from 0.08 to 0.32 cycles/cm (wavelength 12.5–3.125 cm) and other two peaks within the range of 0.32 to 0.9 cycles/cm (wavelength 3.125–1.11 cm). These peaks are definitely due to the gyri structures and associated larger patterns on the cortical surface. Smaller peaks in the range of 1–3 cycles/cm were also observed which are possibly due to sulci structures. These results suggest that the spatial information was present in the EEG and MEG at the spatial frequencies of gyri. This also implies that the practical Nyquist frequency for sampling scalp EEGs should be 3.0 cycles/cm and an optimal interelectrode spacing of about 3 mm is needed for extraction of cortical patterns from scalp EEGs in humans.

Spatial patterning of the neonatal EEG suggests a need for a high number of electrodes

“…Because the spatial texture of EEG is spatially mostly noncyclic, it would be more robust to have the sample frequency 3-5 times higher than the frequency limit (Barlow, 1993), which would yield interelectrode spacing of about 3-5 mm in neonates and 5-8 mm in adults. Simulations with adult head model have even suggested need for interelectrode spacing as low as 3 mm (see Ramon et al, 2009). The inflection point on the left side (0.5 cycles/cm in Fig.…”

Spatiotemporal phase clusters and phase synchronization patterns derived from high density EEG and ECoG recordings

“…A good question to ask is how many electrodes are needed to properly sample the spatial information embedded in EEG or ECoG data which are generated from the electrical activity of the distributed cortical sources in the gray matter. A simulation study has shown that to pick up the gyral structures related effects in the scalp EEG, possibly, a 3.0 mm interelectrode separation will be necessary [21]. This simulation study was performed with a finite element method (FEM) model of an adult male with 2 mm  2 mm  3.2 mm size voxels.…”

“…The x coordinate is in the sagittal plane, increasing from anterior (front) to the posterior (back) of the subject. The spatial PSD in the x direction is named as PSDx [21]. However, often, many authors use the symbol PSDx for any spatial direction, whether it is x, y or z [11,22].…”

“…The second peak (0.32-0.9 cycles/cm) is definitely due to 2-5 cm size gyral structures on the cortical surface. The cortical PSDx has multiple smaller peaks within the 0.32-0.9 cycles/cm which are smoothed out in the PSDx of EEG due to the volume conduction effects [21]. A 2.0 cm interelectrode separation in a dEEG system has a spatial frequency of 0.5 cycles/cm which will be sufficient to map some of the gyral structures on the cortical surface.…”

“…Spatiotemporal plots of analytic amplitude and phase carry a wealth of information in all EEG bands. Generally, EEG bands are classified as: theta (3-7 Hz), alpha (7-12 Hz), beta (12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30), low gamma and high gamma (50-80 Hz). Two nearby channels slip in and out of phase in the temporal direction and form spatial structures.…”