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Similarities between simulated spatial spectra of scalp EEG, MEG and structural MRI.

Ramon C, Freeman WJ, Holmes M, Ishimaru A, Haueisen J, Schimpf PH, Rezvanian E - Brain Topogr (2009)

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Department of Electrical Engineering, University of Washington, Seattle, WA 98195, USA. ceon@u.washington.edu

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.

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Averaged PSDx of cortex, EEG and MEG in sagittal slices. Large broad peaks at mid frequencies (0.3–1.0 cycles/cm) are possibly due to gyral structures
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Fig2: Averaged PSDx of cortex, EEG and MEG in sagittal slices. Large broad peaks at mid frequencies (0.3–1.0 cycles/cm) are possibly due to gyral structures

Mentions: We performed 100 trial runs with different random dipole moments. The PSD of EEG and MEG for each run was computed and then averaged over all the trials. The spatial profiles of scalp potentials were different for each run. However, their spatial PSD profiles were only slightly different from one run to the next. This is because the spatial locations and the orientation of the dipoles are fixed and only their intensities are randomly changing. This will be evident in the errors bars of the PSD of scalp EEGs and MEGs averaged over 100 trials. Refer to Figs. 2 and 3 in the “Results” section. The error bars in the averaged spatial PSD plots were not significantly different from one trial to the next when averaged over more than 20 runs. Thus, averaging over 100 runs is more than adequate for the results presented here. The PSD was computed after taking the fast Fourier transform (FFT) of the 2-D plot of the scalp EEG or MEG data.Fig. 2


Similarities between simulated spatial spectra of scalp EEG, MEG and structural MRI.

Ramon C, Freeman WJ, Holmes M, Ishimaru A, Haueisen J, Schimpf PH, Rezvanian E - Brain Topogr (2009)

Averaged PSDx of cortex, EEG and MEG in sagittal slices. Large broad peaks at mid frequencies (0.3–1.0 cycles/cm) are possibly due to gyral structures
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2749166&req=5

Fig2: Averaged PSDx of cortex, EEG and MEG in sagittal slices. Large broad peaks at mid frequencies (0.3–1.0 cycles/cm) are possibly due to gyral structures
Mentions: We performed 100 trial runs with different random dipole moments. The PSD of EEG and MEG for each run was computed and then averaged over all the trials. The spatial profiles of scalp potentials were different for each run. However, their spatial PSD profiles were only slightly different from one run to the next. This is because the spatial locations and the orientation of the dipoles are fixed and only their intensities are randomly changing. This will be evident in the errors bars of the PSD of scalp EEGs and MEGs averaged over 100 trials. Refer to Figs. 2 and 3 in the “Results” section. The error bars in the averaged spatial PSD plots were not significantly different from one trial to the next when averaged over more than 20 runs. Thus, averaging over 100 runs is more than adequate for the results presented here. The PSD was computed after taking the fast Fourier transform (FFT) of the 2-D plot of the scalp EEG or MEG data.Fig. 2

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Department of Electrical Engineering, University of Washington, Seattle, WA 98195, USA. ceon@u.washington.edu

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.

Show MeSH