Cancellation of EEG and MEG signals generated by extended and distributed sources

Abstract

Extracranial patterns of scalp potentials and magnetic fields, as measured with electro- and magnetoencephalography (EEG, MEG), are spatially widespread even when the underlying source in the brain is focal. Therefore, loss in signal magnitude due to cancellation is expected when multiple brain regions are simultaneously active. We characterized these cancellation effects in EEG and MEG using a forward model with sources constrained on an anatomically accurate reconstruction of the cortical surface. Prominent cancellation was found for both EEG and MEG in the case of multiple randomly distributed source dipoles, even when the number of simultaneous dipoles was small. Substantial cancellation occurred also for locally extended patches of simulated activity, when the patches extended to opposite walls of sulci and gyri. For large patches, a difference between EEG and MEG cancellation was seen, presumably due to selective cancellation of tangentially vs. radially oriented sources. Cancellation effects can be of importance when electrophysiological data are related to hemodynamic measures. Furthermore, the selective cancellation may be used to explain some observed differences between EEG and MEG in terms of focal vs. widespread cortical activity.

Figure 1

Examples of cancellation of EEG and MEG signals. A: Simulated data generated by two current dipoles in the primary visual cortex of the left and right hemispheres. The maps depict spatial patterns of signals over the occipital region when each dipole is active alone (LH: left hemisphere, RH: right hemisphere), and when the two dipoles are active simultaneously. Top: EEG isocontour maps; red and blue indicate positive and negative scalp potentials, respectively. Middle: MEG magnetometer isocontour maps; red and blue indicate positive and negative radial component of the magnetic field. Bottom: MEG gradiometer signals; each arrow depicts the signal in a pair of planar gradiometers. The arrows have been rotated by 90 degrees from orientation of the actual gradient vectors, to match the orientation of the arrows with the direction of underlying source currents. The locations (green dots) and orientations (green lines) of the dipoles are indicated in coronal and sagittal MRI slices. This example illustrates how cancellation can occur in all three types of sensor arrays, when sources of opposite orientation are close to each other. B: Maps of MEG magnetometer signals for bilateral inferior temporal sources. For this particular source configuration, the cancellation is specific to mid‐occipital MEG magnetometer sensors. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

Figure 2

Cancellation index I _C for randomly distributed sources. The index was computed for an array of 60 EEG electrodes, 102 MEG magnetometers, and 204 MEG gradiometers. The mean and standard deviation over 1,000 random selections of n dipoles are shown. For clarity, the standard deviation bars are plotted one‐sided only. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

Figure 3

Examples of EEG and MEG signals for extended patches of uniform cortical activity. A patch with a 4‐mm (A) and 20‐mm (B) radius in the left parietal cortex is shown in red on a cortical surface representation reconstructed from anatomical MRI. The dots and squares indicate the locations of the EEG scalp electrodes and the MEG triple‐sensor units (one magnetometer and two gradiometers), respectively. Cancellation is illustrated by the signal per unit source element, obtained by plotting the maps for the 20‐mm patch normalized by the ratio of the areas of the two patches (C). [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

Figure 4

Cancellation index I _C for cortical patches. The mean and standard deviation over 200 randomly located simulated patches of uniform activity for each of the different radii shown. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

Figure 5

Comparison of the cancellation index I _C and the orientation non‐uniformity measure I _O for cortical patches. Data from simulated patches with different radii up to 50 mm are shown for EEG (A), MEG magnetometers (B), and MEG gradiometers (C). [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]