A geometric correction scheme for spatial leakage effects in MEG/EEG seed‐based functional connectivity mapping

Abstract

Spatial leakage effects are particularly confounding for seed-based investigations of brain networks using source-level electroencephalography (EEG) or magnetoencephalography (MEG). Various methods designed to avoid this issue have been introduced but are limited to particular assumptions about its temporal characteristics. Here, we investigate the usefulness of a model-based geometric correction scheme (GCS) to suppress spatial leakage emanating from the seed location. We analyze its properties theoretically and then assess potential advantages and limitations with simulated and experimental MEG data (resting state and auditory-motor task). To do so, we apply Minimum Norm Estimation (MNE) for source reconstruction and use variation of error parameters, statistical gauging of spatial leakage correction and comparison with signal orthogonalization. Results show that the GCS has a local (i.e., near the seed) effect only, in line with the geometry of MNE spatial leakage, and is able to map spatially all types of brain interactions, including linear correlations eliminated after signal orthogonalization. Furthermore, it is robust against the introduction of forward model errors. On the other hand, the GCS can be affected by local overcorrection effects and seed mislocation. These issues arise with signal orthogonalization too, although significantly less extensively, so the two approaches complement each other. The GCS thus appears to be a valuable addition to the spatial leakage correction toolkits for seed-based FC analyses in source-projected MEG/EEG data.

Age-related differences in practice-dependent resting-state functional connectivity related to motor sequence learning

“…Seed‐based rsFC maps were estimated using the slow Hilbert envelope correlation analysis adapted from the seminal works of MEG rsFC [Brookes et al, ; Hipp et al, ] and previously described in Wens et al []. Briefly, the instantaneous orthogonalization method of [Hipp et al, ] was first applied to sources' analytic signals in order to limit the spurious rsFC due to the spatial leakage effects in MNE [Wens, ; Wens et al, ]. Slow envelope fluctuations (∼1 Hz) were then extracted by averaging their Hilbert envelope over sliding windows (size: 1 s, step: 0.5 s) and their Pearson's correlation coefficient between the seed and orthogonalized sources were finally computed.…”

“…Reported results were significant at P corr < 0.05 after correction for the massive multiple comparisons involved in testing source‐level statistical maps [Barnes et al, ]. Correction for multiple comparisons was applied using a Bonferroni correction for the number of spatial degrees of freedom involved in MNE reconstructions, estimated as in [Wens et al, ]. In practice, the corrected significance level corresponded to P < 0.0009.…”

Optimising experimental design for MEG resting state functional connectivity measurement

“…However, the other methods described above (AEC, Coh, PLV) remain susceptible to leakage and, prior to their calculation, a correction must be undertaken. A number of methods to apply such correction are available (Brookes et al, 2012;Hipp et al, 2012;O'Neill et al, 2015b;Wens et al, 2015) most of which employ orthogonalisation of beamformer projected signals to remove zero lag effects. For the present work, prior to calculation of AEC, Coh and PLV, we adopted two approaches:…”

Automated voxel classification used with atlas-guided diffuse optical tomography for assessment of functional brain networks in young and older adults

“…[54][55][56] This concern is essential when the measurement data are reconstructed into sourcespace. Since such connectivity images or maps are formed by solving an ill-posed inverse problem, there are inherent correlations among adjacent vertices.…”

“…55 Another approach, named geometric correction scheme, was based on subtraction of the spatial leakage geometry model from the reconstructed image data and approved efficient to minimize the spatial leakage effect. 54 …”