areas, whereas other areas may appear as false positives if all of the “activated” fMRI voxels are included in the prior spatial constraint [24, 139]. The fMRI invisible sources are the real EEG/MEG sources but not deemed as active by fMRI. A transient current source may generate observable EEG/MEG signals whereas it may last too briefly to induce a sustained BOLD response. In this condition, the fMRI-derived time-invariant spatial constraint includes false negatives, which often result in the underestimation of fMRI invisible sources as reported in several independent studies [20, 24, 139]. The fMRI displacement refers to the spatial difference between the vascular and electrophysiological sources [134, 139]. Because of these fMRI-EEG/MEG mismatches, it is problematic to constrain the temporally variable current source estimates to “time-invariant” fMRI spatial priors, which may entail both fMRI false positives and false negatives.
