These technical limitations are primarily due to the fundamental mismatches between fMRI and EEG/MEG, owing to highly different temporal scales in which fMRI and ERP/ERF data are generated and collected (see Table 1). The fMRI-EEG/MEG mismatches may be further categorized into three types, namely fMRI extra sources, fMRI invisible sources and the fMRI displacement [20, 134, 139]. The fMRI extra sources represent the source regions that are deemed as active in fMRI but do not contain the sources for the EEG/MEG at a certain time instant. During a short period of interest following the event onset, the fMRI activations are thought of as “static” (or time-invariant) while the EEG/MEG signals are variable and the source imaging is carried out instant by instant. The electrical source activity in a millisecond time scale may only involve a subset of the activated fMRI 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
