In contrast, the fMRI-constrained distributed source imaging is generally applicable regardless of whether the source activity is focal or extended. Perhaps, the most straightforward approach is the fMRI-weighted current density imaging, which uses the fMRI activation map to derive weighting factors for the inverse solutions compatible with the EEG/MEG signals. This approach can be implemented in a Wiener filter [20, 21, 26] and weighted minimum norm frameworks [22, 134–136]. Although seemingly different, these implementations are fundamentally equivalent [23]. Both of them are based upon the identical core assumption that locations inside the fMRI activations are more favored than those outside in terms of the likelihood of being active current sources. The degree of preference to fMRI hightlighted areas is controlled by an fMRI weighting factor.
