Our understanding of the coupling between fMRI and EEG/MEG has been substantially improved in the past decade, providing a great opportunity to combine these modalities in a more fundamental and principled way. Agreements have been reached in general regarding the neurovascular and neurometabolic coupling. Quantitative models of the cross-modal relationship, like those discussed in this review, represent important progress along this line. Although these models can at best approximate the complex interactions between hemodynamics and electrophysiology while subjected to experimental conditions and fundamental assumptions, they do serve as a sound basis for developing multimodal neuroimaging techniques, which promise to enhance the existing imaging capability, at least relative to fMRI and EEG/MEG alone.
