As discussed in this review, the vast majority of methodologies integrating fMRI and EEG/MEG are based upon a linear neurovascular coupling. However, recent studies have proposed alternative hypotheses [38, 191]. That is, a negative correlation between low-frequency (e.g. alpha-band) electrophysiological signals and BOLD fMRI signals [38, 145–147], whereas high-frequency components (e.g. gamma-band) contribute positively to the BOLD signal [38, 192]. Although this relationship remains highly controversial and often varies considerably across subjects [193], it does point to some cross-modal relationship that has been ignored or unexplained by conventional neurovascular coupling models. This alternative view, although being speculative so far, may help in revising the neurovascular model by extending the model from the spatiotemporal domain to the spatial-frequency or spatial-temporal-frequency domain. Along this line, an interesting paper deserves attentions [191]. In this analytical work, Kilner et al. propose a heuristic model suggesting that an increase in hemodynamic signals is associated with a shift of electrophysiological power spectrum from low to high frequencies (i.e. a loss in low-frequency power relative to a gain in high-frequency power). As the authors admit, the model
