Investigators have computed cortical connectivity patterns based on hemodynamic or metabolic measurements such as fMRI [196–199], whereas the indirect nature of fMRI signals confounds the interpretation of fMRI-derived connectivity in terms of neuronal interaction [200]. The cortical networks are formed and characterized by organized neuronal oscillations that span several orders of magnitude in frequency [201]. As discussed in this review, the neurovascular coupling behaves as a temporal low-pass filter with a cut-off frequency at around 0.4 Hz. High-frequency oscillations may be effectively excluded from the connectivity patterns estimated from fMRI. Even though the fMRI connectivity may reveal coherent low-frequency modulations of high-frequency oscillatary neural activities, the sluggishness of BOLD fMRI signals represents a significant challenge, if all possible, to imaging the full spectrum of brain functional connectivity.
