To address the linearity or nonlinearity of the neurovascular coupling, investigators need to monitor both neural activity and hemodynamic response through invasive [39, 121, 122, 124] or noninvasive measurements [125–127]. The measured electrophysiological (e.g. MUA, LFP, EEG and MEG) and hemodynamic (e.g. BOLD and CBF) signals are quantified individually before being compared against a linear function. In this regard, a critical concern lies in the variety of methods for quantifying the multimodal signals. Hemodynamic signals have been quantified as its peak height [124], steady-state height [128] or integral over time [129]. Ways for quantifying electrophysiological signals are even more diverse. No consensus has been reached so far with regard to an appropriate pair of quantitative measures for assessing the cross-modal relationship. It is likely that some reported nonlinear neurovascular coupling may be simply due to the use of mismatched quantitative measures. Additional challenges may be further appreciated by considering the highly different temporal scales of the hemodynamic and electrophysiological responses. Table 1 lists typical values related to the temporal and frequency features of the single-trial ERP/ERF and BOLD signals. A
