For an illustrative overview, we graph the existing noninvasive neuroimaging methodologies, in comparison with relevant invasive recording/imaging techniques, with respect to their spatial and temporal resolution, as shown in Fig. 1. Typical invasive techniques include intracranial electrophysiological recordings of the single-unit activity (SUA), multi-unit activity (MUA) and local field potential (LFP). In addition, intrinsic light optical imaging, which can be applied to exposed brain surface, is also capable of imaging the hemodynamic signals [18,19]. Noticeably, none of existing noninvasive modalities can alone achieve high resolution in both spatial and temporal domains. One technique may have merits in the temporal aspect but limitations in the spatial aspect (such as EEG/MEG), or vice versa (such as fMRI). These complementary features have motivated recent developments in integrating multiple neuroimaging modalities, particularly EEG/MEG and fMRI [20–24]. As illustrated in Fig. 2, such a multimodal functional neuroimaging approach is based on the fundamental relationships between electromagnetic signals generated by neuronal activity via volume conduction, and BOLD-contrast fMRI signals related to the hemodynamic responses. A number of efforts in recording technology, multimodal data fusion and the
