In the context of multimodal neuroimaging, one unique feature of EEG, as opposed to MEG, lies in the feasibility of recording fMRI and EEG simultaneously. This feature is often desirable in studies that integrate both modalities or investigate the cross-modal relationship. For instance, simultaneous fMRI-EEG recordings are indispensable for using fMRI to localize the neural substrates underlying interictal EEG spikes. For this purpose, the fMRI response is not manipulated by user-defined external stimuli or tasks; but rather, it is driven by spontaneous biological events. The onsets of these events can be determined by detecting the interitcal spikes from the EEG recorded during concurrent fMRI scans, serving as the temporal markers necessary for carrying out the event-related fMRI analysis [143, 144]. For similar reasons, simultanoues fMRI-EEG recordings are desirable for mapping the fMRI correlates to continuous rhythmic EEG modulations during the resting state [145–147] or sleep [148, 149].
