All noninvasive neuroimaging modalities are based on biophysical signals related to either brain electrophysiology or hemodynamics/metabolism. Electroencephalography (EEG) [1] and magnetoencephalography (MEG) [2] are based on electrophysiological principles. Functional magnetic resonance imaging (fMRI) [3–5], positron emission tomography (PET) [6, 7], single-photon emission computed tomography (SPECT) and near-infrared spectroscopy (NIRS) are based on hemodynamic and/or metabolic principles. Strengths and limitations of these modalities depend largely upon the spatiotemporal characteristics of the measured “source” signals in relation to neuronal activity, as well as many diverse sensing and imaging methods applied to individual modalities.
