The strength and limitation of fMRI are almost precisely the obverse of those of EEG and MEG. As neural activity elevates, the concomitant alternation of local oxyhemoglobin vs. deoxyhemoglobin content gives rise to a so-called blood oxygen level dependent (BOLD) magnetic resonance (MR) signal [16]. Since its advent in the early 1990's [3–5], the BOLD-contrast fMRI has rapidly gained a dominant position in neuroscience research [17]. The merits of fMRI include its whole brain coverage, relatively uniform sensitivity, high spatial resolution and specificity. These advantages are primarily attributable to well-established MR imaging techniques (e.g. echo-planar imaging) that allow for the frequency and phase encoding of spatially distributed MR signals. However, fMRI is also limited by its poor temporal resolution as well as its indirect nature with respect to neuronal activity. These limitations often pose concerns to any simple interpretation of the BOLD signal as a surrogate index of neural activity.
