Electrophysiological activity and BOLD responses are coupled in a complex way that is not yet completely understood. There is evidence to suggest that the sign of the EEG-BOLD correlate is both a function of space and of frequency, and the sign of the BOLD response itself is affected by the excitation-inhibition balance within local neuronal circuits (Logothetis et al., 2008). It has been shown that low-frequency electrophysiological variations, such as those measured by scalp EEG in the 0–20 Hz range, are negatively correlated with the BOLD response (Goldman et al., 2002; Mukamel et al., 2005; Scheeringa et al., 2011). Additionally, commonly-studied fMRI resting state networks have been shown to correlate with specific frequency bands in magneto-encephalography (MEG) studies (Brookes et al., 2011; de Pascuale et al., 2010) and show some (though lesser) frequency-dependency using EEG-fMRI (Mantini et al., 2007). Since there is still some question about the physiological meaning of directionality of electrophysiological BOLD correlates, the following sections do not attempt to interpret the sign of the correlation between the BOLD signal and the STV regressor.
