On the relationship between slow cortical potentials and BOLD signal changes in humans.

This review summarizes experimental studies that investigated the relationship between DC-recorded slow event-related potentials (slow waves) of the electroencephalogram (EEG) and the hemodynamic BOLD response, as measured with functional magnetic resonance imaging (fMRI). Slow waves have been found to accompany a large number of cognitive processes in a systematic and topographically specific way, and have thus been successfully employed in psychophysiological experiments to dissociate cognitive functions by means of their slow wave topography. Recently, however, several independent studies, using different experimental paradigms, suggest the existence of another feature of slow waves, i.e., a close relationship with the fMRI BOLD response. Some of these studies found couplings between slow waves and BOLD signals in various brain regions, using simultaneous EEG-fMRI recordings. Others found similar task-related activation patterns of slow waves (i.e., scalp topographies) and BOLD responses (i.e., activated voxel profiles), as well as corresponding parametric increases of signal strength with increasing task difficulty. The close relationship between slow waves and BOLD responses reported here concerns a low frequency range of the EEG signal (<1 Hz) that has so far received less attention in studies on the correspondence between EEG and fMRI than the higher frequencies, and therefore adds to the various findings obtained at higher EEG frequencies. Implications for the use of slow waves for neuroscientific research are discussed.