Global signal (GS) regression is a commonly used preprocessing approach in
the analysis of resting-state fMRI data. However GSR should be used with caution
as it can not only induce spurious anti-correlations, but may also remove signal
of neural origin. Here we used simultaneously acquired EEG/fMRI data to study
the relation between the GS and an EEG-based measure of vigilance at rest. We
found that there is a significant negative correlation between the GS and EEG
vigilance. Our results indicate that GS has a significant neuronal component
and further emphasizes the need to exercise caution when regressing out the GS.