Sampling by the EEG recording is not synchronized with the fMRI gradient signal. Successive periods of the fMRI gradient signal are sampled by the EEG recording system with a varying temporal offset (top graph). Period n is drawn in blue and the following period n+1 in red. The bottom left graph shows these fMRI periods overlayed as sampled by the EEG recorder during period n (blue stems) and period n+1 (red stems). Overlaying the EEG samples at the quantized sampling times therefore leads to a vertical difference Δ for every period as denoted in the bottom right graph.
Glaser et al. BMC Neuroscience 2013 14:138 doi:10.1186/1471-2202-14-138