We performed EEG preprocessing offline using Matlab (Mathworks, Natick, MA). In addition to standard EEG artifacts, electrophysiological signals recorded inside the MRI scanner are contaminated with gradient artifacts and ballistocardiogram (BCG) artifacts due to magnetic induction in the EEG wires. First we removed the gradient artifacts by subtracting the mean artifact across all functional volume acquisitions. We then applied a 10 ms median filter to remove any residual spike artifacts. Because we sync the scanner with the EEG we are able to remove the gradient artifact at the commonly-used 1 kHz sampling rate of stand-alone EEG. Next we removed standard EEG artifacts, using the following digital Butterworth filters: 1 Hz high pass to remove DC drift, 60 Hz and 120 Hz notches to remove electrical line noise and its first harmonic, and 100 Hz low pass to remove high frequency artifacts not associated with neurophysiological processes. These filters were applied together in the form of a linear phase finite impulse response (FIR) filter to avoid distortions caused by phase delays.
