However, strong cross-modal interferences pose great challenges to simultaneous fMRI-EEG recordings. Both time-varying and static magnetic fields in the MRI/fMRI environment can introduce large artifacts to EEG recordings. During the fMRI scans, the radio-frequency (RF) excitation pulses and rapidly switching magnetic gradients result in large dB/dt, which in turn induce gradient artifacts (GA) about 1000 times larger than the normal EEG magnitude [152, 153]. The pulsatile motion of EEG leads associated with heart beats causes cardiac ballistic artifacts (CBA) [154]. In addition, subjects' safety is worth special attention, since the tissues surrounding the electrodes may be heated or damaged by large artificial currents [155]. The quality of MRI/fMRI images may also be affected by the EEG electrodes as well as other devices placed inside the scanner [156].
