Cardiac and pulse artifact

The heartbeat contaminates neural recordings in two ways. The electrocardiogram (ECG) is volume-conducted to the scalp and neck and appears as a QRS-locked deflection, strongest on electrodes near large vessels or with a long inter-electrode distance. The pulse or ballistocardiogram (BCG) is a mechanical artifact: each pulse wave moves the electrodes and the head slightly, producing a slower artifact near 1 Hz that is especially severe inside an MRI scanner, where electrode motion in the static field induces large voltages.

Because it is quasi-periodic and time-locked to the R-peak, cardiac artifact is well suited to template-based removal: detect R-peaks from a reference ECG lead, build an average artifact template, and subtract it, or remove the corresponding ICA components. In EEG-fMRI the BCG is often the limiting artifact after gradient removal, and residual BCG contaminates spectral and connectivity estimates if it is left uncorrected.