Figure 3.

Topography and time course of the electrocardiographic (ECG) artifact. (a) Topographic maps (six subjects) showing the characteristic topography of ECG artifacts in EEG data as identified with independent component analysis (ICA) (see also the text in the “Preprocessing and Analysis” section for more detail on the method). A diagonal distribution, with a gradient from the left posterior scalp (where the ECG signal is typically maximal) to the right frontal scalp (e.g., see the first four subjects on the left) is seen. The exact artifact topography depends on the position of the cardiac source in the chest. Some individual variation in scalp ICA maps (e.g., see the last two subjects at the right) can be seen. The color scale depicts arbitrary units. (b) On rare occasions, the artifact appears in two separate ICA components, as shown in the data of one subject. The ICA maps show the diagonal gradient (left) and an additional component with a different gradient. The ICA time courses of the ECG artifacts (right) show clear QRS complexes in the epoched data traces (red vertical lines separate data epochs). a.u. = arbitrary units.