Cardio-respiratory contributions to the magnetoencephalogram

Modern MEG technology provides whole head helmet systems which extend posterior to the base of the skull. While this provides better visualization of low temporal structures it also allows the intrusion of activity that is of cardiac origin. This consists not only of the QRS complex of the MCG but a late component (LC) during the SP interval. This tends to consist of a series of waves and has a different topographic distribution from the magnetic QRS complex. The LC is usually most prominent in the low occipital and sub-occipital sensors but can occur also in other areas. When the high pass filter is set to 0.1 Hz it can be seen that the LC rides on a slow wave of about 0.3 Hz, which is linked to respiration. Principal component analysis of cardiac cycles averaged from the ECG separates the MCG from the LC. The physiology of the LC is unknown at this time but it may be related to the ballistocardiogram. Algorhythms designed to remove the cardiac contributions to the MEG will have to take the LC and its marked variability into account.