Relationship of the magnetoencephalogram to the electroencephalogram. Normal wake and sleep activity.

Abstract 1. 1. The electroencephalogram (EEG) and also the magnetoencephalogram (MEG) were recorded simultaneously from normal subjects (and some patients); the characteristics of the MEG and its relationship to the EEG were investigated with the use of a digital computer. 2. 2. The noise in the MEG is primarily around the frequency component of 1/sec. 3. 3. The EEG and MEG show the same peak frequency only for alpha rhythms and are “locked-in” especially at 10/sec. 4. 4. At alpha frequencies phase reversals are seen in the relationship of the EEG with the MEGs from opposite sides of the head. 5. 5. MEG-EEG comparisons more often show a lead or lag than EEG-EEG comparisons and the MEG usually lags when not synchronous with the EEG. 6. 6. During hyperventilation, slow waves may be seen only in the EEG (and not in the MEG) or only in the MEG (and not in the EEG) or in both the MEG and EEG. These findings emphasize the dissociation between the MEG and EEG when slow waves are recorded. 7. 7. During sleep stages (I and II) there is a high correlation between EEG linkages, as there is in the waking state. The correlation between the MEG and EEG is primarily for alpha activity present during wakefulness, but not during sleep. 8. 8. During sleep the MEG and EEG often show different kinds of rhythms, but the most common in the MEG is theta activity, especially at 4–5/sec, frequently seen in the MEG under different conditions. 9. 9. In comparison with alpha activity during wakefulness, sleep spindles are relatively poorly represented in the MEG as are vertex sharp waves, both showing great variability in amplitude, as compared with their representation in the EEG. 10. 10. The relationship of the MEG and EEG during wakefulness and sleep, their similarities and differences, are discussed, especially with reference to the dipole orientation of various types of activity.