Multiplicity of the α rhythm in normal humans

Previous analyses of the at rhythm in a given single derivation have shown that it is a result of narrowband filtration of a broadband process. As many as three distinct a rhythms within each hemisphere could be differentiated in 65 resting, awake subjects by considering the spatial properties of these rhythms along with their dynamics. The analysis was based on cross-correlation measurements of rhythmic and broadband processes, and comparison of the dynamic characteristics of oscillatory systems underlying the a rhythms. Five-minute epochs of the EEG were recorded to provide 10% precision of the statistical estimates of the variables measured. A frontal a rhythm independent of occipital rhythmic activity was present in 20% of subjects. This third rhythm is distinct from the more familiar alpha and mu rhythms described in the literature, and is attenuated when the eyes are open. The authors demonstrate that the dynamic characteristics of the oscillatory systems underlying the a rhythm, as well as intrahemispheric coefficients of cross-correlation, are reproducible over time in a single subject. These parameters can form the basis for reliable individual EEG characteristics in the description of the "normal" EEG. The high correlation of the a rhythm in symmetric derivations could be explained by symmetric afferent impulse flows rather than by structural interconnection between the oscillatory systems.