Multiplicity of the alpha 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 man y as three distinct a rhythms within each hemisphere could be differentiate d in 65 resting, awake subjects by considering the spatial properties of th ese rhythms along with their dynamics. The analysis was based on cross-corr elation measurements of rhythmic and broadband processes, and comparison of the dynamic characteristics of oscillatory systems underlying the a rhythm s. Five-minute epochs of the EEG were recorded to provide 10% precision of the statistical estimates of the variables measured. A frontal a rhythm ind ependent of occipital rhythmic activity was present in 20% of subjects. Thi s third rhythm is distinct from the more familiar alpha and mu rhythms desc ribed in the literature, and is attenuated when the eyes are open. The auth ors 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 paramet ers can form the basis for reliable individual EEG characteristics in the d escription of the "normal" EEG. The high correlation of the a rhythm in sym metric derivations could be explained by symmetric afferent impulse flows r ather than by structural interconnection between the oscillatory systems.