Comprehensive spectral analysis of human EEG generators in posterior cerebral regions

Abstract

Continuous spectral analyses of several minutes of normal human EEGs were prepared by computer, and converted to compact graphic forms. This new presentation of voluminous data in contour maps gives an overview of the evolution of patterns in the EEG. Parameters graphed were spectral intensity (“power spectral density”), and coherence, a quantity expressing strength of relationship between brain areas. In one subject, the strongest relations were among longitudinally oriented parieto-occipital linkages, representing the subject's alpha wave. Wave activity in a similar frequency band, recorded across the occipital midline, was completely incoherent with the alpha wave. Thus, two independent generation processes in the same frequency band, orthogonally polarized, are required to account for these records. Low frequency activity in the O1O2 leads was related only partially to the heart-beat. In a second subject, there was somewhat greater coherence from side to side than between homolateral centro-parietal and parieto-occipital pairs of leads. The same configuration of two geometrically orthogonal and statistically uncorrelated generators was observed in this subject. Of 28 other analyzed in this way, eighteen showed the same clear perpendicularity, seven were similar but less marked, while three showed no such pattern. The second illustrative subject also showed coherences flanking his alpha-wave peaks, covering a band considerably wider (2–5 c/sec) than those of high alpha intensity (0.5–1.0 c/sec). Eighteen of 25 other subjects showed a similar disparity of width. Mathematical constraints implied by the common finding of two perpendicular alphaband generators were presented. The wide coherent bands (C3-P3/C4-P4) were interpreted as possibly due to shared “side-bands” of the alpha wave; but other features of these wide bands led to the suggestion that much of the local activity not shared between the sides is caused by the shared activity, or at least causally connected with it.