[Spectral analysis of rapid (beta) EEG activity]

Spectral analysis techniques provide high resolution of the beta frequency range and, if a logarithmic transformation is used, also of lower level intensity. This is in contrast to the generally poor resolution of fast activity in the time domain, an effect which may be compared with that of a magnifying glass applied to beta activity. In the frequency domain several different types of beta activity may be distinguished, i.e., (1) narrow; (2) medium; (3) broad-band beta; (4) harmonic beta where the beta component shows a harmonic relationship to a lower frequency component; (5) complex beta as a mixture of 2 or more of the foregoing types; and (6) undefined beta, where the relative increase of beta power is the only assessable feature. If observed, significant coherence occurs most frequently between antero-posterior leads in activities of types 1 and 2, whereas coherence between hemispheres is generally low. Broad-band beta seems to be essentially non-coherent. Coherence of harmonic beta depends largely on the features of the underlying component. Bi-coherence analysis may be of help to further analysis of harmonic or complex beta activity.