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EEG Spectral Patterns

Brain Waves

Brain waves are attributed to electrical activities of the brain which are manifest as alternating potential differences at the scalp surface. When acquired through scalp electrodes, such potential differences result in time-continuous signals termed electroencephalogram (EEG). From a physical point of view, it is opportune to model the wave-like qualities of EEG as a finite sum of harmonic oscillations at discrete vibration rates triggered by a central pace-maker. Hence, brain-waves can analoguously be interpreted like sound waves in acoustics which consist of a fundamental oscillation superimposed by higher harmonics. Each of these partial tones is uniquely characterized by 2 quantities, its frequency ("pitch") and its amplitude ("loudness"). In terms of this model, brain-waves are composed of a series of "partial tones" ranging in frequency between 0.25Hz and 64Hz (7 octaves), whereby the "tonal composition" essentially depends on the state of consciousness, such as wakefulness or sleep stages.

Characteristic Fluctuations

The fluctuations of EEG partial tones display marked between-subject differences together with a high within-subject stability over time. To quantify these characteristic fluctuations we have developed the concept of "spectral patterns" which generalizes the notion "spectrum" in such a way that spectral intensities are regarded as fluctuating rather than being fixed-valued, thus incorporating the non-stationary nature of EEG time series into our model.


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sleep EEG

Fig. 20a: EEG sleep pattern of a male proband: the variability of spectral intensities is plotted on log proportional sclaes along the vertical axis at a frequency resolution of 1/6-tone over 5 octaves. The pattern encompasses theta- and flat alpha-waves superimposed to delta-activity.
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