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Electroencephalography (EEG)

Electrical Activities of the Brain

Brain waves are attributed to electrical activities of the brain that 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 the physical point of view the wave-like qualities of the EEG can be modelled as a finite sum of harmonic oscillations at discrete vibration rates. Each oscillation is uniquely characterized by two quantities, frequency ("pitch") and amplitude ("loudness"). In terms of this model, brain waves are composed of a series of "partial tones" ranging in frequency between 0.25 – 64 Hz (7 octaves), where the tonal composition essentially depends on the subject’s state of consciousness, such as wakefulness or sleep stages.

Natural versus Reactive Fluctuations

The amplitudes of EEG partial tones are not constant, but fluctuate around average values. The same is true, though less pronounced, for frequency. Here we find subtle shifts of partial tones towards lower and higher pitch, analogous to intonation patterns in human speech. Although EEG fluctuations possess a pronounced random character, detailed analysis also reveals strong intrinsic regularities that allow one to distinguish between (1) "natural" fluctuations that reflect the basic activities of brain regions, and (2) "reactive" fluctuations that reflect responses to or interactions with the immediate environment. While natural fluctuations and slow reactive changes can be quantified through standard spectral analysis – which involves a transformation of the EEG signal from the time into the frequency domain ("Fourier transformation") –, fast reactive changes of the EEG can only be assessed in the time domain through an "event-related potential" analysis.

References

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Stassen HH, Lykken DT, Propping P: Zwillingsuntersuchungen zur Genetik des normalen Elektroenzephalogramms. In: P. Baumann (ed): Biologische Psychiatrie der Gegenwart, Wien: Springer 1993, 139-144
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Stassen HH: EEG and evoked potentials. In: D. Cooper (ed) Nature Encyclopedia of the Human Genome. Nature Publishing Group, London 2003; 3: 266-269
Weisbrod M, Hill H, Sauer H, Niethammer R, Guggenbühl S, Stassen HH: Nongenetic pathologic developments of brain-wave patterns in monozygotic twins discordant and concordant for schizophrenia. Am J Med Genetics B 2004; 125: 1-9
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EEG spectral pattern
Fig. 4: EEG spectral pattern derived from a typical Alpha-EEG. The variability of the spectral intensities is estimated from 4 consecutive epochs of 20-sec length under the experimental condition of quiet wakefulness and plotted as shaded area along the verical axis. The spectral resolution is 0.25Hz over the frequency range of 0-30Hz.
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