Chapter 1.5 - Spontaneous Electroencephalogram

Abstract

The EEG is electric field measured from scalp electrodes. For standard placement of electrodes the 10-20 International System is used. The main feature of EEG is oscillatory nature of potential dynamics. The oscillations are seen at different frequency bands: infralow (0.01 -0.1), low (0.1-1), delta (1-4 Hz), theta (4-8 Hz), alpha (8-12 Hz), and beta (>13 Hz) bands. A powerful method of extracting and compressing the information about rhythmicity over time is given by Fourier analysis. For assessing dynamics of oscillations wavelet analysis is used. At EEG spectra the oscillations are seen as peaks at the corresponding frequency. The generators of these rhythms are neuronal networks with different membrane and synaptic mechanisms based on interplay between excitatory and inhibitory processes. This interplay leads to oscillations which in turn control the neuronal network functioning through voltage gated channels. EEG as electrical field is volume-conducted so that a given single current dipole generates a distribution of positive and negative potentials recorded on the scalp. Distributed Source Models (such as LORETA) are used to assess the cortical sources of the scalp recorded potentials. A powerful tool for separating sources from spontaneous EEG is given by a family of blind source separation methods. Independent component analysis (ICA) represents one of such methods. It decomposes multi-channel EEG signals into a linear combination of statistically independent components.