Introduction and overview

Time series of EEG scalp potential differences typically appear to be composed of oscillations at various frequencies. Although the amplitude and spatial distributions of these oscillations may fluctuate in time, the quantification of these oscillations as a function of frequency and location (i.e. the multichannel spectral analysis of the EEG) is very reproducible within and across subjects and systematically varies depending on a series of physiologically interesting factors. To mention a few examples, EEG spectral analysis has been successfully employed to characterize a subject's age, state of arousal, the presence of neurological or psychiatric disorders, drugs or task demand as systematic deviations of spectral power from a norm.

In the present chapter, we will outline the possibilities of quantifying EEG oscillations with a special emphasis on those aspects that are specific for multichannel EEG. First, a methodological primer delineates the interdependencies among spectral amplitude, phase and recording montage. In addition, it also delineates what scalp signals we expect from one or several known oscillating sources in the brain. Next, we describe the currently employed analysis strategies: starting from the classic EEG spectral power mapping of measured EEG, we proceed to methods that take into account the relations of amplitude and phase between the different electrodes, which is an essential prerequisite for discussing the results in terms of sources and interactions of brain regions. Finally, we delineate possible methods for custom-tailored quantitative analyses of multichannel EEG oscillations and source localization in the frequency domain.