2 results
7 - Multichannel frequency and time-frequency analysis
- Edited by Christoph M. Michel, Université de Genève, Thomas Koenig, Daniel Brandeis, Lorena R. R. Gianotti, Universität Zürich, Jiří Wackermann
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- Book:
- Electrical Neuroimaging
- Published online:
- 15 December 2009
- Print publication:
- 23 July 2009, pp 145-168
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Summary
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.
3 - Imaging the electric neuronal generators of EEG/MEG
- Edited by Christoph M. Michel, Université de Genève, Thomas Koenig, Daniel Brandeis, Lorena R. R. Gianotti, Universität Zürich, Jiří Wackermann
-
- Book:
- Electrical Neuroimaging
- Published online:
- 15 December 2009
- Print publication:
- 23 July 2009, pp 49-78
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- Chapter
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Summary
Introduction
In order to try to understand “how the brain works,” one must make measurements of brain function. And ideally, the measurements should be as noninvasive as possible, i.e. the brain should be disturbed as little as possible during the measurement of its functions. One of the first types of noninvasive measurements reported in the literature, by Hans Berger, that directly tapped brain function was the human electroencephalogram (EEG), consisting of scalp electric potential differences as a function of time. In fact, Berger saw the EEG as a “window into the brain.” One of Berger's first observations that showed compelling evidence of having tapped brain function was the alpha rhythm. This oscillatory activity, at around 10–12 Hz, is optimally recorded from a posterior electrode with an anterior reference. The activity is very pronounced when the human subject is with eyes closed, awake, alert, resting. By simply being instructed to perform a mental task such as overtly subtracting the number seven serially, starting at 500, the alpha activity disorganizes and almost disappears.
The main subject matter addressed in this chapter is the use of noninvasive extracranial measurements, i.e. the EEG and the magnetoencephalogram (MEG), for the estimation of the distribution in the brain of their electric neuronal generators. This can be seen as an extension of Berger's initial efforts towards developing a window into the brain.