EEG: Origin and measurement

The existence of the electrical activity of the brain (i.e. the electroencephalogram or EEG) was discovered more than a century ago by Caton. After the demonstration that the EEG could be recorded from the human scalp by Berger in the 1920s, it made a slow start before it became accepted as a method of analysis of brain functions in health and disease. It is interesting to note that this acceptance came only after the demonstration by Adrian and Mathews (1934) that the EEG, namely the alpha rhythm, was likely generated in the occipital lobes in man, and was not artefactual. However, the neuronal sources of the alpha rhythm remained undefined until the 1970s, when we demonstrated, in dog, that the alpha rhythm is generated by a dipole layer cantered at layers IV and V of the visual cortex (Lopes da Silva and Storm van Leeuwen 1977). It may be not surprising that the mechanisms of generation and the functional significance of the EEG remained controversial for a relatively long time considering the complexity of the underlying systems of neuronal generators on the one hand and the rather involved transfer of signals from the cortical surface to the scalp due to the topological and electrical properties of the volume conductor (brain, cerebrospinal fluid, skull, scalp) on the other. The EEG consists of the summed electrical activities of populations of neurons, with a modest contribution from glial cells. The neurons are excitable cells with characteristic intrinsic electrical properties, and their activity produces electrical and magnetic fields. These fields may be recorded by means of electrodes at a short distance from the sources (the local EEG or local field potentials, LFPs), or from the cortical surface (the electrocorticogram or ECoG), or at longer distances, even from the scalp (i.e. the EEG, in the most common sense). The associated MEG is usually recorded via sensors that are highly sensitive to changes in the very weak neuronal magnetic fields, which are placed at short distances around the scalp.
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About this book

* The first book to cover the aspects of the integration of EEG and fMRI
* Contains discussion of the physiological principals, practical aspects of measurement, artefact reduction and analysis
* Reviews all applications, which are mainly in the field of epilepsy sleep research, cognitive neuroscience and clinical use in neurology and psychiatry
* This is the first time that electrophysiological and haemodynamic characteristics of the brain can be measured at the same time

Functional magnetic resonance imaging (fMRI) and electroencephalography (EEG) are very important and complementary modalities since fMRI offers high spatial resolution while EEG provides a direct measurement of neuronal activity with high temporal resolution. Interest in the integration of these two types of data is growing rapidly as it promises to yield important new insights into human brain activity, as has already occurred in the case of epilepsy. Indeed, it seems certain that integrated EEG-fMRI will play an increasing role in neuroscience and in the clinical study of various brain disorders. This book discusses in depth all aspects of EEG-fMRI, including physiological principles and technical and methodological issues such as EEG artefact reduction methods, image quality, and data analysis strategies. Detailed consideration is given to all potential applications, primarily in the fields of sleep research, cognitive neuroscience, and clinical neurology and psychiatry. All of the authors are recognized experts in the field, and the text is supported by numerous informative illustrations.