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Link: Introduction to EEG and MEG

The electrical activity of active nerve cells in the brain produces currents spreading through the head. These currents also reach the scalp surface, and resulting voltage differences on the scalp can be recorded as the electroencephalogram (EEG). The currents inside the head produce magnetic fields which can be measured above the scalp surface as the magnetoencephalogram (MEG). EEG and MEG reflect brain electrical activity with millisecond temporal resolution, and are the most direct correlate of on-line brain processing obtainable non-invasively. Unfortunately, the spatial resolution of these methods is limited for principal physical reasons. Even with an infinite amount of EEG and MEG recordings around the head, a non-ambiguous localisation of the activity inside the brain would not be possible. This “inverse problem” is comparable to reconstructing an object from its shadow: only some features (the shape) are uniquely determined, others have to be deduced on the ground of additional information. However, by imposing reasonable modelling constraints or by focussing on rough features of the activity distribution, useful inferences about the activity of interest can be made.

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