Electrode Placement

In standard clinical practice, 19 recording electrodes are placed uniformly over the scalp (the International 10-20 System). In addition, one or two reference electrodes (often placed on ear lobes) and a ground electrode (often placed on the nose to provide amplifiers with reference voltages) are required. Potential differences between electrode pairs are recorded with EEG machines containing amplifiers, filters, and other hardware. In referential recordings, potentials between each recording electrode and a fixed reference are measured over time. The distinction between ''recording'' and "reference" electrodes is mostly artificial since both electrode categories involve potential differences between body sites, allowing closed current loops through tissue and EEG machine. Bipolar recordings measure potential differences between adjacent scalp electrodes. When such bipolar electrodes are placed close together (e.g., 1 or 2 cm), potential differences are estimates of tangential electric fields (or current densities) in the scalp between the electrodes. Electrode placements and the different ways of combining electrode pairs to measure potential differences on the head constitute the electrode montage.

Many research and some clinical laboratories use more than 21 electrodes to obtain more detailed information about brain sources. However, more electrodes may add very little useful information unless supplemented by sophisticated computer algorithms to reduce raw EEG data to a manageable form. Often, 48-131 recording electrodes are used in research; laboratories may soon use as many as 256 channels. The resulting multichannel data are sub

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