Recording Methods A Eeg Machines

Human EEG is recorded using electrodes with diameters typically in the 0.4- to 1-cm range, held in place on the scalp with special pastes, caps, or nets as illustrated in Fig. 2. EEG recording procedures are

Figure 2 Two kinds of EEG scalp electrode placements are shown in which electrodes are held in place by tension in a supporting structure. (Left) A geodesic net with 128 electrodes making scalp contact with a sponge material (courtesy Electrical Geodesics, Inc.). (Right) An electrode cap containing 131 metal electrodes (courtesy Electro-Cap International, Inc., and the Brain Sciences Institute, Melbourne, Australia). Alternate methods use special pastes to attach electrodes.

Figure 2 Two kinds of EEG scalp electrode placements are shown in which electrodes are held in place by tension in a supporting structure. (Left) A geodesic net with 128 electrodes making scalp contact with a sponge material (courtesy Electrical Geodesics, Inc.). (Right) An electrode cap containing 131 metal electrodes (courtesy Electro-Cap International, Inc., and the Brain Sciences Institute, Melbourne, Australia). Alternate methods use special pastes to attach electrodes.

noninvasive, safe, and painless. Experimental subjects used in research laboratories are often the same students or senior scientists conducting the research. Special gels are applied between electrodes and scalp to improve electrical contact. Wires from scalp electrodes connect to special EEG machines containing amplifiers to boost raw scalp signals, which are typically in the 5- to 200-mV range or approximately 100 times smaller than EKG (heart) signals. With older EEG machines, analog signals are displayed by rotating ink pens writing on chart paper that moves horizontally across machine surfaces. Modern machines typically replace such paper tracing with computer displays (digital EEG) and provide software packages to analyze unprocessed data.

mitted to computer algorithms that estimate potentials on the brain surface by accounting for distortions caused by intervening tissue and the physical separation of electrodes from brain. The combined use of high electrode density and computer algorithms providing such ''inward continuation estimates'' to the brain surface is called high-resolution EEG. Another approach using sophisticated computer methods is dipole localization. This method can estimate the location of source regions in the brain depths in a few specialized applications in which EEG is generated mainly in only one or two isolated source regions. However, in most applications, the sources are distributed over large regions of cerebral cortex and possibly deeper regions as well.

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