30. Basic rhythms of eeg.

Human EEG is recorded using electrodes with diameters typically in the 0.4 to 1.0 cm range, held in place on the scalp with special pastes, caps or nets as in the example of Figure 1. EEG recording procedures are noninvasive, safe and painless. Experimental subjects recruited for research are often the same students or scientists conducting the research. 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. 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 (say 1 or 2 centimeters), 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.

Most research and some clinical laboratories use many more electrodes to obtain detailed information about brain sources. However, additional electrodes may add minimal useful information unless supplemented by computer algorithms to reduce raw EEG data to manageable form; often 64 to 131 recording electrodes or more are used in research. Any single electrode may be chosen as the reference and potentials at other locations recorded with respect to this reference. In EEG practice, inappropriate reference choices have often led to erroneous interpretations of brain source locations and measures of dynamic behavior. When large numbers of electrodes are employed, potential at each location may be measured with respect to the average of all potentials (the common average reference), which often provides a good estimate of potential at infinity. The common average reference is not appropriate when electrode coverage is sparse (perhaps less than 64 electrodes).

The resulting multi-channel data are submitted to 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 computer methods is dipole localization. This method can estimate the location of source regions in the brain depths in a few specialized applications where EEG is mainly generated in only one or two isolated source regions.