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EEG Recording

EEG recording, as it was mentioned in the beginning of the chapter, is a rather routine procedure, particularly in clinics. Therefore the equipment for EEG is manufactured in almost all developed countries and its advertising and specification is presented in the journals of appropriate profile. All this equipment is supplied with detailed instructions for its use. Nevertheless it is worthwhile to present below some details of EEG recording procedure useful for researchers naive in this field.

The EEG recording usually include the follows steps:

  1. A subject is seated in comfortable chair in dimly illuminated room;
  2. Electrodes are placed on his head according to certain scheme;
  3. The reference electrodes are chosen;
  4. Parameters of electroencephalograph and software for EEG acquisition and storage are established;
  5. Calibration of electroencephalograph and data acquisition software is executed;
  6. EEG is recorded;
  7. Artifacts are removed.

EEG cabin. The EEG recordings is performed usually in a room shielded from outer electrical and magnetic fields. But modern amplifiers can reject these effects. During the recording procedure the subject should avoid movements, which can cause artifacts in a record.

Electrodes and their placement schemes. The most appropriate electrodes for the EEG scalp recording are Ag-AgCl which avoid potential shift due to electrode polarization. To get a good (i. e., with impedance below 5 Kilo-Ohms) contact between electrode and skin surface, the skin has to be cleaned with ether or alcohol for fat or dirt removal. Some abrasives were in practice earlier to lower the impedance, but it is unacceptable due to risk of bacterial, HIV and prion infection. An electrode gel or salt solutions are used to improve potential conduction between skin and electrode surface.

    10-20.gif (4575 bytes)

    Figure. 10-20 electrodes placement scheme. According to this scheme three distances are measured: that between two preauricular points, that between the nasion (nose bridge) and inion (the occipital bone mount), both across vertex, and the circumference between the last two point of the skull. These distances are divided in proportion of 10-20-20-20-20-10% in both orthogonal axes and in circumference, and a net of imaging quadrates is built on head surface. The electrodes are placed in a quadrates angles.

     

 

The most popular scheme for electrode placement is the so-called 10/20 scheme (Jasper 1958) (Figure 2). Additional electrodes may be placed between the basic ones. According to “IFCN Standards for digital recording of clinical EEG“ (Nuwer et al 1998), amplification and channel acquisition must be available for at least 24 EEG channels. For artifact removal electrooculogram records are used. Now the most common way to place the electrode array on the scalp is the use of a cap with the electrodes fixed on it. These caps (or helmets) are available with different numbers of electrodes (19, 32, 64, up to 256 electrodes) and in several sizes, including ones for children (see, for example, the catalogues of “Electro Cap”, “Geodesic Sensor Net” and “NeuroScan”). Such devices can be placed and removed rapidly and cause a minimal unpleasant feeling. The latter is especially important for psychophysiological experiments, when a rather long recording is required. These caps automatically provide the electrode placement with appropriate, usually equal, interelectrode distance.

Reference electrodes. One of the important questions in EEG recording is the site of reference electrodes, relative to which the electric brain potentials in all other electrodes is measured. The reference electrodes should be placed on a presumed “inactive” zone. Frequently, this is the left or right earlobe or both of them. If one earlobe electrode is used as a reference, the topography of EEG rhythms is rather close to true, but there is the systematic decrease of EEG amplitude in the electrodes which are closer to the reference side. If “linked” earlobes are used, this kind of asymmetry is avoided but this distorts the EEG picture since the electric current flows inside the linking wire. This affects the intracranial currents that form the EEG potentials. Besides this, low-amplitude EEG is observed in both temporal areas. Alternatively, the EEG may be recorded with any scalp electrode as a reference, and then the average reference is computed as a mean of all electrodes. It avoids all kind of asymmetry and makes the EEG recorded in various laboratories comparable. But in some cases using the common reference may reveal rhythms not at their actual location. Sometimes the so called bipolar recording is used when the potential is measured between two active electrodes. This sheme is good for exact location of some locale potential changes, i.e., pathological activity focus. The comprehensive review of reference problem may be found in Lehmann (1987).

Parameters for computerized EEG acquisition and storage. For acquisition and storage of EEG data “IFCN Standards” recommend a minimum sampling rate of the analog to digital conversion (ADC) of 200 samples/second (Nuwer et al 1998). This rate allows to analyze frequencies up to 100 Hz, as the maximal allowed frequency of the input signal (the Nyquist frequency) should be the half of sampling rate. If the signal is sampled at too low rate, aliasing (falsification of the signal) may occur with unpredictable errors in the digital waveform compared to the original one. Prior to sampling an anti-aliasing low-pass filter must be used. ADC should be done at a resolution of at least 12 bits in order for the EEG to be resolved down to 0.5 µV. Whenever possible, the low-pass filter should be set to 0.16 Hz or less for recording. Routine use of higher settings of this frequency for recording are discouraged, as they should be reserved for specific or difficult clinical recordings only. A 50-60 Hz notch filter should be available, but not routinely used. Interchannel cross-talk must be less than 1%, i.e., 40 dB down or better.

Calibration. The calibration is needed to determine the exact amplitude of EEG signal and to evaluate the amplifier noise and other possible artifacts produced by it as well as by connection wires. Usually sine, triangle, and rectangle impulses of known amplitude are generated for this purpose by a special circuitry on the input of the main amplifier of the electroencephalograph. The calibration signal thus passes through the large part of the EEG signal’s path in the recording system. The calibration impulses should be recorded and then used to measure the true EEG amplitude and to evaluate the equipment noise in quantitative EEG analysis. The modern software usually includes automatical comparison of EEG and calibration signals showing the actual brain electrical potential values.

Artifacts. EEG artifacts appear due to external electrical or magnetic fields and subjects movements during recording procedure. The last are caused both by muscle electrical potentials fields and electrode displacement. Visual and automatic search of high amplitude artifacts usually is not difficult. For example, eye movement artifacts can be eliminated via special algorithms (Gratton et al 1983). Search and rejection of low amplitude artifacts is possible only by collation of results of frequency analysis, topographical mapping, and original EEG records. Topographical distribution of main artifacts discussed by Lee and Buchsbaum (1987). Eye movements are mainly reflected in frontal sites. Muscle activity is high-frequency and has lateral topography. Artifacts due to bad electrode placement have simple forms and are restricted by given EEG derivation.

 
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