Method of electroencephalography

In normal practice, the EEG is removed using electrodes located on intact head covers. Electrical potentials are amplified and recorded. In electroencephalographs, 16-24 or more identical amplifying-recording blocks (channels) are provided, allowing one-time recording of electrical activity from a corresponding number of pairs of electrodes mounted on the patient's head. Modern electroencephalographs are based on computers. Enhanced potentials are digitized; Continuous EEG recording is displayed on the monitor and simultaneously recorded to disk. After processing, the EEG can be printed on paper.

Electrodes that divert potentials are metal plates or rods of various shapes with a contact surface diameter of 0.5-1 cm. Electrical potentials are fed to the input box of an electroencephalograph with 20-40 or more numbered contact sockets through which a suitable number of electrodes. In modern electroencephalographs, the input box combines an electrode switch, an amplifier, and an analogue-digital EEG converter. From the input box, the converted EEG signal is fed to a computer, through which the device functions are controlled, EEG recording and processing is performed.

EEG registers the potential difference between two points of the head. Accordingly, each channel of the electroencephalograph is supplied with voltages assigned by two electrodes: one to "input 1", the other to "input 2" of the gain channel. A multi-contact EEG lead switch allows you to switch the electrodes for each channel in the desired combination. Having established, for example, on any channel the correspondence of the occipital electrode to the socket of the input box "1", and the temporal - to the socket of the box "5", it is thereby possible to register a potential difference between the corresponding electrodes along this channel. Before starting the work, the researcher types with the help of appropriate programs, several lead circuits, which are used in the analysis of the received records. Analog and digital high and low frequency filters are used to specify the bandwidth of the amplifier. The standard bandwidth for recording EEG is 0.5-70 Hz.

[1], [2], [3], [4], [5]

Evolution and recording of the electroencephalogram

The recording electrodes are arranged so that all the main parts of the brain, represented by the initial letters of their Latin names, are represented on a multichannel recording. In clinical practice, two basic systems of EEG leads are used: the international system "10-20" and a modified scheme with a reduced number of electrodes. If a more detailed picture of the EEG is required, a "10-20" scheme is preferred.

The referent refers to such a lead when the "input 1" of the amplifier is supplied with potential from the electrode above the brain, and to "input 2" from the electrode at a distance from the brain. The electrode located above the brain is most often called active. The electrode removed from the brain tissue is called the reference one. As such, use the left (A ₁ ) and right (A ₂ ) lobes of the ear. The active electrode is connected to the "input 1" of the amplifier, the supply of which to the negative potential shift causes the recording pen to move upwards. The reference electrode is connected to the "input 2". In some cases, the reference electrode is used to lead from two shorted electrodes (AA) located on the ear lobes. Since the potential difference between the two electrodes is recorded on the EEG, the position of the point on the curve will be equal, but in the opposite direction, the potential changes under each of the electrode pair will be affected. In the reference lead, an alternating potential of the brain is generated under the active electrode. Under the reference electrode, which is far from the brain, there is a constant potential that does not pass into the ac amplifier and does not affect the recording pattern. The potential difference reflects without distortion the oscillation of the electric potential generated by the brain under the active electrode. However, the head region between the active and reference electrodes forms part of the "amplifier-object" electric circuit, and the presence of a sufficiently intense source of potential on this site, located asymmetrically relative to the electrodes, will significantly affect the readings. Consequently, when referencing leads, the judgment about the localization of the potential source is not completely reliable.

Bipolar refers to the lead in which electrodes placed above the brain are connected to the "input 1" and "input 2" of the amplifier. The position of the EEG recording point on the monitor is equally affected by the potentials under each of the pair of electrodes, and the recorded curve reflects the potential difference of each of the electrodes. Therefore, the judgment about the form of oscillation under each of them based on one bipolar lead is impossible. At the same time, the analysis of the EEG recorded from several pairs of electrodes in various combinations makes it possible to determine the localization of the potential sources that make up the components of the complex total curve obtained with bipolar lead.

For example, if there is a local source of slow oscillations in the posterior temporal region, when connecting to the terminals of the amplifier of the front and rear temporal electrodes (Ta, Tp), a record is obtained containing a slow component corresponding to slow activity in the posterior temporal region (Tp), with superimposed on it more rapid oscillations, generated by the normal medulla of the anterior temporal region (Ta). To clarify the question of which electrode registers this slow component, pairs of electrodes are switched on two additional channels, in each of which one is represented by an electrode from the original pair, that is, Ta or Tp. And the second corresponds to some non-temporal lead, for example, F and O.

It is clear that in the newly formed pair (Tp-O), which includes the posterior temporal electrode Tp, which is above the pathologically altered brain substance, a slow component will again be present. In a couple whose inputs are active from two electrodes that are above a relatively intact brain (Ta-F), a normal EEG will be recorded. Thus, in the case of a local pathological cortical focus, the connection of the electrode standing above this focus, paired with any other, leads to the appearance of a pathological component on the corresponding EEG channels. This allows us to determine the localization of the source of pathological oscillations.

An additional criterion for determining the localization of the source of the potential of interest to the EEG is the phenomenon of the distortion of the phase of oscillations. If you connect three electrodes to the inputs of two channels of an electroencephalograph as follows: electrode 1 to "input 1", electrode 3 to "input 2" of amplifier B, and electrode 2 to both "input 2" of amplifier A and "input 1" of amplifier B; to assume that under the electrode 2 a positive displacement of the electric potential occurs with respect to the potential of the remaining parts of the brain (indicated by the sign "+"), it is obvious that the electric current due to this displacement of the potential will have the opposite direction in the circuits of the amplifiers A and B, will be reflected in oppositely directed displacements of the potential difference - antiphase - on the corresponding EEG recordings. Thus, the electric oscillations under the electrode 2 in the records along the channels A and B will be represented by curves having the same frequencies, amplitudes and shape, but opposite in phase. When the electrodes are switched through several channels of the electroencephalograph in the form of a chain, the antiphase oscillations of the investigated potential will be recorded on those two channels, to which different inputs one common electrode is connected, which is above the source of this potential.

[6], [7], [8], [9], [10], [11]

Rules for recording the electroencephalogram and functional tests

The patient should be in a light and soundproof room in a comfortable armchair with closed eyes during the examination. Observation of the researcher is conducted directly or with the help of a video camera. During the recording markers mark significant events and functional tests.

When the sample opens and closes the eyes on the EEG, characteristic artifacts of the electro-oculogram appear. The evolving changes in the EEG make it possible to reveal the degree of contact of the examinee, the level of his consciousness and tentatively assess the reactivity of the EEG.

Single brain stimuli are used to detect the response of the brain to external influences in the form of a short flash of light, a sound signal. In patients with coma, the use of nociceptive stimuli is permissible by pressing the nail on the base of the nail bed of the index finger of the patient.

For photostimulation, short (150 μs) bursts of light close to the white spectrum are used, of sufficiently high intensity (0.1-0.6 J). Photostimulators allow us to present a series of flares used to study the rhythm assimilation reaction - the ability of electroencephalographic oscillations to reproduce the rhythm of external stimuli. Normally, the rhythm assimilation reaction is well expressed at the flicker frequency, close to the EEG rhythms. Rhythmic waves of assimilation have the greatest amplitude in the occipital regions. In photosensitive epileptic seizures, rhythmic photostimulation reveals a photoparoximal response, a generalized discharge of epileptiform activity.

Hyperventilation is carried out mainly to induce epileptiform activity. The subject is offered a deep rhythmic breathing within 3 minutes. The respiration rate should be within 16-20 per minute. EEG registration begins at least 1 minute before the start of hyperventilation and continues throughout the entire hyperventilation and at least 3 minutes after its end.