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Electroencephalography (EEG) is a method of recording the electrical activity of the brain using electrodes placed on the skin of the scalp.

By analogy with the operation of a computer, from the operation of a single transistor to the functioning of computer programs and applications, the electrical activity of the brain can be considered at different levels: on the one hand, the action potentials of individual neurons, on the other, the general bioelectrical activity of the brain, which is recorded using EEG.

EEG results are used both for clinical diagnosis and for scientific purposes. There is intracranial, or intracranial EEG (intracranial EEG, icEEG), also called subdural EEG (subdural EEG, sdEEG) and electrocorticography (ECoG, or electrocorticography, ECoG). When conducting these types of EEG, the registration of electrical activity is carried out directly from the surface of the brain, and not from the scalp. ECoG is characterized by a higher spatial resolution compared to the surface (percutaneous) EEG, since the bones of the skull and scalp somewhat "soften" the electrical signals.

However, transcranial electroencephalography is used much more frequently. This method is key in the diagnosis of epilepsy, and also provides additional valuable information for many other neurological disorders.

Historical reference

In 1875, the Liverpool medical practitioner Richard Caton (1842-1926) presented in the British Medical Journal the results of an electrical phenomenon observed during his examination of the cerebral hemispheres of rabbits and monkeys. In 1890, Beck published a study of the spontaneous electrical activity of the brain of rabbits and dogs, which manifested itself in the form of rhythmic oscillations that change when exposed to light. In 1912, the Russian physiologist Vladimir Vladimirovich Pravdich-Neminsky published the first EEG and evoked potentials of a mammal (dog). In 1914, other scientists (Cybulsky and Jelenska-Macieszyna) photographed an EEG recording of an artificially induced seizure.

The German physiologist Hans Berger (1873-1941) began researching the human EEG in 1920. He gave the device its modern name, and although other scientists had previously performed similar experiments, Berger is sometimes considered the discoverer of the EEG. In the future, his ideas were developed by Edgar Douglas Adrian.

In 1934, a pattern of epileptiform activity was first demonstrated (Fisher and Lowenback). The beginning of clinical encephalography is considered to be 1935, when Gibbs, Davis and Lennox described interictal activity and the pattern of a small epileptic seizure. Subsequently, in 1936, Gibbs and Jasper characterized interictal activity as a focal feature of epilepsy. In the same year, the first EEG laboratory was opened at Massachusetts General Hospital.

Franklin Offner (Franklin Offner, 1911-1999), a professor of biophysics at Northwestern University, developed a prototype electroencephalograph that included a piezoelectric recorder called a kristograph (the entire device was called Offner's Dynograph).

In 1947, in connection with the founding of the American Society of Electroencephalography (The American EEG Society), the first International Congress on EEG was held. And already in 1953 (Aserinsky and Kleitmean) discovered and described the phase of sleep with rapid eye movement.

In the 1950s, the English physician William Gray Walter developed a method called EEG topography, which made it possible to map the electrical activity of the brain on the surface of the brain. This method is not applicable to clinical practice, it is used only in scientific research. The method gained particular popularity in the 1980s and was of particular interest to researchers in the field of psychiatry.

Physiological basis of EEG

When conducting an EEG, the total postsynaptic currents are measured. An action potential (AP, short-term change in potential) in the presynaptic membrane of the axon causes the release of a neurotransmitter into the synaptic cleft. A neurotransmitter, or neurotransmitter, is a chemical that transmits nerve impulses across synapses between neurons. After passing through the synaptic cleft, the neurotransmitter binds to receptors on the postsynaptic membrane. This causes ionic currents in the postsynaptic membrane. As a result, compensatory currents arise in the extracellular space. It is these extracellular currents that form the EEG potentials. EEG is insensitive to AP of axons.

Although postsynaptic potentials are responsible for the formation of the EEG signal, surface EEG is not able to capture the activity of a single dendrite or neuron. It is more correct to say that the surface EEG is the sum of the synchronous activity of hundreds of neurons with the same orientation in space, located radially to the scalp. Currents directed tangentially to the scalp are not recorded. Thus, during the EEG, the activity of apical dendrites located radially in the cortex is recorded. Since the voltage of the field decreases in proportion to the distance to its source to the fourth power, the activity of neurons in the deep layers of the brain is much more difficult to fix than the currents directly near the skin.

The currents recorded on the EEG are characterized by different frequencies, spatial distribution and relationship with different brain states (for example, sleep or wakefulness). Such potential fluctuations represent the synchronized activity of a whole network of neurons. Only a few neural networks responsible for the recorded oscillations have been identified (for example, thalamocortical resonance underlying "sleep spindles" - accelerated alpha rhythms during sleep), while many others (for example, the system that forms the occipital basic rhythm) have not yet been established. .

EEG technique

To obtain a traditional surface EEG, the recording is performed using electrodes placed on the scalp using an electrically conductive gel or ointment. Usually, before placing the electrodes, if possible, dead skin cells are removed, which increase the resistance. The technique can be improved by using carbon nanotubes that penetrate into the upper layers of the skin and improve electrical contact. Such a sensor system is called ENOBIO; however, the presented methodology general practice(neither in scientific research, nor even in the clinic) is not yet used. Typically, many systems use electrodes, each with a separate wire. Some systems use special caps or helmet-like mesh structures that enclose the electrodes; most often, this approach justifies itself when a set with a large number of densely spaced electrodes is used.

For most clinical and research applications (with the exception of sets with a large number of electrodes), the location and name of the electrodes are determined by the International "10-20" system. The use of this system ensures that electrode names are strictly consistent between different laboratories. In the clinic, a set of 19 electrodes (plus ground and reference electrode) is most commonly used. Fewer electrodes are usually used to record the EEG of newborns. Additional electrodes can be used to obtain an EEG of a specific area of ​​the brain with higher spatial resolution. A set with a large number of electrodes (usually in the form of a cap or a mesh helmet) can contain up to 256 electrodes located on the head at more or less the same distance from each other.

Each electrode is connected to one input of the differential amplifier (that is, one amplifier per pair of electrodes); in the standard system, the reference electrode is connected to the other input of each differential amplifier. Such an amplifier increases the potential between the measuring electrode and the reference electrode (typically 1,000-100,000 times, or a voltage gain of 60-100 dB). In the case of an analog EEG, the signal then passes through a filter. At the output, the signal is recorded by the recorder. However, many recorders these days are digital, and the amplified signal (after passing through a noise filter) is converted using an analog-to-digital converter. For clinical surface EEG, the A/D conversion frequency occurs at 256-512 Hz; conversion frequency up to 10 kHz is used for scientific purposes.

In a digital EEG, the signal is stored in in electronic format; for display, it also passes through the filter. The usual settings for the low pass filter and the high pass filter are 0.5-1 Hz and 35-70 Hz, respectively. The low pass filter usually removes slow wave artifacts (eg motion artifacts) and the high pass filter desensitizes the EEG channel to high frequency fluctuations (eg electromyographic signals). In addition, an optional notch filter can be used to eliminate noise caused by power lines (60 Hz in the US and 50 Hz in many other countries). The notch filter is often used if the EEG recording is carried out in the intensive care unit, that is, in extremely unfavorable technical conditions for the EEG.

To assess the possibility of surgical treatment of epilepsy, it becomes necessary to place electrodes on the surface of the brain, under the dura mater. To carry out this EEG variant, a craniotomy is performed, that is, a burr hole is formed. This EEG variant is called intracranial, or intracranial EEG (intracranial EEG, icEEG), or subdural EEG (subdural EEG, sdEEG), or electrocorticography (ECoG, or electrocorticography, ECoG). The electrodes can be immersed in brain structures, such as the amygdala (amygdala) or the hippocampus, brain regions where epileptic foci are formed, but whose signals cannot be recorded during a surface EEG. The electrocorticogram signal is processed in the same way as the routine EEG digital signal (see above), however, there are several features. Usually, ECoG is recorded at higher frequencies compared to the surface EEG, since, according to the Nyquist theorem, high frequencies predominate in the subdural signal. In addition, many of the artifacts that affect surface EEG results do not affect ECoG, and therefore the use of an output signal filter is often unnecessary. Typically, the amplitude of the EEG signal of an adult is about 10-100 μV when measured on the scalp and about 10-20 mV when measured subdurally.

Since the EEG signal is the potential difference between the two electrodes, the EEG results can be displayed in several ways. The order of simultaneous display of a certain number of leads when recording an EEG is called editing.

Bipolar montage

Each channel (that is, a separate curve) represents the potential difference between two adjacent electrodes. Installation is a collection of such channels. For example, the channel "Fp1-F3" is the potential difference between the Fp1 electrode and the F3 electrode. The next montage channel, "F3-C3", reflects the potential difference between electrodes F3 and C3, and so on for the entire set of electrodes. There is no common electrode for all leads.

Referential mounting

Each channel represents the potential difference between the selected electrode and the reference electrode. There is no standard location for the reference electrode; however, its location is different from that of the measuring electrodes. Often, electrodes are placed in the area of ​​projections of the median structures of the brain on the surface of the skull, since in this position they do not amplify the signal from any of the hemispheres. Another popular electrode fixation system is the attachment of electrodes to the earlobes or mastoid processes.

Laplace montage

Used when recording a digital EEG, each channel is the potential difference of the electrode and the weighted average value for the surrounding electrodes. The averaged signal is then called the averaged reference potential. When using an analog EEG during recording, the specialist switches from one type of editing to another in order to maximally reflect all the characteristics of the EEG. In the case of a digital EEG, all signals are stored according to a certain type of montage (usually referential); since any type of montage can be mathematically constructed from any other, the EEG can be observed by a specialist in any montage.

Normal EEG activity

The EEG is usually described using terms such as (1) rhythmic activity and (2) transient components. Rhythmic activity changes in frequency and amplitude, in particular, forming an alpha rhythm. But some changes in rhythmic activity parameters may be of clinical significance.

Most of the known EEG signals correspond to the frequency range from 1 to 20 Hz (under standard recording conditions, rhythms whose frequency is outside this range are most likely artifacts).

Delta waves (δ-rhythm)

The frequency of the delta rhythm is up to about 3 Hz. This rhythm is characterized by high-amplitude slow waves. Usually present in adults during non-REM sleep. It also occurs normally in children. The delta rhythm can occur in foci in the area of ​​subcortical lesions or spread everywhere with diffuse lesions, metabolic encephalopathy, hydrocephalus, or deep lesions of the midbrain structures. Usually this rhythm is most noticeable in adults in the frontal region (frontal intermittent rhythmic delta activity, or FIRDA - Frontal Intermittent Rhythmic Delta) and in children in the occipital region (occipital intermittent rhythmic delta activity or OIRDA - Occipital Intermittent Rhythmic Delta).

Theta waves (θ-rhythm)

Theta rhythm is characterized by a frequency of 4 to 7 Hz. Usually seen in young children. It can occur in children and adults in a state of drowsiness or during activation, as well as in a state of deep thought or meditation. An excess of theta rhythms in elderly patients indicates pathological activity. It can be observed as a focal disorder with local subcortical lesions; and in addition, it can spread in a generalized manner with diffuse disorders, metabolic encephalopathy, lesions of the deep structures of the brain, and in some cases with hydrocephalus.

Alpha waves (α-rhythm)

For the alpha rhythm, the characteristic frequency is from 8 to 12 Hz. The name of this type of rhythm was given by its discoverer, the German physiologist Hans Berger. Alpha waves are observed in back departments heads on both sides, and their amplitude is higher in the dominant part. This type of rhythm is detected when the subject closes his eyes or is in a relaxed state. It is noticed that the alpha rhythm fades if you open your eyes, and also in a state of mental stress. Now this type of activity is called the "basic rhythm", "occipital dominant rhythm" or "occipital alpha rhythm". In fact, in children, the basic rhythm has a frequency of less than 8 Hz (that is, technically falls into the range of theta rhythm). In addition to the main occipital alpha rhythm, there are normally several more of its normal variants: mu rhythm (μ rhythm) and temporal rhythms - kappa and tau rhythms (κ and τ rhythms). Alpha rhythms can also occur in pathological situations; for example, if a patient in a coma has a diffuse alpha rhythm on the EEG that occurs without external stimulation, such a rhythm is called "alpha coma".

Sensorimotor rhythm (μ-rhythm)

The mu rhythm is characterized by the frequency of the alpha rhythm and is observed in the sensorimotor cortex. The movement of the opposite hand (or the representation of such a movement) causes the mu rhythm to decay.

Beta waves (β-rhythm)

The frequency of the beta rhythm is from 12 to 30 Hz. Usually the signal has a symmetrical distribution, but is most evident in the frontal region. A low-amplitude beta rhythm with varying frequency is often associated with restless and fussy thinking and active concentration. Rhythmic beta waves with a dominant set of frequencies are associated with various pathologies and the action of drugs, especially the benzodiazepine series. A rhythm with a frequency of more than 25 Hz, observed during the removal of a surface EEG, is most often an artifact. It may be absent or mild in areas of cortical damage. The beta rhythm dominates the EEG of patients who are in a state of anxiety or worry, or in patients whose eyes are open.

Gamma waves (γ-rhythm)

The frequency of gamma waves is 26-100 Hz. Due to the fact that the scalp and skull bones have filtering properties, gamma rhythms are recorded only during electrocortigraphy or, possibly, magnetoencephalography (MEG). It is believed that gamma rhythms are the result of the activity of various populations of neurons united in a network to perform a certain motor function or mental work.

For research purposes, with a DC amplifier, activity close to DC or which is characterized by extremely slow waves is recorded. Typically, such a signal is not recorded in a clinical setting, since a signal with such frequencies is extremely sensitive to a number of artifacts.

Some EEG activities may be transient and do not recur. Peaks and sharp waves may be the result of an attack or interictal activity in patients with or predisposed to epilepsy. Other temporary phenomena (vertex potentials and sleep spindles) are considered normal variants and are observed during normal sleep.

It is worth noting that there are some types of activity that are statistically very rare, but their manifestation is not associated with any disease or disorder. These are the so-called "normal variants" of the EEG. An example of such a variant is the mu-rhythm.

EEG parameters depend on age. The EEG of a newborn is very different from the EEG of an adult. The EEG of a child usually includes lower frequency oscillations compared to the EEG of an adult.

Also, EEG parameters vary depending on the state. EEG is recorded along with other measurements (electrooculogram, EOG and electromyogram, EMG) to determine sleep stages during a polysomnography study. The first stage of sleep (drowsiness) on the EEG is characterized by the disappearance of the occipital main rhythm. In this case, an increase in the number of theta waves can be observed. There is a whole catalog of different EEG patterns during drowsiness (Joan Santamaria, Keith H. Chiappa). In the second stage of sleep, sleep spindles appear - short-term series of rhythmic activity in the frequency range of 12-14 Hz (sometimes called the "sigma band"), which are most easily recorded in the frontal region. The frequency of most waves in the second stage of sleep is 3-6 Hz. The third and fourth stages of sleep are characterized by the presence of delta waves and are commonly referred to as non-REM sleep. Stages one through four constitute so-called non-Rapid Eye Movements (non-REM, NREM) sleep. EEG during sleep with rapid eye movement (REM) is similar in its parameters to the EEG in the waking state.

The results of an EEG performed under general anesthesia depend on the type of anesthetic used. With the introduction of halogenated anesthetics, such as halothane, or intravenous agents, such as propofol, a special “fast” EEG pattern (alpha and weak beta rhythms) is observed in almost all leads, especially in the frontal region. According to the former terminology, this EEG variant was called the frontal, widespread fast (Widespread Anterior Rapid, WAR) as opposed to the widespread slow pattern (Widespread Slow, WAIS) that occurs with the introduction of large doses of opiates. Only recently, scientists have come to understand the mechanisms of the effect of anesthetic substances on EEG signals (at the level of the interaction of a substance with various types of synapses and understanding of the circuits due to which the synchronized activity of neurons is carried out).

Artifacts

biological artifacts

Artifacts are called EEG signals that are not associated with brain activity. Such signals are almost always present on the EEG. Therefore, the correct interpretation of the EEG requires great experience. The most common types of artifacts are:

• artifacts caused by eye movement (including the eyeball, eye muscles and eyelid);
• artifacts from the ECG;
• artifacts from EMG;
• artifacts caused by the movement of the tongue (glossokinetic artifacts).

Artifacts caused by eye movement are due to the potential difference between the cornea and the retina, which turns out to be quite large compared to the potentials of the brain. No problems arise if the eye is in a state of complete rest. However, reflex eye movements are almost always present, generating a potential, which is then recorded by the frontopolar and frontal leads. Eye movements - vertical or horizontal (saccades - rapid jerky eye movements) - occur due to the contraction of the eye muscles, which create an electromyographic potential. Regardless of whether this blinking of the eyes is conscious or reflex, it leads to the emergence of electromyographic potentials. However, in this case, when blinking, it is the reflex movements that are more important. eyeball, because they cause a number of characteristic EEG artifacts.

Artifacts of a characteristic type, arising from trembling of the eyelids, were previously called the kappa rhythm (or kappa waves). They are usually recorded by the prefrontal leads, which are directly above the eyes. Sometimes they can be found during mental work. They usually have a theta (4-7 Hz) or alpha (8-13 Hz) frequency. This species The activity was named because it was thought to be the result of brain activity. Later it was found that these signals are generated as a result of movements of the eyelids, sometimes so subtle that they are very difficult to notice. In fact, they should not be called a rhythm or a wave, because they are noise or an "artifact" of the EEG. Therefore, the term kappa rhythm is no longer used in electroencephalography, and the specified signal should be described as an artifact caused by eyelid trembling.

However, some of these artifacts turn out to be useful. Eye movement analysis is essential in polysomnography and is also useful in conventional EEG to evaluate possible changes in anxiety, wakefulness, or sleep.

Very often there are ECG artifacts that can be confused with spike activity. The modern way of EEG recording usually includes one ECG channel coming from the extremities, which makes it possible to distinguish the ECG rhythm from spike waves. This method also makes it possible to determine various variants of arrhythmia, which, along with epilepsy, can be the cause of syncope (fainting) or other episodic disorders and seizures. Glossokinetic artifacts are caused by the potential difference between the base and the tip of the tongue. Small movements of the tongue "clog" the EEG, especially in patients suffering from parkinsonism and other diseases that are characterized by tremor.

Artifacts of external origin

In addition to artifacts of internal origin, there are many artifacts that are external. Moving near the patient and even adjusting the position of the electrodes can cause EEG interference, bursts of activity due to a short-term change in the resistance under the electrode. Poor grounding of the EEG electrodes can cause significant artifacts (50-60 Hz) depending on the parameters of the local power system. An intravenous drip can also be a source of interference, since such a device can cause rhythmic, fast, low-voltage bursts of activity that are easily confused with real potentials.

Artifact correction

Recently, to correct and eliminate EEG artifacts, the decomposition method was used, which consists in decomposing EEG signals into a number of components. There are many algorithms for decomposing a signal into parts. Each method is based on the following principle: it is necessary to carry out such manipulations that will allow obtaining a “clean” EEG as a result of neutralization (zeroing) of unwanted components.

pathological activity

Pathological activity can be roughly divided into epileptiform and non-epileptiform. In addition, it can be divided into local (focal) and diffuse (generalized).

Focal epileptiform activity is characterized by fast, synchronous potentials of a large number of neurons in a certain area of ​​the brain. It may occur outside of a seizure and indicate an area of ​​the cortex (an area of ​​increased excitability) that is predisposed to the onset of epileptic seizures. Registration of interictal activity is still not enough to establish whether the patient really suffers from epilepsy, or to localize the area in which the attack originates in the case of focal or focal epilepsy.

The maximum generalized (diffuse) epileptiform activity is observed in the frontal zone, but it can also be observed in all other projections of the brain. The presence of signals of this nature on the EEG suggests the presence of generalized epilepsy.

Focal non-epileptiform pathological activity may be observed at sites of cortical injury or white matter brain. It contains more low-frequency rhythms and/or is characterized by the absence of normal high-frequency rhythms. In addition, such activity can manifest itself as a focal or unilateral decrease in the amplitude of the EEG signal. Diffuse non-epileptiform pathological activity may manifest as scattered abnormally slow rhythms or bilateral slowing of normal rhythms.

Advantages of the method

The EEG as a tool for brain research has several significant benefits, for example, EEG is characterized by a very high resolution in time (at the level of one millisecond). For other methods of studying brain activity, such as positron emission tomography (positron emission tomography, PET) and functional MRI (fMRI, or Functional Magnetic Resonance Imaging, fMRI), time resolution is between seconds and minutes.

The EEG method measures the electrical activity of the brain directly, while other methods capture changes in blood flow velocity (for example, single-photon emission computed tomography, SPECT, or Single-Photon Emission Computed Tomography, SPECT; and fMRI), which are indirect indicators of brain activity. EEG can be performed simultaneously with fMRI to co-record both high temporal and high spatial resolution data. However, since the events recorded as a result of the study by each of the methods occur in different periods time, it is not at all necessary that the data set reflects the same brain activity. There are technical difficulties in combining these two methods, which include the need to eliminate EEG artifacts of radiofrequency impulses and the movement of pulsating blood. In addition, currents can occur in the wires of the EEG electrodes due to magnetic field generated by MRI.

EEG can be recorded simultaneously with MEG, so the results of these complementary studies with high time resolution can be compared with each other.

Method Limitations

The EEG method has several limitations, the most important of which is poor spatial resolution. The EEG is especially sensitive to a certain set of postsynaptic potentials: to those that form in the upper layers of the cortex, on the tops of the convolutions directly adjacent to the skull, directed radially. Dendrites located deeper in the cortex, inside the sulci, located in deep structures (for example, the cingulate gyrus or the hippocampus), or whose currents are directed tangentially to the skull, have a significantly lesser effect on the EEG signal.

membranes of the brain, cerebrospinal fluid and the bones of the skull "blur" the EEG signal, obscuring its intracranial origin.

It is not possible to mathematically recreate a single intracranial current source for a given EEG signal because some currents create potentials that cancel each other out. A lot of scientific work is being done on the localization of signal sources.

Clinical Application

A standard EEG recording usually takes 20 to 40 minutes. In addition to the state of wakefulness, the study can be carried out in a state of sleep or under the influence of various kinds of stimuli on the test. This contributes to the emergence of rhythms that are different from those that can be observed in a state of relaxed wakefulness. These actions include periodic light stimulation with flashes of light (photostimulation), increased deep breathing (hyperventilation), and opening and closing of the eyes. When examining a patient suffering from epilepsy or at risk, the encephalogram is always viewed for the presence of interictal discharges (that is, abnormal activity resulting from "epileptic brain activity", which indicates a predisposition to epileptic seizures, lat. inter - between, among, ictus - seizure, attack).

In some cases, video-EEG monitoring is performed (simultaneous recording of EEG and video / audio signals), while the patient is hospitalized for a period of several days to several weeks. While in the hospital, the patient does not take antiepileptic drugs, which makes it possible to record the EEG during the onset period. In many cases, recording the onset of an attack provides the clinician with much more specific information about the patient's illness than does an interictal EEG. Continuous EEG monitoring involves the use of a portable electroencephalograph connected to a patient in an intensive care unit to observe seizure activity that is not clinically evident (i.e., not detectable by observing the patient's movements or mental state). When a patient is put into an artificial, drug-induced coma, the EEG pattern can be used to judge the depth of the coma, and depending on EEG indicators drugs are titrated. The "amplitude-integrated EEG" uses a special type of EEG signal representation and is used in conjunction with continuous monitoring of the brain function of newborns in the intensive care unit.

Various types of EEG are used in the following clinical situations:

• in order to distinguish an epileptic seizure from other types of seizures, for example, from psychogenic seizures of a non-epileptic nature, syncope (fainting), movement disorders and migraine variants;
• to describe the nature of seizures in order to select treatment;
• to localize the area of ​​the brain in which the attack originates, for the implementation of surgical intervention;
• for monitoring non-convulsive seizures / non-convulsive variant of epilepsy;
• to differentiate organic encephalopathy or delirium (acute mental disorder with elements of excitation) from primary mental illness, such as catatonia;
• for monitoring the depth of anesthesia;
• as an indirect indicator of brain perfusion during carotid endarterectomy (removal of the inner wall of the carotid artery);
• How additional research to confirm brain death;
• in some cases for prognostic purposes in patients in a coma.

The use of quantitative EEG (mathematical interpretation of EEG signals) to assess primary mental, behavioral and learning disorders seems to be rather controversial.

The use of EEG for scientific purposes

The use of EEG in neuroscience research has a number of advantages over others. instrumental methods. First, EEG is a non-invasive way to study an object. Secondly, there is no such rigid need to remain still, as during a functional MRI. Thirdly, during the EEG, spontaneous brain activity is recorded, so the subject is not required to interact with the researcher (as, for example, is required in behavioral testing as part of a neuropsychological study). In addition, EEG has high temporal resolution compared to techniques such as functional MRI and can be used to identify millisecond fluctuations in brain electrical activity.

Many studies of cognitive abilities using EEG use potentials associated with events (event-related potential, ERP). Most models of this type of research are based on the following statement: when exposed to the subject, he reacts either in an open, explicit form, or in a veiled way. During the study, the patient receives some kind of stimulus, and an EEG is recorded. Event-related potentials are isolated by averaging the EEG signal for all studies in a particular condition. Then the average values ​​for different states can be compared with each other.

Other EEG possibilities

EEG is performed not only during the traditional examination for clinical diagnosis and studying the work of the brain from the point of view of neuroscience, but also for many other purposes. The neurofeedback variant of neurotherapy is still an important complementary application of the EEG, which in its most advanced form is regarded as the basis for the development of the Brain Computer Interfaces. There are a number of commercial products that are mainly based on EEG. For example, on March 24, 2007, an American company (Emotiv Systems) introduced a thought-controlled video game device based on the electroencephalography method.

The introduction of this method into clinical practice and experimental neurophysiology made it possible to obtain fundamentally new data on the functional organization of the brain: on the so-called non-specific systems - activating and deactivating (synchronizing), on the organization of sleep (slow and fast sleep) and the role of dysfunction of non-specific systems in many pathological processes.

The method of electroencephalography has played a major role in the development of modern ideas about the pathogenesis of epilepsy. For the diagnosis of the latter, it is the most important method of instrumental research.

To register the EEG, special devices are used - electroencephalographs, which amplify the bioelectric activity removed from the brain by hundreds of thousands, a million times and register it on a paper tape or in a computer processor with subsequent visual or automatic analysis.

Electroencephalography is recorded in a relaxed state of the subject, with his eyes closed.

EEG with functional tests

After recording the background activity, functional tests are applied: short-term opening of the eyes (causes an activation reaction - the disappearance of the a-rhythm), light rhythmic stimulation (normally, the assimilation of frequencies of light flickers in the range of 6-18 Hz is noted); hyperventilation - deep breathing ("inflating the ball") - causes synchronization, i.e. slowing down the frequency of oscillations and increasing their amplitude. This phenomenon is especially pronounced in children and usually becomes insignificant after the age of 20.

Evoked Potentials

A special method of electroencephalographic research is the method of recording evoked responses of the brain (evoked potentials - EP) to discrete stimulation (light, sound, etc.), EEG registers a regular response, however, with the usual recording method, an insignificant amplitude of the response against the background of rhythmic activity a huge mass of neurons does not allow you to select the answer. The creation of special devices that allow summing repeated responses and leveling background activity made it possible to introduce the method of evoked potentials into clinical and experimental practice.

The evoked potentials are rhythmic fluctuations, in which early and late components are distinguished (Fig. 1.9.14). It is believed that the early components reflect the processes associated with the excitation and passage of an impulse along the corresponding sensory path with its switching in relay structures; late components are associated with afferent from non-specific structures activated by specific impulses.

There are negative (directed upwards from the isoline) and positive (directed downwards) oscillations, which are marked with the corresponding numbers or numbers indicating the latent periods of oscillations in milliseconds.

Investigate responses to flashes of light - visual evoked potentials (VEP, sound clicks - auditory evoked potentials (AEP) and electrical stimulation of peripheral nerves or receptors - somatosensory evoked potentials (SSEPs).

In clinical practice, the method of evoked potentials is used in diagnosing the level and localization of damage to the nervous system and, accordingly, certain diseases, in particular multiple sclerosis (the early components of VEP are disturbed), hysterical blindness (VEP do not change), etc.

In recent years, new methods of computer processing of electroencephalography have entered clinical practice: amplitude mapping, spectral power estimation, the method of multi-step dipole localization, and the method of low-resolution electromagnetic tomography.

Amplitude mapping of bioelectrical activity of the brain

This method allows you to visualize the distribution of potential differences on the surface of the brain at any time, evaluate the polarity, spatial distribution of certain phenomena, as well as the correspondence of potential maps to the dipole model (namely, the presence of 1 or 2 extrema of the opposite sign) .

Spectral Power Estimation

This method is used to analyze the spatial distribution of the spectral power according to the main EEC rhythms: α, β 1 , β 2 , θ and δ on the given artifact-free sections of the records (analysis epochs). The choice of epochs is determined by the presence of phenomena of interest to the researcher on the EEG.

Multi-step dipole localization method

Based on the analysis of the distribution of potential differences on the surface of the head, the BranLoc program makes it possible to solve the inverse EEG problem, i.e., to determine the three-dimensional localization of the sources of bioelectrical activity of the brain. The source of activity is represented as a dipole in three-dimensional space (Cartesian coordinate system), where the X axis runs along the inion-nason line, the Y axis is parallel to the line connecting the auditory canals, and the Z axis is from the base to the artex. The program features allow you to display the results of dipole localization on real and standardized CT or MRI slices.

EEG norm

Bioelectric potentials are normally characterized by symmetry. EEG reflects the total functional activity of neurons in the cerebral cortex. However, this activity is under the influence of non-specific stem-cortical systems, activating and deactivating, is rhythmically organized and has a different age characteristic.

On the electroencephalography of an awake adult (Fig. 1.9.10), bioelectric activity consists mainly of rhythm and weighty vili with a frequency of 8-12 Hz and an amplitude of 50-100 μV (a-rhythm), mainly expressed in the posterior parts of the brain, maximum - in occipital leads, and from more frequent fluctuations in the anterior parts of the brain with a frequency of 13-40 Hz and an amplitude of up to 15 μV (p-rhythm). material from the site

child's EEG

The EEG of the newborn is characterized by the absence of rhythmic activity. Irregular slow waves are registered. By the age of 3 months, rhythmic activity is formed, mainly in the 5-band. By 6 months, the 0-rhythm (5-6 Hz) dominates. In the future, the so-called slow a-rhythm (7-8 Hz) appears and grows, which becomes dominant by the age of 12 months.

11.02.2002

Momot T.G.

What is the reason for the need for an electroencephalographic study?

The need to use the EEG is due to the fact that its data should be taken into account both in healthy people in professional selection, especially in people working in stressful situations or with harmful production conditions, and when examining patients to solve differential diagnostic problems, which is especially important in the early stages of the disease for choosing the most effective methods of treatment and monitoring ongoing therapy.

What are the indications for electroencephalography?

Undoubted indications for the examination should be considered the presence of the patient: epilepsy, non-epileptic crises, migraine, volumetric process, vascular lesions of the brain, traumatic brain injury, inflammatory disease of the brain.

In addition, in other cases that are difficult for the attending physician, the patient can also be referred for an electroencephalographic examination; often multiple repeated EEG examinations are performed to monitor the effect of drugs and clarify the dynamics of the disease.

What does the preparation of the patient for the examination include?

The first requirement when conducting EEG examinations is a clear understanding by the electrophysiologist of his goals. For example, if a doctor only needs an assessment of the general functional state of the CNS, the examination is carried out according to a standard protocol, if it is necessary to identify epileptiform activity or the presence of local changes, the examination time and functional loads change individually, a long-term monitoring record can be used. Therefore, the attending physician, referring the patient to an electroencephalographic study, must collect the patient's history, ensure, if necessary, a preliminary examination by a radiologist and an ophthalmologist, and clearly formulate the main tasks diagnostic search neurophysiologist. When conducting a standard study, a neurophysiologist at the stage of the initial assessment of the electroencephalogram needs to have data on the age and state of consciousness of the patient, and additional clinical information may affect the objective assessment of certain morphological elements.

How to achieve flawless EEG recording quality?

The efficiency of computer analysis of an electroencephalogram depends on the quality of its registration. An impeccable EEG recording is the key to its subsequent correct analysis.

EEG registration is carried out only on a pre-calibrated amplifier. Calibration of the amplifier is carried out according to the instructions attached to the electroencephalograph.

For the examination, the patient is comfortably seated in a chair or laid down on a couch, a rubber helmet is put on his head and electrodes are applied that are connected to an electroencephalographic amplifier. This procedure is described in more detail below.

Scheme of the location of the electrodes.

Mounting and application of electrodes.

Electrode care.

EEG registration conditions.

Artifacts and their removal.

EEG recording procedure.

A. Electrode layout

For EEG recording, the "10-20%" electrode arrangement system, which includes 21 electrodes, or the modified "10-20%" system, which contains 16 active electrodes with a reference averaged common electrode, is used. A feature of the latter system, which is used by the company "DX Systems" is the presence of an unpaired occipital electrode Oz and an unpaired central Cz. Some versions of the program provide for a system of 16 electrodes with two occipital leads O1 and O2, in the absence of Cz and Oz. The ground electrode is located in the center of the anterior frontal region. Alphabetic and digital designations of electrodes correspond to the international layout "10-20%". The removal of electrical potentials is carried out in a monopolar way with an averaged total. The advantage of this system is a less time-consuming process of applying electrodes with sufficient information content and the ability to convert to any bipolar leads.

b. Mounting and application of electrodes is carried out in the following order:

The electrodes are connected to the amplifier. To do this, the electrode plugs are inserted into the electrode sockets of the amplifier.

The patient is wearing a helmet. Depending on the size of the patient's head, the dimensions of the helmet are adjusted by tightening and loosening the rubber bands. The locations of the electrodes are determined according to the system of location of the electrodes, and helmet harnesses are installed at the intersection with them. It must be remembered that the helmet should not cause discomfort to the patient.

With a cotton swab dipped in alcohol, the places intended for setting the electrodes are degreased.

According to the designations indicated on the amplifier panel, the electrodes are installed in the places provided by the system, the paired electrodes are arranged symmetrically. Immediately before placing each electrode, the electrode gel is applied to the surface in contact with the skin. It must be remembered that the gel used as a conductor must be intended for electrodiagnostics.

C. Electrode care.

Particular attention should be paid to the care of the electrodes: after the end of work with the patient, the electrodes should be washed warm water and dry with a clean towel, avoid kinks and excessive pulling of the electrode cables, as well as water and saline solution on the electrode cable connectors.

D. EEG registration conditions.

The conditions for recording an electroencephalogram should provide a state of relaxed wakefulness for the patient: a comfortable chair; light and soundproof chamber; correct placement of electrodes; the location of the phonophotostimulator at a distance of 30-50 cm from the eyes of the subject.

After applying the electrodes, the patient should sit comfortably in a special chair. The muscles of the upper shoulder girdle should be relaxed. The quality of the recording can be checked by turning on the electroencephalograph in recording mode. However, an electroencephalograph can register not only the electrical potentials of the brain, but also extraneous signals (the so-called artifacts).

E. Artifacts and their removal.

The most important step in the use of computers in clinical electroencephalography is the preparation of the original electroencephalographic signal, which is stored in the computer's memory. The main requirement here is to ensure the input of artifact-free EEG (Zenkov L.R., Ronkin M.A., 1991).

To eliminate artifacts, it is necessary to determine their cause. Depending on the cause of occurrence, artifacts are divided into physical and physiological.

Physical artifacts are due to technical reasons, which include:

Unsatisfactory quality of grounding;

Possible influence from various equipment used in medicine (X-ray, physiotherapy, etc.);

Uncalibrated electroencephalographic signal amplifier;

Poor quality electrode placement;

Damage to the electrode (the part in contact with the surface of the head and the connecting wire);

Pickup from a working phonophotostimulator;

Violation of electrical conductivity when water and saline get on the connectors of the electrode cables.

To troubleshoot problems related to unsatisfactory grounding quality, interference from nearby equipment and a working phonophotostimulator, the assistance of an installation engineer is required to properly ground medical equipment and install the system.

In case of poor-quality application of electrodes, reinstall them according to p.B. the present recommendations.

A damaged electrode must be replaced.

Clean the connectors of the electrode cables with alcohol.

Physiological artifacts that are caused by the biological processes of the organism of the subject include:

Electromyogram - artifacts of muscle movement;

Electrooculogram - eye movement artifacts;

Artifacts associated with recording the electrical activity of the heart;

Artifacts associated with the pulsation of blood vessels (with a close location of the vessel from the recording electrode;

Artifacts related to breathing;

Artifacts associated with changes in the resistance of the skin;

Artifacts associated with the patient's restless behavior;

It is not always possible to completely avoid physiological artifacts, so if they are short-term (rare blinking of the eyes, masticatory muscle tension, short-term anxiety), it is recommended to remove them using a special mode provided by the program. The main task of the researcher at this stage is the correct recognition and timely removal of artifacts. In some cases, filters are used to improve the quality of the EEG.

Electromyogram registration can be associated with masticatory muscle tension and is reproduced in the form of high-amplitude beta-range oscillations in the temporal leads. Similar changes are found when swallowing. Certain difficulties also arise when examining patients with ticoid twitches, because there is a layering of the electromyogram on the electroencephalogram, in these cases it is necessary to apply antimuscular filtration or prescribe appropriate drug therapy.

If the patient blinks for a long time, you can ask him to keep his eyelids closed by lightly pressing the index and thumb fingers. This procedure can be performed by a nurse. The oculogram is recorded in the frontal leads in the form of bilaterally synchronous oscillations of the delta range, exceeding the background level in amplitude.

The electrical activity of the heart can be recorded mainly in the left posterior temporal and occipital leads, coincides in frequency with the pulse, is represented by single fluctuations in the theta range, slightly exceeding the level of background activity. Does not cause a noticeable error in automatic analysis.

Artifacts associated with vascular pulsation are represented mainly by delta-range oscillations, exceed the level of background activity and are eliminated by moving the electrode to an adjacent region not located above the vessel.

With artifacts associated with the patient's breathing, regular slow-wave oscillations are recorded, coinciding in rhythm with respiratory movements and due to mechanical movements of the chest, which are more often manifested during a hyperventilation test. To eliminate it, it is recommended to ask the patient to switch to diaphragmatic breathing and avoid extraneous movements during breathing.

With artifacts associated with a change in the resistance of the skin, which may be due to a violation of the emotional state of the patient, irregular oscillations of slow waves are recorded. To eliminate them, it is necessary to calm the patient, wipe the skin areas under the electrodes again with alcohol and scarify them with chalk.

The question of the appropriateness of the study and the possibility of using drugs in patients in a state of psychomotor agitation is decided jointly with the attending physician individually for each patient.

In cases where the artifacts are slow waves that are difficult to eliminate, it is possible to record with a time constant of 0.1 s.

F. What is the EEG recording procedure?

The procedure for recording the EEG during a routine examination lasts about 15-20 minutes and includes recording the "background curve" and recording the EEG in various functional states. It is convenient to have several pre-created registration protocols, including functional tests of different duration and sequence. If necessary, a long-term monitoring record can be used, the duration of which is initially limited only by the reserves of paper or free space on the disk where the database is located. protocol record. A log entry may contain multiple functional probes. A research protocol is selected individually or a new one is created, which indicates the sequence of samples, their type and duration. The standard protocol includes an eye-opening test, 3-minute hyperventilation, photostimulation at a frequency of 2 and 10 Hz. If necessary, phono- or photo-stimulation is performed at frequencies up to 20 Hz, trigger stimulation on a given channel. In special cases, in addition, clenching fingers into a fist, sound stimuli, taking various pharmacological drugs, psychological tests are used.

What are standard functional tests?

The "open-close eyes" test is usually carried out for a duration of about 3 seconds with intervals between successive tests from 5 to 10 seconds. It is believed that the opening of the eyes characterizes the transition to activity (more or less inertia of the processes of inhibition); and closing the eyes characterizes the transition to rest (more or less inertia of excitation processes).

Normally, when the eyes are opened, there is a suppression of alpha activity and an increase (not always) of beta activity. Closing the eyes increases the index, amplitude and regularity of alpha activity.

The latent period of the response with open and closed eyes varies from 0.01-0.03 seconds and 0.4-1 seconds, respectively. It is believed that the response to opening the eyes is a transition from a state of rest to a state of activity and characterizes the inertness of the processes of inhibition. And the response to closing the eyes is a transition from the state of activity to rest and characterizes the inertness of the excitation processes. Response parameters for each patient are usually stable on repeat trials.

When conducting a test with hyperventilation, the patient needs to breathe with rare, deep breaths and exhalations for 2-3 minutes, sometimes longer. In children under 12-15 years of age, hyperventilation by the end of the 1st minute naturally leads to a slowdown in the EEG, which increases during further hyperventilation simultaneously with the frequency of oscillations. The effect of EEG hypersynchronization during hyperventilation is more pronounced, the younger the subject. Normally, such hyperventilation in adults does not cause any special changes in the EEG or sometimes leads to an increase in the percentage contribution of the alpha rhythm to the total electrical activity and the amplitude of alpha activity. It should be noted that in children under 15-16 years of age, the appearance of regular slow high-amplitude generalized activity during hyperventilation is the norm. The same reaction is seen in young (under 30) adults. When evaluating the response to a hyperventilation test, one should take into account the degree and nature of the changes, the time of their occurrence after the onset of hyperventilation and the duration of their persistence after the end of the test. There is no consensus in the literature on how long EEG changes after the end of hyperventilation. According to the observations of N.K. Blagosklonova, the persistence of EEG changes for more than 1 minute should be regarded as a sign of pathology. However, in some cases, hyperventilation leads to the appearance of a special form of electrical activity of the brain - paroxysmal. Back in 1924, O. Foerster showed that intense deep breathing for several minutes provokes the appearance of an aura or an extended epileptic seizure in patients with epilepsy. With the introduction of electroencephalographic examination into clinical practice, it was found that in a large number of patients with epilepsy, epileptiform activity appears and intensifies already in the first minutes of hyperventilation.

Light rhythmic stimulation.

In clinical practice, the appearance on the EEG of rhythmic responses of varying severity, repeating the rhythm of light flashes, is analyzed. As a result of neurodynamic processes at the level of synapses, in addition to the unambiguous repetition of the flickering rhythm, the EEG may exhibit stimulation frequency conversion phenomena, when the frequency of EEG responses is higher or lower than the stimulation frequency, usually by an even number of times. It is important that in any case, the effect of synchronization of brain activity with an external rhythm sensor occurs. Normally, the optimal stimulation frequency for detecting the maximum assimilation reaction lies in the region of EEG natural frequencies, amounting to 8–20 Hz. The amplitude of the potentials during the assimilation reaction usually does not exceed 50 μV and most often does not exceed the amplitude of spontaneous dominant activity. The reaction of rhythm assimilation is best expressed in the occipital regions, which is obviously due to the corresponding projection of the visual analyzer. The normal reaction of assimilation of the rhythm stops no later than 0.2-0.5 seconds after the cessation of stimulation. characteristic feature brain in epilepsy is an increased tendency to excitatory reactions and synchronization of neural activity. In this regard, at certain, individual for each examined frequency, the brain of a patient with epilepsy gives hypersynchronous high-amplitude responses, sometimes called photoconvulsive reactions. In some cases, responses to rhythmic stimulation increase in amplitude, acquire a complex form of peaks, sharp waves, peak-wave complexes, and other epileptic phenomena. In some cases, the electrical activity of the brain in epilepsy under the influence of flickering light acquires the autorhythmic nature of a self-sustaining epileptic discharge, regardless of the frequency of stimulation that caused it. The discharge of epileptic activity may continue after the cessation of stimulation and sometimes turn into a petit mal or grand mal seizure. These types of epileptic seizures are called photogenic.

In some cases, special samples are used with dark adaptation(stay in a darkened room up to 40 minutes), partial and complete (from 24 to 48 hours) sleep deprivation, as well as joint EEG and ECG monitoring, and night sleep monitoring.

How does an electroencephalogram occur?

On the origin of the electrical potentials of the brain.

Over the years, theoretical ideas about the origin of brain potentials have repeatedly changed. Our task does not include a deep theoretical analysis of the neurophysiological mechanisms of the generation of electrical activity. Gray Walter's figurative statement about the biophysical significance of the information received by an electrophysiologist is given in the following quote: "The electrical changes that cause the alternating currents of different frequencies and amplitudes that we register occur in the cells of the brain itself. Undoubtedly, this is their only source. The brain should be described as an extensive aggregate of electrical elements as numerous as the stellar population of the galaxy. In the ocean of the brain, the restless tides of our electrical being rise, thousands of times relatively more powerful than those of the oceans of the earth. This occurs when millions of elements are jointly excited, which makes it possible to measure the rhythm of their repeated discharges in frequency and amplitude.

It is not known what causes these millions of cells to work together and what causes the discharge of one cell. We are still very far from explaining these basic brain mechanisms. Future research will perhaps give us a dynamic perspective of amazing discoveries, similar to that which opened up before physicists in their attempts to understand the atomic structure of our being. Perhaps, as in physics, these discoveries can be described in terms of mathematical language. But even today, as we move in line with new ideas, the adequacy of the language used and the clear definition of the assumptions we make are of increasing importance. Arithmetic is an adequate language for describing the height and time of the tide, however, if we want to predict its rise and fall, we must use another language, the language of algebra with its special symbols and theorems. Similarly, electrical waves and flushes in the brain can be adequately described by counting, arithmetic; but as our pretensions increase and we want to understand and predict the behavior of the brain, there are many unknown "x's" and "y's" of the brain. It is thus necessary to have its algebra as well. Some people find this word intimidating. But it means nothing more than "connecting the pieces of the broken."

The EEG records can therefore be regarded as particles, fragments of the mirror of the brain, its speculum speculorum. Attempts to combine them with fragments of other origin must be preceded by careful sorting. Electroencephalographic information comes, like a regular report, in encrypted form. You can open the cipher, but that doesn't mean that the information you get will necessarily be of great value...

The function of the nervous system is to perceive, compare, store and generate many signals. The human brain is not only a mechanism much more complex than any other, but also a mechanism with a long individual history. In this regard, to investigate only the frequencies and amplitudes of the wavy line components over a limited period of time would be at least an oversimplification. "(Gray Walter. Living Brain. M., Mir, 1966).

Why do we need a computer analysis of the electroencephalogram?

Historically, clinical electroencephalography has evolved from the visual phenomenological analysis of the EEG. However, already at the beginning of the development of electroencephalography, physiologists arose the desire to evaluate the EEG using quantitative objective indicators, to apply the methods of mathematical analysis.

At first, EEG processing and calculation of its various quantitative parameters were carried out manually by digitizing the curve and calculating the frequency spectra, the difference in which in different areas was explained by the cytoarchitectonics of the cortical zones.

TO quantitative methods EEG evaluation should also include planimetric and histographic methods of EEG analysis, which were also performed by manually measuring the amplitude of oscillations. The study of the spatial relationships of the electrical activity of the human cerebral cortex was carried out using a toposcope, which made it possible to study the signal intensity in dynamics, the phase relationships of activity and to select the selected rhythm. The use of the correlation method for EEG analysis was first proposed and developed by N. Wiener in the 1930s, and the most detailed justification for the application of spectral correlation analysis to EEG is given in the work of G. Walter.

With the introduction of digital computers into medical practice, it became possible to analyze electrical activity at a qualitatively new level. Currently, the most promising direction in the study of electrophysiological processes is the direction of digital electroencephalography. Modern methods computer processing of the electroencephalogram allows for a detailed analysis of various EEG phenomena, viewing any part of the curve in an enlarged form, performing its amplitude-frequency analysis, presenting the data obtained in the form of maps, numbers, graphs, diagrams, and obtaining probabilistic characteristics of the spatial distribution of factors that cause the occurrence of convexital surface of electrical activity.

Spectral analysis, which is most widely used in the analysis of electroencephalograms, was used to assess the background standard EEG characteristics in different groups pathologies (Ponsen L., 1977), chronic effects of psychotropic drugs (Saito M., 1981), prognosis for cerebrovascular accidents (Saimo K. et al., 1983), hepatogenic encephalopathy (Van der Rijt C.C. et al ., 1984). A feature of spectral analysis is that it represents the EEG not as a temporal sequence of events, but as a spectrum of frequencies over a certain period of time. Obviously, the spectra will reflect the background stable characteristics of the EEG to a greater extent than they were recorded over a longer period of analysis in similar experimental situations. Long epochs of analysis are also preferable due to the fact that deviations in the spectrum caused by short-term artifacts are less pronounced in them, if they do not have a significant amplitude.

When evaluating the generalized characteristics of the background EEG, most researchers choose analysis epochs of 50 - 100 sec, although according to J. Mocks and T. Jasser (1984), the 20 sec epoch also gives fairly well reproducible results if it is selected according to the criterion of minimum activity in the band 1.7 - 7.5 Hz in the EEG lead. Regarding the reliability of the results of spectral analysis, the opinions of the authors vary depending on the composition of the investigated and specific problems solved using this method. R. John et al. (1980) came to the conclusion that absolute EEG spectra in children are unreliable, and only relative spectra recorded with the subject's eyes closed are highly reproducible. At the same time, G. Fein et al. (1983), examining the EEG spectra of normal and dyslexic children, came to the conclusion that the absolute spectra are informative and more valuable, giving not only the power distribution over frequencies, but also its real value. When assessing the reproducibility of the EEG spectra in adolescents during repeated studies, the first of which was carried out at the age of 12.2 years, and the second at the age of 13, reliable correlations were found only in the alpha1 (0.8) and alpha2 (0.72) bands, while time, as for the rest of the spectral bands, the reproducibility is less reliable (Gasser T. et al., 1985). In ischemic stroke, out of 24 quantitative parameters obtained on the basis of spectra from 6 EEG derivations, only the absolute power of local delta waves was a reliable predictor of the prognosis (Sainio K. et al., 1983).

Due to the sensitivity of the EEG to changes in cerebral blood flow, a number of works are devoted to the spectral analysis of the EEG during transient ischemic attacks, when changes detected by manual analysis seem insignificant. V. Kopruner et al. (1984) studied EEG in 50 healthy and 32 patients with cerebrovascular accidents at rest and when squeezing the ball with the right and left hands. The EEG was subjected to computer analysis with the calculation of the power from the main spectral bands. Based on these initial data, we obtain 180 parameters that were processed by the method of multivariate linear discriminant analysis. On this basis, a multiparametric asymmetry index (MPA) was obtained, which made it possible to differentiate healthy and sick people, groups of patients according to the severity of the neurological defect and the presence and size of the lesion on a computed tomogram. The greatest contribution to the MPA was given by the ratio of the theta power to the delta power. Additional significant skewness parameters were theta and delta power, peak frequency, and event-related desynchronization. The authors noted a high degree of symmetry of the parameters in healthy people and the main role of asymmetry in the diagnosis of pathology.

Of particular interest is the use of spectral analysis in the study of the mu-rhythm, which, when visually analyzed, is found only in a small percentage of individuals. Spectral analysis combined with the technique of averaging the spectra obtained over several epochs makes it possible to identify it in all subjects.

Since the distribution of the mu rhythm coincides with the area of ​​blood supply to the middle cerebral artery, its changes can serve as an index of disturbances in the corresponding area. diagnostic criteria are differences in the peak frequency and power of the mu-rhythm in the two hemispheres (Pfurtschillir G., 1986).

The method of calculating the spectral power on the EEG is highly appreciated by C.S. Van der Rijt et al. (1984) in staging hepatic encephalopathy. An indicator of the severity of encephalopathy is a decrease in the average dominant frequency in the spectrum, and the degree of correlation is so close that it makes it possible to establish the classification of encephalopathies according to this indicator, which turns out to be more reliable than the clinical picture. In the control, the average dominant frequency is greater than or equal to 6.4 Hz, and the percentage of theta is below 35; in stage I encephalopathy, the average dominant frequency lies in the same range, but the number of theta is equal to or higher than 35%, in stage II, the average dominant frequency is below 6.4 Hz, the content of theta waves is in the same range and the number of delta waves does not exceed 70 %; V Stage III the number of delta waves is more than 70%.

Another area of ​​application of the mathematical analysis of the electroencephalogram by the fast Fourier transform method concerns the control of short-term EEG changes under the influence of some external and internal factors. Thus, this method is used to monitor the state of cerebral blood flow during endaterectomy or heart surgery, taking into account high sensitivity EEG to disorders of cerebral circulation. In the work of M. Myers et al. (1977), the EEG, previously passed through a filter with restrictions in the range of 0.5 - 32 Hz, was digitized and subjected to fast Fourier transform successive epochs lasting 4 seconds. Spectral diagrams of successive epochs were placed on the display one under the other. The resulting picture was a three-dimensional graph, where the X axis corresponded to the frequency, Y - to the registration time, and an imaginary coordinate corresponding to the height of the peaks, displayed the spectral power. The method provides a demonstrative display of temporal fluctuations in the spectral composition in the EEG, which, in turn, is highly correlated with fluctuations in cerebral blood flow, which is determined by the arteriovenous pressure difference in the brain. The authors concluded that EEG data could be effectively used to correct cerebral circulation disorders during surgery by an anesthesiologist who did not specialize in EEG analysis.

The EEG spectral power method is of interest in assessing the influence of certain psychotherapeutic influences, mental stress, and functional tests. R.G. Biniaurishvili et al. (1985) observed an increase in total power and especially power in the delta and theta bands during hyperventilation in patients with epilepsy. In renal failure studies, effective technique analysis of EEG spectra during light rhythmic stimulation. The subjects were presented with successive 10-s series of light flashes from 3 to 12 Hz with simultaneous continuous recording of successive power spectra for epochs of 5 seconds. The spectra were placed in the form of a matrix to obtain a pseudo-three-dimensional image, in which time is represented along the axis moving away from the observer when viewed from above, frequency - along the X-axis, amplitude - along the Y-axis. Normally, a clearly defined peak was observed at the dominant harmonic and less clear at the subharmonic stimulation, gradually shifting to the right in the course of increasing stimulation frequency. When uremia was observed a sharp decline power at the fundamental harmonic, the predominance of peaks at low frequencies with a total power dispersion. In more precise quantitative terms, this manifested itself in a decrease in activity at lower-frequency harmonics below the main one, which correlated with the worsening of the patients' condition. There was a restoration of the normal picture of the spectra of assimilation of rhythms with improvement due to dialysis or kidney transplantation (Amel B. et al., 1978). Some studies use the method of isolating a certain frequency of interest on the EEG.

When studying dynamic shifts on the EEG, usually short analysis epochs are used: from 1 to 10 seconds. The Fourier transform has some features that partly make it difficult to match the data obtained with its help with the data of visual analysis. Their essence lies in the fact that on the EEG slow phenomena have a greater amplitude and duration than high-frequency ones. In this regard, in the spectrum constructed according to the classical Fourier algorithm, there is a certain predominance of slow frequencies.

The evaluation of the frequency components of the EEG is used for local diagnostics, since this EEG characteristic is one of the main criteria in the visual search for local brain lesions. This raises the question of choosing significant parameters for EEG assessment.

In an experimental clinical study, attempts to apply spectral analysis to the nosological classification of brain lesions, as expected, were unsuccessful, although its usefulness as a method for detecting pathology and localizing lesions was confirmed (Mies G., Hoppe G., Hossman K.A.., 1984). In this mode of the program, the spectral array is displayed with varying degrees of overlap (50-67%), the range of change in equivalent amplitude values ​​(color coding scale) is presented in μV. The capabilities of the mode allow you to display 2 spectral arrays at once, using 2 channels or hemispheres for comparison. The histogram scale is automatically calculated so that the white color corresponds to the maximum equivalent amplitude value. Floating parameters of the color coding scale allow you to present any data on any range without a scale, as well as compare a fixed channel with the rest.

What methods of mathematical analysis of EEG are the most common?

EEG mathematical analysis is based on the transformation of initial data by the fast Fourier transform method. The original electroencephalogram, after its conversion into a discrete form, is divided into successive segments, each of which is used to build the appropriate number of periodic signals, which are then subjected to harmonic analysis. Output forms are presented in the form of numerical values, graphs, graphic maps, compressed spectral regions, EEG tomograms, etc. (J. Bendat, A. Peirsol, 1989, Applied Random Data Analysis, ch.11)

What are the main aspects of the application of computer EEG?

Traditionally, EEG is most widely used in the diagnosis of epilepsy, which is due to the neurophysiological criteria included in the definition of an epileptic seizure as a pathological electrical discharge of brain neurons. It is possible to objectively fix the corresponding changes in electrical activity during a seizure only by electroencephalographic methods. However, the old problem of diagnosing epilepsy remains relevant in cases where direct observation of an attack is not possible, history data are inaccurate or unreliable, and routine EEG data do not give direct indications in the form of specific epileptic discharges or epileptic seizure patterns. In these cases, the use of methods of multiparametric statistical diagnostics allows not only to obtain a reliable diagnosis of epilepsy from unreliable clinical and electroencephalographic data, but also to resolve the issues of the need for treatment with anticonvulsants for traumatic brain injury, isolated epileptic seizure, febrile seizures and others. Thus, the use of automatic methods for EEG processing in epileptology is currently the most interesting and promising direction. Objective assessment of the functional state of the brain in the presence of a patient with paroxysmal seizures of non-epileptic origin, vascular pathology, inflammatory diseases of the brain, etc. with the possibility of longitudinal studies allows you to observe the dynamics of the disease and the effectiveness of therapy.

The main directions of the mathematical analysis of the EEG can be reduced to several main aspects:

Transformation of primary electroencephalographic data into a more rational form adapted to specific laboratory tasks;

Automatic analysis of EEG frequency and amplitude characteristics and elements of EEG analysis by pattern recognition methods, partially reproducing operations performed by a person;

Converting analysis data into the form of graphs or topographic maps (Rabending Y., Heydenreich C., 1982);

The method of probabilistic EEG-tomography, which allows to investigate with a certain degree of probability the location of the factor that caused the electrical activity on the scalp EEG.

What are the main processing modes contained in the program "DX 4000 practic"?

When considering various methods of mathematical analysis of an electroencephalogram, it is possible to show what information this or that method gives to a neurophysiologist. However, none of the methods available in the arsenal can fully illuminate all aspects of such a complex process as the electrical activity of the human brain. Only a complex of different methods makes it possible to analyze EEG patterns, describe and quantify the totality of its different aspects.

Methods such as frequency, spectral, and correlation analysis have been widely used, which make it possible to estimate the spatiotemporal parameters of electrical activity. Among the latest software developments of the DX-systems company is an automatic EEG analyzer that determines local rhythmic changes that differ from the typical picture for each patient, synchronous flashes caused by the influence of the median structures, paroxysmal activity with display of its focus and distribution pathways. The method of probabilistic EEG tomography has proven itself well, allowing with a certain degree of reliability to display on the functional section the location of the factor that caused the electrical activity on the scalp EEG. Currently, a 3-dimensional model of a functional focus of electrical activity is being tested with its spatial and layer-by-layer mapping in planes and alignment with sections taken in the study of the anatomical structures of the brain using NMRI methods. This method is used in the software version of "DX 4000 Research".

An increasing use in clinical practice in assessing the functional state of the brain is found by the method of mathematical analysis of evoked potentials in the form of mapping, spectral and correlation methods of analysis.

Thus, the development of digital EEG is the most promising method for studying the neurophysiological processes of the brain.

The use of correlation-spectral analysis makes it possible to study the spatio-temporal relationships of EEG potentials.

The morphological analysis of various EEG patterns is evaluated visually by the user, however, the possibility of viewing it with different speed and scale can be implemented programmatically. Moreover, recent developments make it possible to expose electroencephalogram recordings to the mode of an automatic analyzer, which evaluates the background rhythmic activity characteristic of each patient, monitors periods of EEG hypersynchronization, localization of certain pathological patterns, paroxysmal activity, its source and distribution pathways. EEG registration provides objective information about the state of the brain in various functional states.

The main methods of computer analysis of the electroencephalogram presented in the "DX 4000 PRACTIC" program are EEG tomography, EEG mapping and representation of the characteristics of the electrical activity of the brain in the form of compressed spectral regions, digital data, histograms, correlation and spectral tables and maps.

Short-lived (from 10 ms) and relatively constant electroencephalographic patterns ("electroencephalographic syndromes"), as well as the electroencephalographic pattern characteristic of each person and its changes associated with age and (normally) and with pathology, according to the degree of involvement, have diagnostic value in the study of the EEG. in the pathological process of different parts of the brain structures. Thus, a neurophysiologist must analyze EEG patterns of different duration, but not in significance, and obtain the most complete information about each of them, and about the electroencephalographic picture as a whole. Therefore, when analyzing an EEG pattern, it is necessary to take into account the time of its existence, since the time period subjected to analysis should be commensurate with the studied EEG phenomenon.

The types of data representation of the fast Fourier transform depend on the field of application of this method, as well as the interpretation of the data.

EEG tomography.

The author of this method is A.V. Kramarenko. The first software developments of the "DX-systems" problem laboratory were equipped with the EEG tomograph mode, and now it is already successfully used in more than 250 medical institutions. The essence and areas of practical application of this method are described in the author's work.

EEG mapping.

For digital electroencephalography, it has become traditional to transform the information received in the form of maps: frequency, amplitude. Topographic maps reflect the distribution of the spectral power of electrical potentials. The advantages of this approach are that some recognition tasks, according to the psychologist, are better solved by a person based on visual-spatial perception. In addition, the presentation of information in the form of a picture that reproduces the real spatial relationships in the brain of the subject is also assessed as more adequate from a clinical point of view, by analogy with such research methods as NMR, etc.

To obtain a map of the power distribution in a certain spectral range, the power spectra are calculated for each of the leads, and then all the values ​​lying spatially between the electrodes are calculated by the method of multiple interpolation; the spectral power in a certain band is coded for each point by the color intensity in a given color scale on a color display. An image of the subject’s head is obtained on the screen (top view), on which color variations correspond to the power of the spectral band in the corresponding area (Veno S., Matsuoka S., 1976; Ellingson R.J.; Peters J.F., 1981; Buchsbaum M.S. et al., 1982; Matsuoka S., Nedermeyer E., Lopes de Silva F., 1982; Ashida H. et al., 1984). K. Nagata et al., (1982), using the system of representing the spectral power in the main EEG spectral bands in the form of color maps, came to the conclusion that it is possible to obtain additional useful information using this method in the study of patients with ischemic cerebrovascular accident with aphasia.

The same authors in the study of patients with transient ischemic attacks found that topographic maps provide information on the presence of residual changes in the EEG even for a long time after an ischemic attack and represent some advantage over conventional visual analysis of the EEG. The authors note that subjectively, pathological asymmetries in topographic maps were perceived more convincingly than in conventional EEG, and diagnostic values ​​had changes in the alpha rhythm band, which, as is known, are the least supported in conventional EEG analysis (Nagata K. et. al., 1984).

Amplitude topographic maps are useful only in the study of event-related brain potentials, since these potentials have sufficiently stable phase, amplitude, and spatial characteristics that can be adequately reflected on a topographic map. Since spontaneous EEG at any recording point is a stochastic process, any instantaneous potential distribution recorded by a topographic map turns out to be unrepresentative. Therefore, the construction of amplitude maps for the given spectrum bands more adequately corresponds to the tasks of clinical diagnostics (Zenkov L.R., 1991).

The median normalization mode includes matching the color scale to the average amplitude values ​​for 16 channels (50 μV span).

Normalization by minimum colors the minimum values ​​of the amplitudes with the coldest color of the scale, and the rest with the same step of the color scale.

Normalization to the maximum includes staining the areas with the maximum amplitude values ​​with the warmest color, and staining the remaining areas with colder tones in increments of 50 μV.

Gradation scales of frequency maps are constructed accordingly.

In the mapping mode, topographic maps can be multiplied in alpha, beta, theta, delta frequency ranges; the median frequency of the spectrum and its deviation. The ability to view sequential topographic maps allows you to determine the localization of the source of paroxysmal activity and the way it spreads with visual and temporal (using an automatic timer) comparison with traditional EEG curves. When recording an electroencephalogram according to a given research protocol, viewing the summary maps corresponding to each sample in four frequency ranges makes it possible to quickly and figuratively assess the dynamics of the electrical activity of the brain during functional loads, identify constant, but not always pronounced asymmetry.

Sector diagrams visually show with the display of digital characteristics the percentage contribution of each frequency range to the total electrical activity for each of the sixteen EEG channels. This mode allows you to objectively assess the predominance of any of the frequency ranges and the level of interhemispheric asymmetry.

Representation of the EEG as a two-dimensional differential distribution law of the median frequency and signal amplitude. Fourier analysis data are presented on a plane, the horizontal axis of which is the median frequency of the spectrum in Hz, and the vertical axis is the amplitude in μV. Color gradation characterizes the probability of a signal appearing at a selected frequency with a selected amplitude. The same information can be represented as a three-dimensional figure, along the Z-axis of which the probability is plotted. Nearby is indicated the area occupied by the figure as a percentage of the total area. The two-dimensional differential law of the distribution of the median frequency and signal amplitude is also constructed for each hemisphere separately. To compare these images, the absolute difference of these two distribution laws is calculated and displayed on the frequency plane. This mode makes it possible to estimate the total electrical activity and gross interhemispheric asymmetry.

Representation of EEG in the form of digital values. Presentation of the electroencephalogram in digital form allows obtaining the following information about the study: equivalent values ​​of the average wave amplitude of each frequency range corresponding to its power spectral density (these are estimates of the mathematical expectation of the spectral composition of the signal based on Fourier realizations, analysis epoch 640 ms, overlap 50%); values ​​of the median (average effective) frequency of the spectrum, calculated from the averaged Fourier implementation, expressed in Hz; deviation of the median frequency of the spectrum in each channel from its average value, i.e. from mathematical expectation (expressed in Hz); standard deviation equivalent values ​​of the average amplitude per channel in the current range from the mathematical expectation (values ​​in the averaged Fourier implementation, expressed in μV).

Histograms. One of the most common and illustrative ways of presenting Fourier analysis data is the distribution histograms of the equivalent values ​​of the average wave amplitude of each frequency range and the histograms of the median frequency of all channels. In this case, the equivalent values ​​of the average wave amplitude of each frequency range are tabulated in 70 intervals with a width of 1.82 in the range from 0 to 128 μV. In other words, the number of values ​​(accordingly, realizations) belonging to each interval (hit frequency) is counted. This array of numbers is smoothed with a Hamming filter and normalized around the maximum value (after that, the maximum in each channel is 1.0). When determining the average effective (median) frequency of the power spectral density, the values ​​for the Fourier realizations are tabulated in 70 intervals with a width of 0.2 Hz in the range from 2 to 15 Hz. The values ​​are smoothed with a Hamming filter and normalized to the maximum. In the same mode, it is possible to build hemispheric histograms and a general histogram. For hemispheric histograms, 70 intervals are taken with a width of 1.82 μV for ranges and 0.2 Hz for the average effective frequency of the spectrum; for the general histogram, the values ​​in all channels are used, and for the construction of hemispheric histograms, only the values ​​in the channels of one hemisphere are used (channels Cz and Oz are not taken into account for any hemisphere). On the histograms, the interval with the maximum frequency value is marked and it is indicated what corresponds to it in μV or Hz.

Compressed spectral regions. Compressed spectral regions represent one of the traditional methods of EEG processing. Its essence lies in the fact that the original electroencephalogram, after converting it into a discrete form, is divided into successive segments, each of which is used to construct the appropriate number of periodic signals, which are then subjected to harmonic analysis. At the output, spectral power curves are obtained, where the EEG frequencies are plotted along the X axis, and the power released at a given frequency over the analyzed time interval along the Y axis. The duration of the epochs is 1 second. The EEG power spectra are displayed sequentially, plotted one under the other with coloring. warm colors maximum values. As a result, a pseudo-three-dimensional landscape of successive spectra is built on the display, which makes it possible to visually see changes in the spectral composition of the EEG over time. The most commonly used method for assessing the spectral power of the EEG is used to characterize the EEG in general in cases of non-specific diffuse lesions brain, such as malformations, various kinds of encephalopathy, impaired consciousness, some psychiatric diseases.
The second field of application of this method is long-term observation of patients in a coma or under therapeutic effects (Fedin AI, 1981).

Bispectral analysis with normalization is one of the special modes of electroencephalogram processing by the fast Fourier transform method and is a repeated spectral analysis of the results of EEG spectral analysis in a given range for all channels. The results of EEG spectral analysis are presented on time histograms of power spectral density (PSD) for the selected frequency range. This mode is designed to study the PSD oscillation spectrum and its dynamics. Bispectral analysis is performed for frequencies from 0.03 to 0.540 Hz with a step of 0.08 Hz on the entire PSD array. Since the PSD is a positive value, the original data for respectral analysis contains some constant component, which shows up in its results at low frequencies. Often there is a maximum. To eliminate the constant component, it is necessary to center the data. This is the mode of bispectral analysis with centering. The essence of the method lies in the fact that their average value is subtracted from the initial data for each channel.

Correlation analysis. The matrix of the correlation coefficient of the values ​​of the power spectral density in the specified range is constructed for all pairs of channels and, on its basis, the vector of the average correlation coefficients of each channel with the others. The matrix has an upper triangular form. Marking its rows and columns gives all possible pairs for 16 channels. The coefficients for a given channel are in the row and in the column with its number. The values ​​of the correlation coefficients range from -1000 to +1000. The sign of the coefficient is written in the cell of the matrix above the values. Correlation of channels i, j is estimated by absolute value correlation coefficient Rij , and the cell of the matrix is ​​coded with the corresponding color: the cell of the coefficient with the maximum absolute value, and black - with a minimum. Based on the matrix for each channel, the average correlation coefficient with the remaining 15 channels is calculated. The resulting vector of 16 values ​​is displayed below the matrix according to the same principles.

There are many mysteries in the human body, and not all of them are subject to doctors yet. The most complex and confusing of them, perhaps, is the brain. Various methods of brain research, such as electroencephalography, help doctors lift the veil of secrecy. What is it and what can the patient expect from the procedure?

Who is eligible for an electroencephalography test?

Electroencephalography (EEG) allows you to clarify many diagnoses associated with infections, injuries and brain disorders.

The doctor may refer you for an examination if:

1. There is a possibility of epilepsy. The brain waves in this case show a special epileptiform activity, which is expressed in the modified form of the graphs.
2. It is required to establish the exact location of the injured part of the brain or tumor.
3. There are some genetic diseases.
4. There are serious violations of sleep and wakefulness.
5. The work of the vessels of the brain is disrupted.
6. An assessment of the effectiveness of the treatment is needed.

The electroencephalography method is applicable to both adults and children, it is non-traumatic and painless. A clear picture of the work of brain neurons in its different parts makes it possible to clarify the nature and causes of neurological disorders.

Method of brain research electroencephalography - what is it?

Such an examination is based on the registration of bioelectric waves emitted by the neurons of the cerebral cortex. With the help of electrodes, the activity of nerve cells is captured, amplified, and the device is translated into a graphic form.

The resulting curve characterizes the process of work of different parts of the brain, its functional state. In the normal state, it has a certain shape, and deviations are diagnosed taking into account changes appearance graphic arts.

EEG can be performed in various ways. The room for him is isolated from extraneous sounds and light. The procedure usually takes 2-4 hours and is performed in a clinic or laboratory. In some cases, electroencephalography with sleep deprivation requires more time.

The method allows doctors to obtain objective data about the state of the brain, even when the patient is unconscious.

How is an EEG performed?

If a doctor prescribes electroencephalography, what is it for the patient? He will be offered to sit in a comfortable position or lie down, put on his head a helmet made of elastic material that fixes the electrodes. If the recording is supposed to be long, then a special conductive paste or collodion is applied at the points of contact of the electrodes with the skin. The electrodes do not cause any discomfort.

EEG does not imply any violations of the integrity of the skin or the introduction of drugs (premedication).

Routine recording of brain activity occurs for a patient in a state of passive wakefulness, when he lies quietly or sits with his eyes closed. It's quite difficult, time drags on slowly and you have to fight sleep. The laboratory assistant periodically checks the patient's condition, asks to open his eyes and perform certain tasks.

During the study, the patient should minimize any motor activity that would interfere. It’s good if the laboratory manages to fix the doctors of interest neurological manifestations(convulsions, tics, epileptic seizure). Sometimes an attack in epileptics is provoked purposefully in order to understand its type and origin.

Preparation for the EEG

On the eve of the study, it is worth washing your hair. It is better not to braid your hair and not to use any styling products. Leave hairpins and clips at home, and collect long hair in a ponytail, if necessary.

Metal jewelry should also be left at home: earrings, chains, lip and eyebrow piercings. Before entering the office, disable mobile phone(not only sound, but completely), so as not to interfere with sensitive sensors.

Before the examination, you need to eat so as not to feel hungry. It is advisable to avoid any unrest and strong feelings, but you should not take any sedatives.

You may need a tissue or towel to wipe off any remaining fixative gel.

Samples during the EEG

In order to track the reaction of brain neurons in various situations, and to expand the demonstrative capabilities of the method, the electroencephalography examination includes several tests:

1. Eye opening-closing test. The laboratory assistant makes sure that the patient is conscious, hears him, and follows the instructions. The absence of patterns on the chart at the time of opening the eyes indicates pathology.

2. Test with photostimulation, when flashes of bright light are directed into the eyes of the patient during recording. Thus, epileptimorphic activity is revealed.

3. A test with hyperventilation, when the subject breathes deeply voluntarily for several minutes. The frequency of respiratory movements at this time decreases slightly, but the oxygen content in the blood rises and, accordingly, the supply of oxygenated blood to the brain increases.

4. Sleep deprivation, when the patient is immersed in a short sleep with the help of sedatives or stay in the hospital for daily observation. This allows you to obtain important data on the activity of neurons at the time of awakening and falling asleep.

5. Stimulation mental activity is to solve simple problems.

6. Stimulation of manual activity, when the patient is asked to perform a task with an object in his hands.

All this gives a more complete picture of the functional state of the brain and notice violations that have a slight external manifestation.

The duration of the electroencephalogram

The time of the procedure may vary depending on the goals set by the doctor and the conditions of a particular laboratory:

• 30 minutes or more if you can quickly register the activity you are looking for;
• 2-4 hours in standard version when the patient is examined reclining in a chair;
• 6 or more hours on EEG with daytime sleep deprivation;
• 12-24 hours, when all phases of night sleep are examined.

The scheduled time of the procedure can be changed at the discretion of the doctor and laboratory assistant in any direction, because if there are no characteristic patterns corresponding to the diagnosis, the EEG will have to be repeated, spending extra time and money. And if all the necessary records are obtained, there is no point in tormenting the patient with forced inactivity.

What is video monitoring during an EEG?

Sometimes the electroencephalography of the brain is duplicated by a video recording, which records everything that happens during the study with the patient.

Video monitoring is prescribed for patients with epilepsy to correlate how behavior during an attack correlates with brain activity. Timed matching of characteristic waves with the picture can clarify gaps in the diagnosis and help the clinician understand the condition of the subject for more accurate treatment.

The result of electroencephalography

When the patient underwent electroencephalography, the conclusion is handed out along with printouts of all graphs of the wave activity of various parts of the brain. In addition, if video monitoring was also carried out, the recording is saved on a disk or flash drive.

At a consultation with a neurologist, it is better to show all the results so that the doctor can assess the features of the patient's condition. Electroencephalography of the brain is not the basis for the diagnosis, but significantly clarifies the picture of the disease.

To ensure that all the smallest teeth are clearly visible on the graphs, it is recommended to store the printouts flattened in a hard folder.

Encryption from the brain: types of rhythms

When an electroencephalography is passed, which each graph shows, it is extremely difficult to understand on your own. The doctor will make a diagnosis based on the study of changes in the activity of areas of the brain during the study. But if the EEG was prescribed, then the reasons were good, and it would not hurt to consciously approach your results.

So, we have in our hands a printout of such an examination, like electroencephalography. What are these - rhythms and frequencies - and how to determine the limits of the norm? The main indicators that appear in the conclusion:

1. Alpha rhythm. The frequency normally ranges from 8-14 Hz. Between the cerebral hemispheres, a difference of up to 100 μV can be observed. The pathology of the alpha rhythm is characterized by asymmetry between the hemispheres exceeding 30%, the amplitude index is above 90 μV and below 20.

2. Beta rhythm. It is mainly fixed on the anterior leads (in the frontal lobes). For most people, a typical frequency is 18-25 Hz with an amplitude of no more than 10 μV. The pathology is indicated by an increase in amplitude over 25 μV and a persistent spread of beta activity to the posterior leads.

3. Delta rhythm and Theta rhythm. Fixed only during sleep. The appearance of these activities during the period of wakefulness signals a malnutrition of the brain tissues.

5. Bioelectric activity (BEA). A normal indicator demonstrates synchrony, rhythm, and the absence of paroxysms. Deviations are manifested in early childhood epilepsy, predisposition to convulsions and depression.

In order for the results of the study to be indicative and informative, it is important to follow the prescribed treatment regimen exactly, without canceling the drugs before the study. Alcohol or energy drinks taken the day before can distort the picture.

What is electroencephalography used for?

For the patient, the benefits of the study are obvious. The doctor can check the correctness of the prescribed therapy and change it if necessary.

In people with epilepsy, when a period of remission is established by observation, the EEG may show seizures that are not superficially observable and still require medical intervention. Or avoid unreasonable social restrictions, specifying the features of the course of the disease.

The study can also contribute to the early diagnosis of neoplasms, vascular pathologies, inflammation and brain degeneration.

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Introduction

Electroencephalography (EEG - diagnostics) is a method for studying the functional activity of the brain, which consists in measuring the electrical potentials of brain cells, which are subsequently subjected to computer analysis.

Electroencephalography makes it possible to qualitatively and quantitatively analyze the functional state of the brain and its reactions to stimuli, and also significantly helps in the diagnosis of epilepsy, tumor, ischemic, degenerative and inflammatory diseases of the brain. Electroencephalography allows you to evaluate the effectiveness of the treatment with an already established diagnosis.

The EEG method is promising and indicative, which allows it to be considered in the field of diagnosing mental disorders. Application mathematical methods analysis of EEG and their implementation in practice allows you to automate and simplify the work of doctors. EEG is an integral part of the objective criteria for the course of the disease under study in the general system of assessments developed for a personal computer.

1. Method of electroencephalography

The use of the electroencephalogram for the study of brain function and diagnostic purposes is based on knowledge gained from observations of patients with various lesions brain, as well as on the results of experimental studies on animals. The entire experience of the development of electroencephalography, starting from the first studies of Hans Berger in 1933, indicates that certain electroencephalographic phenomena or patterns correspond to certain states of the brain and its individual systems. The total bioelectrical activity recorded from the surface of the head characterizes the state of the cerebral cortex, both as a whole and its individual areas, as well as the functional state of deep structures at different levels.

Changes in intracellular membrane potentials (MPs) of cortical pyramidal neurons underlie the potential fluctuations recorded from the head surface in the form of an EEG. When the intracellular MF of a neuron changes in the extracellular space, where glial cells are located, a potential difference arises - the focal potential. The potentials that arise in the extracellular space in a population of neurons are the sum of such individual focal potentials. Total focal potentials can be recorded using electrically conductive sensors from different brain structures, from the surface of the cortex or from the surface of the skull. The voltage of the currents of the brain is about 10-5 Volts. The EEG is a record of the total electrical activity of the cells of the cerebral hemispheres.

1.1 Leading and recording an electroencephalogram

The recording electrodes are placed in such a way that all the main parts of the brain are represented on the multichannel recording, denoted by the initial letters of their Latin names. In clinical practice, two main EEG lead systems are used: the international "10-20" system (Fig. 1) and a modified scheme with a reduced number of electrodes (Fig. 2). If it is necessary to obtain a more detailed picture of the EEG, the "10-20" scheme is preferable.

Rice. 1. International layout of electrodes "10-20". Letter indices mean: O - occipital abduction; P - parietal lead; C - central lead; F - frontal lead; t - temporal abduction. Numerical indices specify the position of the electrode within the corresponding area.

Rice. Fig. 2. Scheme of EEG recording with monopolar leads (1) with a reference electrode (R) on the earlobe and with bipolar leads (2). In a system with a reduced number of leads, the letter indices mean: O - occipital lead; P - parietal lead; C - central lead; F - frontal lead; Ta - anterior temporal lead, Tr - posterior temporal lead. 1: R - voltage under the reference ear electrode; O - voltage under the active electrode, R-O - record obtained with monopolar lead from the right occipital region. 2: Tr - voltage under the electrode in the area of ​​the pathological focus; Ta - voltage under the electrode, standing above the normal brain tissue; Ta-Tr, Tr-O and Ta-F - records obtained with bipolar lead from the corresponding pairs of electrodes

Such a lead is called a reference lead when a potential is applied to "input 1" of the amplifier from an electrode located above the brain, and to "input 2" - from an 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 electrode.

As such, the left (A1) and right (A2) earlobes are used. The active electrode is connected to "input 1" of the amplifier, the supply of a negative potential shift to which causes the recording pen to deflect upwards.

The reference electrode is connected to "input 2". In some cases, a lead from two shorted electrodes (AA) located on the earlobes is used as a reference electrode. Since the potential difference between the two electrodes is recorded on the EEG, the position of the point on the curve will be equally, but in the opposite direction, affected by changes in the potential under each of the pair of electrodes. In the reference lead under the active electrode, an alternating potential of the brain is generated. Under the reference electrode, which is away 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 fluctuations in the electrical potential generated by the brain under the active electrode. However, the region of the head between the active and reference electrodes is part of the "amplifier-object" electrical circuit, and the presence of a sufficiently intense source of potential in this area, located asymmetrically with respect to the electrodes, will significantly affect the readings. Therefore, in the case of a referential assignment, the judgment about the localization of the potential source is not entirely reliable.

Bipolar is called a lead, in which electrodes 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 of the form of oscillation under each of them on the basis of one bipolar assignment 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 potential sources that make up the components of a complex total curve obtained with bipolar derivation.

For example, if there is a local source of slow oscillations in the posterior temporal region (Tp in Fig. 2), when the anterior and posterior temporal electrodes (Ta, Tr) are connected to the amplifier terminals, a recording is obtained containing a slow component corresponding to slow activity in the posterior temporal region ( Tr), superimposed on it by faster 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 Tr, and the second corresponds to some non-temporal lead, for example F and O.

It is clear that in the newly formed pair (Tr-O), including the posterior temporal electrode Tr, located above the pathologically altered medulla, there will again be a slow component. In a pair whose inputs are fed with activity from two electrodes placed over 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 an electrode located above this focus, paired with any other one, leads to the appearance of a pathological component in the corresponding EEG channels. This allows you to determine the localization of the source of pathological fluctuations.

An additional criterion for determining the localization of the source of the potential of interest on the EEG is the phenomenon of oscillation phase distortion.

Rice. Fig. 3. Phase relationship of records at different localization of the potential source: 1, 2, 3 - electrodes; A, B - channels of the electroencephalograph; 1 - the source of the recorded potential difference is located under the electrode 2 (records on channels A and B are in antiphase); II - the source of the recorded potential difference is located under the electrode I (the records are in phase)

The arrows indicate the direction of the current in the channel circuits, which determines the corresponding directions of the deviation of the curve on the monitor.

If you connect three electrodes to the inputs of two channels of the electroencephalograph as follows (Fig. 3): electrode 1 - to "input 1", electrode 3 - to "input 2" of amplifier B, and electrode 2 - simultaneously to "input 2" of amplifier A and "input 1" amplifier B; Assuming that under electrode 2 there is a positive shift of the electric potential relative to the potential of the remaining parts of the brain (indicated by the sign "+"), then it is obvious that the electric current due to this potential shift will have the opposite direction in the circuits of amplifiers A and B, which will be reflected in oppositely directed displacements of the potential difference - antiphases - on the corresponding EEG records. Thus, the electrical oscillations under electrode 2 in the records on channels A and B will be represented by curves having the same frequencies, amplitudes and shape, but opposite in phase. When switching electrodes through several channels of the electroencephalograph in the form of a chain, antiphase oscillations of the investigated potential will be recorded through those two channels, to the opposite inputs of which one common electrode is connected, standing above the source of this potential.

1.2 Electroencephalogram. Rhythms

The nature of the EEG is determined by the functional state of the nervous tissue, as well as the metabolic processes occurring in it. Violation of the blood supply leads to the suppression of the bioelectric activity of the cerebral cortex. Important EEG feature is its spontaneous nature and autonomy. The electrical activity of the brain can be recorded not only during wakefulness, but also during sleep. Even with deep coma and anesthesia, a special characteristic pattern of rhythmic processes (EEG waves) is observed. In electroencephalography, four main ranges are distinguished: alpha, beta, gamma and theta waves (Fig. 4).

Rice. 4. EEG wave processes

The existence of characteristic rhythmic processes is determined by the spontaneous electrical activity of the brain, which is due to the total activity of individual neurons. Electroencephalogram rhythms differ from each other in duration, amplitude and form. The main components of the EEG of a healthy person are shown in Table 1. The grouping is more or less arbitrary, it does not correspond to any physiological categories.

Table 1 - The main components of the electroencephalogram

Alpha(b)-rhythm: frequency 8-13 Hz, amplitude up to 100 μV. Registered in 85-95% of healthy adults. It is best expressed in the occipital regions. The b-rhythm has the greatest amplitude in a state of calm relaxed wakefulness with closed eyes. In addition to changes associated with the functional state of the brain, in most cases spontaneous changes in the amplitude of the β-rhythm are observed, expressed in an alternating increase and decrease with the formation of characteristic "Spindles", lasting 2-8 s. With an increase in the level of functional activity of the brain (intense attention, fear), the amplitude of the b-rhythm decreases. High-frequency, low-amplitude irregular activity appears on the EEG, reflecting the desynchronization of neuronal activity. With a short-term, sudden external stimulus (especially a flash of light), this desynchronization occurs abruptly, and if the stimulus is not of an emotiogenic nature, the b-rhythm is restored quite quickly (after 0.5-2 s). This phenomenon is called "activation reaction", "orientation reaction", "b-rhythm extinction reaction", "desynchronization reaction".

· Beta(b)-rhythm: frequency 14-40 Hz, amplitude up to 25 μV. Best of all, the B-rhythm is recorded in the region of the central gyri, but it also extends to the posterior central and frontal gyri. Normally, it is very weakly expressed and in most cases has an amplitude of 5-15 μV. β-Rhythm is associated with somatic sensory and motor cortical mechanisms and gives an extinction response to motor activation or tactile stimulation. Activity with a frequency of 40-70 Hz and an amplitude of 5-7 μV is sometimes called the g-rhythm; it has no clinical significance.

Mu(m)-rhythm: frequency 8-13 Hz, amplitude up to 50 μV. The parameters of the m-rhythm are similar to those of the normal b-rhythm, but the m-rhythm differs from the latter in its physiological properties and topography. Visually, the m-rhythm is observed only in 5-15% of the subjects in the rolandic region. The amplitude of the m-rhythm (in rare cases) increases with motor activation or somatosensory stimulation. In routine analysis, the m-rhythm has no clinical significance.

Theta (I) -activity: frequency 4-7 Hz, amplitude of pathological I-activity? 40 μV and most often exceeds the amplitude normal rhythms brain, reaching at some pathological conditions 300 uV or more.

· Delta (d) -activity: frequency 0.5-3 Hz, the amplitude is the same as that of I-activity. I- and d-oscillations can be present in a small amount on the EEG of an awake adult and are normal, but their amplitude does not exceed that of the b-rhythm. An EEG is considered pathological if it contains i- and d-oscillations with an amplitude of ?40 μV and takes up more than 15% of the total recording time.

Epileptiform activity is a phenomenon typically observed on the EEG of patients with epilepsy. They arise as a result of highly synchronized paroxysmal depolarization shifts in large populations of neurons, accompanied by the generation of action potentials. As a result, high-amplitude sharp-shaped potentials arise, which have the appropriate names.

Spike (eng. Spike - tip, peak) - a negative potential of an acute form, lasting less than 70 ms, amplitude? 50 μV (sometimes up to hundreds or even thousands of μV).

· An acute wave differs from a spike in its extension in time: its duration is 70-200 ms.

· Sharp waves and spikes can combine with slow waves, forming stereotypical complexes. Spike-slow wave - a complex of a spike and a slow wave. The frequency of spike-slow wave complexes is 2.5-6 Hz, and the period, respectively, is 160-250 ms. An acute-slow wave is a complex of an acute wave and a slow wave following it, the period of the complex is 500-1300 ms (Fig. 5).

An important characteristic of spikes and sharp waves is their sudden appearance and disappearance, and a clear difference from the background activity, which they exceed in amplitude. Acute phenomena with appropriate parameters that do not clearly differ from background activity are not designated as sharp waves or spikes.

Rice. 5 . The main types of epileptiform activity: 1 - adhesions; 2 - sharp waves; 3 - sharp waves in the P-band; 4 - spike-slow wave; 5 - polyspike-slow wave; 6 - sharp-slow wave. The value of the calibration signal for "4" is 100 µV, for the rest of the records - 50 µV.

Flare is a term for a group of waves with sudden appearance and disappearance, clearly different from background activity in frequency, shape and / or amplitude (Fig. 6).

Rice. 6. Flashes and discharges: 1 - flashes of b-waves of high amplitude; 2 - bursts of high-amplitude B-waves; 3 - flashes (discharges) of sharp waves; 4 - flashes of polyphase oscillations; 5 - bursts of q-waves; 6 - flashes of i-waves; 7 - flashes (discharges) of spike-slow wave complexes

Discharge - a flash of epileptiform activity.

The pattern of an epileptic seizure is a discharge of epileptiform activity, typically coinciding with a clinical epileptic seizure.

2. Electroencephalography in epilepsy

Epilepsy is a disease characterized by two or more epileptic seizures (seizures). An epileptic seizure is a short, usually unprovoked, stereotypical disturbance of consciousness, behavior, emotions, motor or sensory functions, which even clinical manifestations can be associated with the discharge of an excess number of neurons in the cerebral cortex. The definition of an epileptic seizure through the concept of a discharge of neurons determines the most important significance of the EEG in epileptology.

Clarification of the form of epilepsy (more than 50 variants) includes a description of the EEG pattern characteristic of this form as an obligatory component. The value of the EEG is determined by the fact that epileptic discharges, and, consequently, epileptiform activity, are also observed on the EEG outside of an epileptic seizure.

Reliable signs of epilepsy are discharges of epileptiform activity and epileptic seizure patterns. In addition, high-amplitude (more than 100-150 μV) bursts of b-, I-, and d-activity are characteristic, however, by themselves they cannot be considered evidence of the presence of epilepsy and are evaluated in the context of the clinical picture. In addition to the diagnosis of epilepsy, EEG plays an important role in determining the form of epileptic disease, which determines the prognosis and choice of drug. EEG allows you to choose the dose of the drug by assessing the decrease in epileptiform activity and predict side effects by the appearance of additional pathological activity.

To detect epileptiform activity on the EEG, light rhythmic stimulation is used (mainly in photogenic seizures), hyperventilation, or other influences, based on information about the factors provoking seizures. Long-term recording, especially during sleep, helps to identify epileptiform discharges and epileptic seizure patterns.

Sleep deprivation contributes to the provocation of epileptiform discharges on the EEG or the seizure itself. Epileptiform activity confirms the diagnosis of epilepsy, but it is also possible under other conditions; at the same time, it cannot be registered in some patients with epilepsy.

Long-term registration of the electroencephalogram and EEG video monitoring, as well as epileptic seizures, epileptiform activity on the EEG is not constantly recorded. In some forms of epileptic disorders, it is observed only during sleep, sometimes provoked by certain life situations or forms of patient activity. Consequently, the reliability of diagnosing epilepsy directly depends on the possibility of long-term EEG recording under conditions of fairly free behavior of the subject. For this purpose, special portable systems have been developed for long-term (12-24 hours or more) EEG recording under conditions close to normal life.

The recording system consists of an elastic cap with electrodes of a special design built into it, which make it possible to obtain high-quality EEG recording for a long time. The output electrical activity of the brain is amplified, digitized and recorded on flash cards by a cigarette case-sized recorder that fits in a convenient bag on the patient. The patient can perform normal household activities. Upon completion of the recording, the information from the flash card in the laboratory is transferred to a computer system for recording, viewing, analyzing, storing and printing electroencephalographic data and is processed as a regular EEG. The most reliable information is provided by EEG - video monitoring - simultaneous registration of the EEG and video recording of the patient during the stupa. The use of these methods is required in the diagnosis of epilepsy, when routine EEG does not reveal epileptiform activity, as well as in determining the form of epilepsy and the type of epileptic seizure, for the differential diagnosis of epileptic and non-epileptic seizures, clarifying the goals of surgery in surgical treatment, and diagnosing epileptic non-paroxysmal disorders associated with epileptiform seizures. activity during sleep, control of the correct choice and dose of the drug, side effects of therapy, reliability of remission.

2.1. Characteristics of the electroencephalogram in the most common forms of epilepsy and epileptic syndromes

· Benign childhood epilepsy with centrotemporal spikes (benign rolandic epilepsy).

Rice. Fig. 7. EEG of a 6-year-old patient with idiopathic childhood epilepsy with centrotemporal spikes

Regular sharp-slow wave complexes with an amplitude of up to 240 μV are seen in the right central (C4) and anterior temporal regions (T4), which form a phase distortion in the corresponding leads, indicating their generation by a dipole in the lower parts of the precentral gyrus at the border with the superior temporal gyrus.

Outside the attack: focal spikes, sharp waves and/or spike-slow wave complexes in one hemisphere (40-50%) or two with unilateral predominance in the central and middle temporal leads, forming antiphases over the rolandic and temporal regions (Fig. 7).

Sometimes epileptiform activity is absent during wakefulness, but appears during sleep.

During an attack: focal epileptic discharge in the central and middle temporal leads in the form of high-amplitude spikes and sharp waves combined with slow waves, with possible spread beyond the original location.

Benign occipital epilepsy of childhood with early onset (Panayotopoulos form).

Outside of an attack: in 90% of patients, mainly multifocal high- or low-amplitude acute-slow wave complexes are observed, often bilateral-synchronous generalized discharges. In two-thirds of cases, occipital adhesions are observed, in a third of cases - extraoccipital.

Complexes occur in series when closing the eyes.

Blocking of epileptiform activity is noted by opening the eyes. Epileptiform activity on the EEG and sometimes seizures are provoked by photostimulation.

During an attack: epileptic discharge in the form of high-amplitude spikes and sharp waves, combined with slow waves, in one or both occipital and posterior parietal leads, usually extending beyond the initial localization.

Idiapathic generalized epilepsy. EEG patterns characteristic of childhood and juvenile idiopathic epilepsy with

Absences, as well as for idiopathic juvenile myoclonic epilepsy, are given above.

EEG characteristics in primary generalized idiopathic epilepsy with generalized tonic-clonic seizures are as follows.

Outside the attack: sometimes within the normal range, but usually with moderate or severe changes with I-, d-waves, flashes of bilaterally synchronous or asymmetric spike-slow wave complexes, spikes, sharp waves.

During an attack: a generalized discharge in the form of rhythmic activity of 10 Hz, gradually increasing in amplitude and decreasing in frequency in the clonic phase, sharp waves of 8-16 Hz, spike-slow wave and polyspike-slow wave complexes, groups of high-amplitude I- and d- waves, irregular, asymmetric, in the tonic phase I- and d-activity, sometimes culminating in periods of lack of activity or low-amplitude slow activity.

· Symptomatic focal epilepsies: characteristic epileptiform focal discharges are observed less regularly than in idiopathic ones. Even seizures may present not with typical epileptiform activity, but with flashes of slow waves or even desynchronization and flattening of the EEG associated with the seizure.

With limbic (hippocampal) temporal lobe epilepsy, there may be no changes in the interictal period. Usually, focal complexes of an acute-slow wave are observed in the temporal leads, sometimes bilaterally synchronous with one-sided amplitude predominance (Fig. 8.). During an attack - outbreaks of high-amplitude rhythmic "steep" slow waves, or sharp waves, or sharp-slow wave complexes in the temporal leads with spread to the frontal and posterior. At the beginning (sometimes during) a seizure, a unilateral flattening of the EEG may be observed. With lateral-temporal epilepsy with auditory and less often visual illusions, hallucinations and dream-like states, speech and orientation disorders, epileptiform activity on the EEG is observed more often. The discharges are localized in the middle and posterior temporal leads.

With non-convulsive temporal seizures, proceeding according to the type of automatism, a picture of an epileptic discharge is possible in the form of rhythmic primary or secondary generalized high-amplitude I activity without acute phenomena, and in rare cases in the form of diffuse desynchronization, manifested by polymorphic activity with an amplitude of less than 25 μV.

Rice. 8. Temporal lobar epilepsy in a 28-year-old patient with complex partial seizures

Bilateral-synchronous acute-slow wave complexes in the anterior temporal region with amplitude predominance on the right (electrodes F8 and T4) indicate the localization of the source of pathological activity in the anterior mediobasal regions of the right temporal lobe.

EEG in frontal lobe epilepsy in the interictal period does not reveal focal pathology in two thirds of cases. In the presence of epileptiform oscillations, they are recorded in the frontal leads from one or both sides, bilateral-synchronous spike-slow wave complexes are observed, often with a lateral predominance in the frontal regions. During a seizure, bilaterally synchronous spike-slow wave discharges or high-amplitude regular I- or d-waves can be observed, mainly in the frontal and / or temporal leads, sometimes sudden diffuse desynchronization. With orbitofrontal foci, three-dimensional localization reveals the appropriate location of the sources of the initial sharp waves of the epileptic seizure pattern.

2.2 Interpretation of results

EEG analysis is carried out during the recording and finally upon its completion. During recording, the presence of artifacts is assessed (induction of mains current fields, mechanical artifacts of electrode movement, electromyogram, electrocardiogram, etc.), and measures are taken to eliminate them. The frequency and amplitude of the EEG are assessed, characteristic graph elements are identified, and their spatial and temporal distribution is determined. The analysis is completed by the physiological and pathophysiological interpretation of the results and the formulation of a diagnostic conclusion with clinical and electroencephalographic correlation.

Rice. 9. Photoparoxysmal EEG response in epilepsy with generalized seizures

Background EEG was within normal limits. With increasing frequency from 6 to 25 Hz of light rhythmic stimulation, an increase in the amplitude of responses at a frequency of 20 Hz is observed with the development of generalized spike discharges, sharp waves, and spike-slow wave complexes. d- right hemisphere; s - left hemisphere.

Basic medical document according to EEG - a clinical and electroencephalographic report written by a specialist based on the analysis of a "raw" EEG.

The EEG conclusion should be formulated in accordance with certain rules and consist of three parts:

1) description of the main types of activity and graph elements;

2) a summary of the description and its pathophysiological interpretation;

3) correlation of the results of the previous two parts with clinical data.

The basic descriptive term in EEG is "activity", which defines any sequence of waves (b-activity, activity of sharp waves, etc.).

The frequency is determined by the number of vibrations per second; it is written in the corresponding number and expressed in hertz (Hz). The description gives the average frequency of the estimated activity. Usually, 4-5 EEG segments with a duration of 1 s are taken and the number of waves on each of them is calculated (Fig. 10).

Amplitude - range of electric potential fluctuations on the EEG; measured from the peak of the preceding wave to the peak of the subsequent wave in opposite phase, expressed in microvolts (µV). A calibration signal is used to measure the amplitude. So, if the calibration signal corresponding to a voltage of 50 µV has a height of 10 mm on the record, then, accordingly, 1 mm of pen deflection will mean 5 µV. To characterize the amplitude of activity in the description of the EEG, the most typical of its maximum values ​​are taken, excluding jumping ones.

· The phase determines the current state of the process and indicates the direction of the vector of its changes. Some EEG phenomena are evaluated by the number of phases they contain. Monophasic is an oscillation in one direction from the isoelectric line with a return to the initial level, biphasic is such an oscillation when, after the completion of one phase, the curve passes the initial level, deviates in the opposite direction and returns to the isoelectric line. Polyphasic vibrations are vibrations containing three or more phases. in a narrower sense, the term "polyphase wave" defines the sequence of b- and slow (usually e) waves.

Rice. 10. Measurement of frequency (1) and amplitude (II) on the EEG

Frequency is measured as the number of waves per unit time (1 s). A is the amplitude.

Conclusion

electroencephalography epileptiform cerebral

With the help of EEG, information is obtained about the functional state of the brain at different levels of the patient's consciousness. The advantage of this method is its harmlessness, painlessness, non-invasiveness.

Electroencephalography has found wide application in the neurological clinic. EEG data are especially significant in the diagnosis of epilepsy; their definite role in the recognition of tumors of intracranial localization, vascular, inflammatory, degenerative diseases of the brain, and coma is possible. EEG using photostimulation or sound stimulation can help to differentiate between true and hysterical visual and hearing disorders or simulation of such disorders. EEG can be used for monitoring the patient. The absence of signs of bioelectrical activity of the brain on the EEG is one of the most important criteria for his death.

The EEG is easy to use, cheap, and does not involve exposure to the subject, i.e. non-invasive. EEG can be recorded near the patient's bed and used to control the stage of epilepsy, long-term monitoring of brain activity.

But there is another, not so obvious, but very valuable advantage of the EEG. In fact, PET and fMRI are based on measuring secondary metabolic changes in brain tissue, rather than primary ones (i.e. electrical processes in nerve cells). EEG can show one of the main parameters of the nervous system - the property of rhythm, which reflects the consistency of the work of different brain structures. Therefore, by recording an electrical (as well as magnetic) encephalogram, the neurophysiologist has access to the actual information processing mechanisms of the brain. This helps to reveal the blueprint of the processes involved in the brain, showing not only "where", but also "how" information is processed in the brain. It is this possibility that makes the EEG a unique and, of course, a valuable diagnostic method.

Electroencephalographic examinations reveal how human brain uses its functional reserves.

Bibliography

1. Zenkov, L.R. Clinical electroencephalography (with elements of epileptology). Guide for doctors - 3rd ed. - M.: MEDpress-inform, 2004. - 368s.

2. Chebanenko A.P., Textbook for students of the Faculty of Physics of the department "Medical Physics", Applied thermo- and electrodynamics in medicine - Odessa. - 2008. - 91s.

3. Kratin Yu.G., Guselnikov, V.N. Technique and methods of electroencephalography. - L .: Nauka, 1971, p. 71.

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