Registration of visual evoked potentials of the cerebral cortex. Diagnosis of visual evoked potentials
Course work
on the topic "Cerebral evoked potentials"
1. INTRODUCTION
Over the past 20 years, the level of use of computers in medicine has increased tremendously. Practical medicine is becoming more and more automated.
Complex modern research in medicine is unthinkable without the use of computer technology. Such studies include computed tomography, tomography using the phenomenon of nuclear magnetic resonance, ultrasonography, studies using isotopes. The amount of information that is obtained during such research is so huge that without a computer a person would be unable to perceive and process it.
Computers have found wide application in electroencephalography. There is no doubt that with the help of computer technology it is already possible to significantly improve the method of recording, storing and retrieving EEG information, to obtain a number of new data that are inaccessible to manual methods of analysis, to convert EEG data into visual-spatial topographic images that open up additional possibilities for local diagnosis of cerebral lesions. .
This paper describes a software tool for analyzing evoked potentials of the brain. The program presented in the thesis allows you to conduct a component analysis of the IP: the search for peaks and peak-to-peak latencies. This analysis can help diagnose diseases such as epilepsy, multiple sclerosis, and detect violations of sensory, visual and auditory functions.
Registration of evoked potentials (EP) of the brain is an objective and non-invasive method for testing the functions of the human CNS. The use of VP is an invaluable tool for the early detection and prognosis of neurological disorders in various diseases, such as stroke, brain tumors, and the consequences of traumatic brain injury.
2. GENERAL
One of the main methods for analyzing brain activity is the study of the bioelectric activity of various structures, the comparison of records simultaneously taken from different parts of the brain, both in the case of spontaneous activity of these structures, and in the case of electrical reactions to short-term single and rhythmic afferent stimuli. Single or rhythmic electrical stimulation of various brain formations is also often used with a recording of reactions in other structures.
The method of evoked potentials (EP) has long been one of the leading methods in experimental neurophysiology; With the help of this method, convincing data have been obtained that reveal the essence of a number of the most important mechanisms of the brain. It can be safely assumed that most of the information about the functional organization of the nervous system was obtained using this method. The development of methods to record EP in humans opens up bright prospects for the study of mental illness.
Registration of the responses of nerves and individual nerve fibers to electrical stimuli made it possible to study the main patterns of the occurrence and conduction of nerve impulses in nerve conductors. An analysis of the responses of individual neurons and their clusters to stimulation revealed the basic laws governing the occurrence of inhibition and excitation in the nervous system. The EP method is the main way to establish the presence of functional connections between the periphery and the central nervous mechanisms and to study intercentral relationships in the nervous system. By registering EP, it was possible to establish the main patterns of functioning of the specific and nonspecific afferent systems and their interaction with each other.
The EP method was used to study the characteristics of changes in the reactivity of the CNS to afferent stimuli depending on the level of functional activity of the brain; The patterns of interaction between synchronizing and desynchronizing systems of the brainstem, thalamus, and forebrain were studied.
ERP studies at various levels of the nervous system are the main method for testing the action of pharmacological neurotropic drugs. With the help of the EP method, the processes of higher nervous activity are successfully studied in experiments: the development of conditioned reflexes, complex forms of learning, emotional reactions, decision-making processes.
The EP technique is primarily applicable for objective testing of sensory functions (vision, hearing, somatic sensitivity), for obtaining more accurate information about the localization of organic cerebral lesions, for studying the state of the brain pathways and the reactivity of various cerebral systems during pathological processes.
The study of EP has found the widest application as a method for assessing the state of the sensory system in the field of studying auditory function disorders; The technique was called objective audiometry. Its advantages are obvious: it becomes possible to study hearing in infants, in persons with impaired consciousness and contact with others, in cases of hysterical and simulated deafness. Also, by registering the EP from the abdominal wall of the mother in the area corresponding to the fetal head, it is possible to identify the degree of development of hearing functions in human fetuses.
The study of visual EPs (VEPs) seems quite promising, given the great importance of assessing the state of the visual systems in the topical diagnosis of cerebral lesions.
The study of somatosensory EPs (SSEPs) makes it possible to determine the state of sensory conductors throughout from the periphery to the cortex. Since SSEPs have a somatotopic corresponding to the cortical projections of the body, their study is of particular interest in cases of damage to sensory systems at the level of the brain. The study of EP for the purpose of differentiating organic and functional (neurotic) sensory disturbances can be of great practical importance. This gives grounds to use the SSEP technique in forensic medicine.
Of great interest is the study of EP in epilepsy, given the important role played by afferent impulses in the pathogenesis of the development of epileptic seizures. The high sensitivity of EPs to changes in the functional state of the brain under the influence of pharmacological agents makes it possible to use them to test the effects of treatment in epilepsy.
In addition to the study of EP for relatively simple stimuli (a short flash of light, a sound click, a short pulse of electric current), a number of studies of EP for more complex types of stimulation have recently appeared using also more complex methods for isolating and analyzing EP. In particular, EPs for the presentation of visual stimuli representing an image are widely studied. The most commonly used image is a sinusoidal brightness-modulated or contrast grating or checkerboard pattern with different spatial frequencies and contrast measures. The image is presented as a relatively long exposure. In addition, presentation is used with the help of a sinusoidally modulated in time in terms of brightness of the light flux. Using this method, the so-called constant-state VP is obtained. This EP is an oscillatory sinusoidal process with constant frequency-amplitude characteristics, which is in a certain frequency-amplitude ratio with the frequency and intensity of the light flux that provides visual stimulation. Such potentials are most often used in testing the function of vision, and at present, research does not go mainly beyond laboratory experiments.
EPs for perversions of the visual pattern (when black elements on the screen change places with white ones) acquire significant practical importance in clinical research. Data have been obtained showing a regular relationship between the amplitude and latent periods of some components of these EPs and the size of a chess field and a correlation with visual acuity. From the point of view of clinical neurology, EPs for perversion of the visual pattern in studies of demyelinating diseases are of the greatest interest.
In recent years, an analysis has been carried out both of EPs in the norm in terms of their connection with various parts of the afferent systems, and a study of changes in EPs in pathology in terms of the connection of these changes with general and particular rearrangements arising in the CNS under the influence of the pathological process.
EAP research finds application in many areas of clinical practice:
Local destructive lesions of the nervous system:
Damage to the peripheral nervous system;
spinal cord injury;
Damage to the brain stem;
Damage to the cerebral hemispheres;
The defeat of the thalamus;
Supratalamic lesions;
Nervous diseases:
Epilepsy;
swelling of the central nervous system;
Cerebrovascular disorders;
Traumatic brain injury;
Deminitions;
Metabolic disorders;
Coma and vegetative state;
Resuscitation monitoring.
The possibilities of the EP method allow not only to detect the structural level of the analyzer damage, but also to quantify the nature of the damage to the human sensory function in various parts of the analyzer. The EP registration method is of particular value and uniqueness for detecting sensory impairments in very young children. Systems using the EP method are used in neurology, neurosurgery, defectology, clinical audiometry, psychiatry, forensic psychiatry, military and labor examination.
3. CHARACTERISTICS OF THE EP
The evoked potentials of the cortex, or caused by responses, are called gradual electrical reactions of the cortex to a single afferent stimulation of any section of the nervous system. The amplitude, which normally reaches 15 μV - long latency (up to 400 ms) and 1 μV - short latency (up to 15 ms).
Somatosensory potentials are afferent responses from various structures of the sensorimotor system in response to electrical stimulation of peripheral nerves. A great contribution to the introduction of evoked potentials was made by Dawson precisely by studying SSEP during stimulation of the ulnar nerve. SSEPs are divided into long-latency and short-latency in response to stimulation of the nerves of the upper or lower extremities. In clinical practice, short-latency SSEPs (SSEPs) are more commonly used. If the necessary technical and methodological conditions are met during registration of SSEPs, clear answers can be obtained from all levels of the somatosensory pathway and cortex, which is quite adequate information about damage to both the conduction tracts of the brain and spinal cord, and the sensorimotor cortex. The stimulating electrode is most often placed on the projection of n.medianus, n.ulnaris, n.tibialis, n.perineus.
KSSVP during stimulation of the upper limbs. When n.medianus is stimulated, the signal passes along the afferent pathways through the brachial plexus (the first switch in the ganglia), then to the posterior horns of the spinal cord at the level of C5-C7, through the medulla oblongata to the Gol-Burdach nuclei (second switch), and through the spinal-thalamic the path to the thalamus, where, after switching, the signal passes to the primary sensorimotor cortex (1-2 field according to Brodmann). SSEP during stimulation of the upper limbs is used in the clinic in the diagnosis and prognosis of diseases such as multiple sclerosis, various traumatic lesions of the brachial plexus, brachial ganglion, injuries of the cervical spinal cord in spinal cord injuries, brain tumors, vascular diseases, evaluation of sensory sensory disorders in hysterical patients , evaluation and prognosis of coma to determine the severity of brain damage and brain death.
Registration conditions. Active recording electrodes are installed on C3-C4 according to the international system "10-20%", on the level of the neck in the projection between C6-C7 vertebrae, in the region of the middle part of the clavicle at Erb's point. The reference electrode is placed on the forehead at point Fz. Cup electrodes are usually used, and in the conditions of the operating room or intensive care unit, needle electrodes. Before cup electrodes are applied, the skin is treated with an abrasive paste and then a conductive paste is applied between the skin and the electrode.
The stimulating electrode is placed in the area of the wrist joint, in the projection n.medianus, the ground electrode is slightly higher than the stimulating one. A current of 4-20 mA is used, with a pulse duration of 0.1-0.2 ms. By gradually increasing the current strength, the stimulation threshold is adjusted to a motor response from the thumb. Stimulation rate 4-7 per sec. Pass filters from 10-30 Hz to 2-3 kHz. Analysis epoch 50 ms. The number of averagings is 200-1000. The signal rejection ratio allows you to get the cleanest responses in the shortest period of time and improve the signal-to-noise ratio. Two series of responses should be recorded.
Response options. After verification, the following components are analyzed in KSSVP: N10 - the level of impulse transmission in the composition of the fibers of the brachial plexus; N11 - reflects the passage of the afferent signal at the level of C6-C7 vertebrae along the posterior horns of the spinal cord; N13 is associated with the passage of an impulse through the Gol-Burdach nuclei in the medulla oblongata. N19 – distant field potential, reflects the activity of neurogenerators in the thalamus; N19-P23 - thalamo-cortical pathways (registered from the contralateral side), P23 responses generated in the postcentral gyrus of the contralateral hemisphere (Fig. 1).
The negative N30 component is generated in the precentral frontal region and recorded in the fronto-central region of the contralateral hemisphere. The positive P45 component is registered in the ipsilateral hemisphere of its central region and is generated in the region of the central sulcus. The negative component of N60 is recorded contralaterally and has the same sources of generation as P45.
SSEP parameters are influenced by factors such as height and age, as well as the sex of the subject.
The following response rates are measured and evaluated:
Fig. 1. Time characteristics of responses at Erb's point (N10), components N11 and N13 during ipsi- and contralateral abduction.
2. Latent time of components N19 and P23.
3. P23 amplitude (between N19-P23 peaks).
4. The speed of the impulse along the afferent sensorimotor peripheral pathways, calculated by dividing the distance from the stimulation point to the Erb point by the time the impulse traveled to the Erb point.
5. Difference between N13 latency and N10 latency.
6. Central conduction time - the conduction time from the Gol-Burdakh nuclei N13 to the thalamus N19-N20 (lemniscal pathway to the cortex).
7. The time of conduction of afferent nerve impulses from the brachial plexus to the primary sensory cortex - the difference between the components N19-N10.
Tables 1 and 2 show the amplitude-time characteristics of the main components of SSEP in healthy people.
Table 1.
Temporal values of SSEP during stimulation of the median nerve are normal (ms).
| Men | Women | |||
| Average value | Upper limit of normal | Average value | Upper limit of normal | |
| N10 | 9,8 | 11,0 | 9,5 | 10,5 |
| N10-N13 | 3,5 | 4,4 | 3,2 | 4,0 |
| N10-N19 | 9,3 | 10,5 | 9,0 | 10,1 |
| N13-N19 | 5,7 | 7,2 | 5,6 | 7,0 |
table 2
Amplitude values of SSEP during stimulation of the median nerve are normal (μV).
| Men and women | ||
| Average value | Lower limit of normal | |
| N10 | 4,8 | 1,0 |
| N13 | 2,9 | 0,8 |
| N19-P23 | 3,2 | 0,8 |
The main criteria for abnormal SSEP during stimulation of the upper limbs are the following changes:
1. The presence of amplitude-time asymmetry of responses during stimulation of the right and left hands.
2. The absence of components N10, N13, N19, P23, which may indicate damage to the processes of generating responses or a violation of the conduction of a sensorimotor impulse in a certain section of the somatosensory pathway. For example, the absence of the N19-P23 component may indicate damage to the cortex or subcortical structures. It is necessary to differentiate true violations of the somatosensory signal from technical errors in the registration of SSEP.
3. The absolute values of the latencies depend on the individual characteristics of the subject, for example, on growth and temperature, and, accordingly, this must be taken into account when analyzing the results.
4. The presence of an increase in peak-to-peak latencies compared to the normative indicators can be assessed as pathological and indicate a delay in the conduction of a sensorimotor impulse at a certain level. On fig. 2. there is an increase in the latency of the N19, P23 components and the central conduction time in a patient with a traumatic lesion in the midbrain.
KSSEP during stimulation of the lower extremities. Most often in clinical practice, n.tibialis stimulation is used to obtain the most stable and clear responses.
Registration conditions. A stimulating electrode with electrically conductive paste is fixed on the inner surface of the ankle. The ground electrode is placed proximal to the stimulating one. In case of two-channel registration of responses, the recording electrodes are set: active in the projection L3 and reference L1, active scalp electrode Cz and reference Fz. The stimulation threshold is selected until the muscle response is flexion of the foot. Stimulation rate 2-4 per sec. at a current strength of 5-30 mA and a pulse duration of 0.2-0.5 ms, the number of averagings is up to 700-1500, depending on the purity of the responses received. Analyzed epoch 70-100ms
The following SSEP components are verified and analyzed: N18, N22 - peaks reflecting the passage of a signal at the level of the spinal cord in response to peripheral stimulation, P31 and P34 - components of subcortical origin, P37 and N45 - components of cortical origin, which reflect activation of the primary somatosensory cortex of the leg projection (Fig. 3).
Parameters of responses of SSEPs during stimulation of the lower extremities are affected by height, age of the subject, body temperature, and a number of other factors. Sleep, anesthesia, impaired consciousness mainly affect the late components of SSEP. In addition to the main peak latencies, interpeak latencies N22-P37 are evaluated - the conduction time from LIII to the primary somatosensory cortex. The conduction time from LIII to the brainstem and between the brainstem and the cortex is also estimated (N22-P31 and P31-P37, respectively).
The following parameters of SSEP responses are measured and evaluated:
1. Temporal characteristics of the N18-N22 components, reflecting the action potential in the LIII projection.
2. Timing characteristics of components P37-N45.
3. Peak-to-peak latencies N22-P37, conduction time from the lumbar spine (root exit site) to the primary sensorimotor cortex.
4. Assessment of the conduction of nerve impulses separately between the lumbar region and the brain stem and the stem and cortex, respectively N22-P31, P31-P37.
The following changes in SSEP are considered the most significant deviations from the norm:
1. The absence of the main components that are stably recorded in healthy subjects N18, P31, P37. The absence of the P37 component may indicate damage to the cortical or subcortical structures of the somatosensory pathway. The absence of other components may indicate dysfunction of both the generator itself and the ascending pathways.
2. Increased peak-to-peak latency N22-P37. An increase of more than 2-3 ms compared to normal indicates a delay in conduction between the corresponding structures and is assessed as pathological. On fig. 4. shows an increase in peak-to-peak latency in multiple sclerosis.
3. The values of latencies and amplitudes, as well as the configuration of the main components, cannot serve as a reliable criterion for deviation from the norm, as they are influenced by factors such as growth. Peak-to-peak latencies are a more reliable indicator.
4. Asymmetry during stimulation of the right and left sides is an important diagnostic indicator.
In the KSSVP clinic, when stimulating the lower extremities, they use: for multiple sclerosis, spinal cord injuries (the technique can be used to assess the level and degree of damage), assess the state of the sensory cortex, assess sensory sensory dysfunctions in hysterical patients, with neuropathies, in prognosis and evaluation coma and brain death. In multiple sclerosis, one can observe an increase in the latencies of the main components of SSEP, peak-to-peak latencies, and a decrease in amplitude characteristics by 60% or more. When stimulating the lower extremities, the changes in SSEP are more pronounced, which can be explained by the passage of a nerve impulse over a greater distance than when stimulating the upper extremities and with a greater likelihood of detecting pathological changes.
In traumatic spinal cord injury, the severity of SSEP changes depends on the severity of the injury. With a partial violation, changes in SSEP are in the nature of minor violations in the form of a change in the configuration of the response, changes in the early components. In the event of a complete interruption of the pathways, the components of SSEP from the higher located departments disappear.
In case of neuropathies, SSEP can be used to stimulate the lower extremities to determine the cause of the disease, for example, cauda equina syndrome, spinal clonus, compression syndrome, etc. The SSEP technique in cerebral lesions is of great clinical importance. Many authors, based on the results of numerous studies, consider it appropriate to conduct a study at 2-3 weeks or 8-12 weeks of ischemic stroke. In patients with reversible neurological symptoms in case of cerebrovascular accidents in the carotid and vertebrobasilar basins, only small deviations from normal SSEP values are detected, and in patients who, upon further observation, have more pronounced consequences of the disease, changes in SSEP turned out to be more significant in subsequent studies.
Long-latency somatosensory evoked potentials. DSSEP make it possible to evaluate the processes of processing sensorimotor information not only in the primary cortex, but also in the secondary cortex. The technique is especially informative in assessing the processes associated with the level of consciousness, the presence of pain of central origin, etc.
Registration conditions. Active recording electrodes are set to Cz, the reference electrode is placed in the forehead at point Fz. The stimulating electrode is placed in the area of the wrist joint, in the projection n.medianus, the ground electrode is slightly higher than the stimulating one. A current of 4-20 mA is used, with a pulse duration of 0.1-0.2 ms. Frequency during stimulation with single pulses 1-2 per second, with stimulation in series 1 series per second. 5-10 pulses with an interstimulus interval of 1-5 ms. Frequency pass filters from 0.3-0.5 to 100-200 Hz. The epoch of analysis is at least 500 ms. The number of averaged single responses is 100-200. For the correct interpretation and analysis of the data obtained, it is necessary to record two series of answers.
Response options. In DSSVP, the most stable component is P250 with a latency of 230-280 ms (Fig. 5), after verification of which the amplitude and latency are determined.
A change in the amplitude-temporal characteristics of DSSEP was shown in patients with chronic pain syndromes of various origins in the form of an increase in amplitude and a decrease in latent time. With impaired consciousness, the P250 component may not be registered or registered with a significant increase in latent time.
Electroencephalography - method of registration and analysis of the electroencephalogram (EEG), i.e. total bioelectrical activity taken both from the scalp and from the deep structures of the brain. The last at the person is possible only in clinical conditions. In 1929, an Austrian psychiatrist. Berger discovered that "brain waves" could be recorded from the surface of the skull. He found that the electrical characteristics of these signals depend on the condition of the subject. The most noticeable were synchronous waves of relatively large amplitude with a characteristic frequency of about 10 cycles per second. Berger called them alpha waves and contrasted them with the high-frequency "beta waves" that occur when a person goes into a more active state. Berger's discovery led to the creation of an electroencephalographic method for studying the brain, which consists in recording, analyzing and interpreting the biocurrents of the brain of animals and humans. One of the most striking features of the EEG is its spontaneous, autonomous nature. Regular electrical activity of the brain can be recorded already in the fetus (that is, before the birth of the organism) and stops only with the onset of death. Even with deep coma and anesthesia, a special characteristic pattern of brain waves is observed. Today, the EEG is the most promising, but still the least deciphered source of data for the psychophysiologist.
Registration conditions and methods of EEG analysis. The stationary complex for recording EEG and a number of other physiological parameters includes a soundproof shielded chamber, an equipped place for the test subject, monochannel amplifiers, recording equipment (ink encephalograph, multichannel tape recorder). Usually, from 8 to 16 EEG recording channels are used simultaneously from different parts of the skull surface. EEG analysis is carried out both visually and with the help of a computer. In the latter case, special software is required.
delta rhythm (0.5-4 Hz);
theta rhythm (5-7 Hz);
alpha rhythm(8-13 Hz) - the main rhythm of the EEG, prevailing at rest;
mu-rhythm - in terms of frequency-amplitude characteristics, it is similar to the alpha rhythm, but prevails in the anterior sections of the cerebral cortex;
beta rhythm (15-35 Hz);
gamma rhythm (above 35 Hz).
According to the frequency in the EEG, the following types of rhythmic components are distinguished:
It should be emphasized that such a division into groups is more or less arbitrary; it does not correspond to any physiological categories. Slower frequencies of electrical potentials of the brain were also registered up to periods of the order of several hours and days. Recording at these frequencies is performed using a computer.
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Basic rhythms and parameters of the encephalogram. 1. Alpha wave - a single two-phase oscillation of the potential difference with a duration of 75-125 ms., It approaches a sinusoidal in shape. 2. Alpha rhythm - rhythmic fluctuation of potentials with a frequency of 8-13 Hz, expressed more often in the posterior parts of the brain with closed eyes in a state of relative rest, the average amplitude is 30-40 μV, usually modulated into spindles. 3. Beta wave - a single two-phase oscillation of potentials with a duration of less than 75 ms and an amplitude of 10-15 μV (no more than 30). 4. Beta rhythm - rhythmic oscillation of potentials with a frequency of 14-35 Hz. It is better expressed in the fronto-central areas of the brain. 5. Delta wave - a single two-phase oscillation of the potential difference with a duration of more than 250 ms. 6. Delta rhythm - rhythmic oscillation of potentials with a frequency of 1-3 Hz and an amplitude of 10 to 250 μV or more. 7. Theta wave - a single, more often two-phase oscillation of the potential difference with a duration of 130-250 ms. 8. Theta rhythm - rhythmic oscillation of potentials with a frequency of 4-7 Hz, more often bilateral synchronous, with an amplitude of 100-200 μV, sometimes with spindle-shaped modulation, especially in the frontal region of the brain. |
Another important characteristic of the electrical potentials of the brain is the amplitude, i.e. the amount of fluctuation. The amplitude and frequency of oscillations are related to each other. The amplitude of high-frequency beta waves in the same person can be almost 10 times lower than the amplitude of slower alpha waves. The location of the electrodes is important in EEG recording, while the electrical activity simultaneously recorded from different points of the head can vary greatly. When recording EEG, two main methods are used: bipolar and monopolar. In the first case, both electrodes are placed in electrically active points of the scalp, in the second case, one of the electrodes is located at a point that is conventionally considered electrically neutral (earlobe, bridge of the nose). With bipolar recording, an EEG is recorded, representing the result of the interaction of two electrically active points (for example, frontal and occipital leads), with monopolar recording - the activity of a single lead relative to an electrically neutral point (for example, frontal or occipital leads relative to the earlobe). The choice of one or another recording option depends on the objectives of the study. In research practice, the monopolar variant of registration is more widely used, since it makes it possible to study the isolated contribution of one or another area of the brain to the process being studied. The International Federation of Societies for Electroencephalography has adopted the so-called "10-20" system to accurately indicate the location of the electrodes. In accordance with this system, the distance between the middle of the bridge of the nose (nasion) and the hard bony tubercle on the back of the head (inion), as well as between the left and right ear fossae, is accurately measured in each subject. The possible locations of the electrodes are separated by intervals of 10% or 20% of these distances on the skull. At the same time, for the convenience of registration, the entire skull is divided into regions indicated by the letters: F - frontal, O - occipital region, P - parietal, T - temporal, C - region of the central sulcus. Odd numbers of abduction sites refer to the left hemisphere, and even numbers to the right hemisphere. The letter Z - denotes the assignment from the top of the skull. This place is called the vertex and is used especially often (see Reader 2.2). 
Clinical and static methods for studying EEG. Since its inception, two approaches to EEG analysis have stood out and continue to exist as relatively independent: visual (clinical) and statistical. Visual (clinical) EEG analysis usually used for diagnostic purposes. The electrophysiologist, relying on certain methods of such an analysis of the EEG, solves the following questions: does the EEG correspond to generally accepted standards of the norm; if not, what is the degree of deviation from the norm, whether the patient has signs of focal brain damage and what is the localization of the lesion. Clinical analysis of the EEG is always strictly individual and is predominantly qualitative. Despite the fact that there are generally accepted methods for describing the EEG in the clinic, the clinical interpretation of the EEG largely depends on the experience of the electrophysiologist, his ability to "read" the electroencephalogram, highlighting hidden and often very variable pathological signs in it. However, it should be emphasized that gross macrofocal disturbances or other distinct forms of EEG pathology are rare in wide clinical practice. Most often (70-80% of cases), there are diffuse changes in the bioelectrical activity of the brain with symptoms that are difficult to formally describe. Meanwhile, it is precisely this symptomatology that may be of particular interest for the analysis of the contingent of subjects who are included in the group of so-called "minor" psychiatry - conditions that border on the "good" norm and obvious pathology. It is for this reason that special efforts are now being made to formalize and even develop computer programs for clinical EEG analysis. Statistical research methods electroencephalograms proceed from the fact that the background EEG is stationary and stable. Further processing in the overwhelming majority of cases is based on the Fourier transform, the meaning of which is that a wave of any complex shape is mathematically identical to the sum of sinusoidal waves of different amplitudes and frequencies. The Fourier transform allows you to transform the wave pattern background EEG to frequency and set the power distribution for each frequency component. Using the Fourier transform, the most complex EEG oscillations can be reduced to a series of sinusoidal waves with different amplitudes and frequencies. On this basis, new indicators are distinguished that expand the meaningful interpretation of the rhythmic organization of bioelectric processes. For example, a special task is to analyze the contribution, or relative power, of different frequencies, which depends on the amplitudes of the sinusoidal components. It is solved by constructing power spectra. The latter is a set of all power values of EEG rhythmic components calculated with a certain discretization step (in the amount of tenths of a hertz). Spectra can characterize the absolute power of each rhythmic component or relative, i.e. the severity of the power of each component (in percent) in relation to the total power of the EEG in the analyzed segment of the record.
EEG power spectra can be subjected to further processing, for example, correlation analysis, while calculating auto- and cross-correlation functions, as well as coherence
, which characterizes the measure of synchronism of EEG frequency bands in two different leads. Coherence ranges from +1 (completely matching waveforms) to 0 (completely different waveforms). Such an assessment is carried out at each point of the continuous frequency spectrum or as an average within the frequency subbands. Using the calculation of coherence, one can determine the nature of the intra- and interhemispheric relationships of EEG parameters at rest and during different types of activity. In particular, using this method, it is possible to establish the leading hemisphere for a particular activity of the subject, the presence of stable interhemispheric asymmetry, etc. Due to this, the spectral-correlation method for assessing the spectral power (density) of EEG rhythmic components and their coherence is currently one of the most common. 
Sources of EEG generation. Paradoxically, but the actual impulse activity neurons is not reflected in the fluctuations of the electrical potential recorded from the surface of the human skull. The reason is that the impulse activity of neurons is not comparable with the EEG in terms of time parameters. The duration of the impulse (action potential) of the neuron is no more than 2 ms. The time parameters of the rhythmic components of the EEG are calculated in tens and hundreds of milliseconds. It is generally accepted that electrical processes recorded from the surface of an open brain or scalp reflect synaptic neuron activity. We are talking about potentials that arise in the postsynaptic membrane of a neuron that receives an impulse. Excitatory postsynaptic potentials have a duration of more than 30 ms, and inhibitory postsynaptic potentials of the cortex can reach 70 ms or more. These potentials (in contrast to the action potential of a neuron, which arises according to the "all or nothing" principle) are gradual in nature and can be summed up. Simplifying the picture somewhat, we can say that positive potential fluctuations on the surface of the cortex are associated either with excitatory postsynaptic potentials in its deep layers, or with inhibitory postsynaptic potentials in the surface layers. Negative potential fluctuations on the surface of the crust presumably reflect the opposite ratio of sources of electrical activity. The rhythmic nature of the bioelectrical activity of the cortex, and in particular the alpha rhythm, is mainly due to the influence of subcortical structures, primarily the thalamus (interbrain). It is in the thalamus that the main, but not the only, pacemakers or pacemakers. Unilateral removal of the thalamus or its surgical isolation from the neocortex leads to the complete disappearance of the alpha rhythm in the areas of the cortex of the operated hemisphere. At the same time, nothing changes in the rhythmic activity of the thalamus itself. The neurons of the nonspecific thalamus have the property of authoritativeness. These neurons, through appropriate excitatory and inhibitory connections, are able to generate and maintain rhythmic activity in the cerebral cortex. An important role in the dynamics of the electrical activity of the thalamus and cortex is played by reticular formation brain stem. It can have a synchronizing effect, i.e. contributing to the generation of a steady rhythmic pattern, and dissynchronizing, disrupting the coordinated rhythmic activity (see Reader. 2.3).
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Synaptic activity of neurons |
The functional significance of the ECG and its components. The question of the functional significance of individual components of the EEG is of great importance. The greatest attention of researchers here has always attracted alpha rhythm is the dominant resting EEG rhythm in humans. There are many assumptions regarding the functional role of the alpha rhythm. The founder of cybernetics N. Wiener and after him a number of other researchers believed that this rhythm performs the function of temporal scanning ("reading") of information and is closely related to the mechanisms of perception and memory. It is assumed that the alpha rhythm reflects the reverberation of excitations that encode intracerebral information and create an optimal background for the process of receiving and processing. afferent signals. Its role consists in a kind of functional stabilization of the states of the brain and ensuring readiness to respond. It is also assumed that the alpha rhythm is associated with the action of brain selective mechanisms that act as a resonant filter and thus regulate the flow of sensory impulses. At rest, other rhythmic components may be present in the EEG, but their significance is best clarified when the functional states of the body change ( Danilova, 1992). So, the delta rhythm in a healthy adult at rest is practically absent, but it dominates the EEG at the fourth stage of sleep, which got its name from this rhythm (slow-wave sleep or delta sleep). On the contrary, the theta rhythm is closely associated with emotional and mental stress. It is sometimes referred to as the stress rhythm or tension rhythm. In humans, one of the EEG symptoms of emotional arousal is an increase in theta rhythm with an oscillation frequency of 4-7 Hz, which accompanies the experience of both positive and negative emotions. When performing mental tasks, both delta and theta activity may increase. Moreover, the strengthening of the last component is positively correlated with the success of solving problems. In its origin, the theta rhythm is associated with cortico-limbic interaction. It is assumed that the increase in theta rhythm during emotions reflects the activation of the cerebral cortex from the limbic system. The transition from a state of rest to tension is always accompanied by a desynchronization reaction, the main component of which is high-frequency beta activity. Mental activity in adults is accompanied by an increase in the power of the beta rhythm, and a significant increase in high-frequency activity is observed during mental activity that includes elements of novelty, while stereotypical, repetitive mental operations are accompanied by its decrease. It was also found that the success of performing verbal tasks and tests for visual-spatial relations is positively associated with high activity of the EEG beta range of the left hemisphere. According to some assumptions, this activity is associated with a reflection of the activity of the mechanisms for scanning the structure of the stimulus, carried out by neural networks that produce high-frequency EEG activity (see Reader 2.1; Reader 2.5).
Magnetoencephalography-registration of magnetic field parameters determined by the bioelectrical activity of the brain. These parameters are recorded using superconducting quantum interference sensors and a special camera that isolates the magnetic fields of the brain from stronger external fields. The method has a number of advantages over the registration of a traditional electroencephalogram. In particular, the radial components of magnetic fields recorded from the scalp do not undergo such strong distortions as the EEG. This makes it possible to more accurately calculate the position of the generators of EEG activity recorded from the scalp.
2.1.2. evoked potentials of the brain
Evoked Potentials (EP)-bioelectric oscillations that occur in the nervous structures in response to external stimulation and are in a strictly defined temporal connection with the onset of its action. In humans, EPs are usually included in the EEG, but against the background of spontaneous bioelectrical activity, they are difficult to distinguish (the amplitude of single responses is several times less than the amplitude of the background EEG). In this regard, the recording of the EP is carried out by special technical devices that allow you to select a useful signal from the noise by sequentially accumulating it, or summing it. In this case, a certain number of EEG segments, timed to coincide with the onset of the stimulus, are summed up.
The widespread use of the EP registration method became possible as a result of the computerization of psychophysiological studies in the 1950s and 1960s. Initially, its use was mainly associated with the study of human sensory functions in normal conditions and with various types of anomalies. Subsequently, the method began to be successfully applied to the study of more complex mental processes that are not a direct response to an external stimulus. Methods for separating a signal from noise make it possible to mark changes in the potential in the EEG record, which are quite strictly related in time to any fixed event. In this regard, a new designation for this range of physiological phenomena has appeared - event-related potentials (ECPs).
fluctuations associated with the activity of the motor cortex (motor potential, or potential associated with movement);
the potential associated with the intention to perform a certain action (the so-called E-wave);
the potential that arises when an expected stimulus is missed.
The examples here are:
These potentials are a sequence of positive and negative oscillations, usually recorded in the range of 0-500 ms. In some cases, later oscillations in the interval up to 1000 ms are also possible. Quantitative methods for estimating EP and SSP provide, first of all, an assessment of the amplitudes and latencies. Amplitude - the range of oscillations of the components, measured in μV, latency - the time from the beginning of stimulation to the peak of the component, measured in ms. In addition, more complex analysis options are used.
phenomenological;
physiological;
functional.
Three levels of analysis can be distinguished in the study of EP and SSP:
Phenomenological level includes a description of VP as a multicomponent reaction with an analysis of the configuration, component composition, and topographic features. In fact, this is the level of analysis from which any study using the IP method begins. The possibilities of this level of analysis are directly related to the improvement of methods for quantitative processing of EP, which include various techniques, ranging from estimating latencies and amplitudes to derivatives, artificially constructed indicators. The mathematical apparatus for processing VP is also diverse, including factorial, dispersion, taxonomic and other types of analysis. Physiological level. According to these results, at the physiological level of analysis, the sources of generation of EP components are identified, i.e. the question is solved in which brain structures the individual components of EP arise. The localization of EP generation sources makes it possible to establish the role of individual cortical and subcortical formations in the origin of certain EP components. The most recognized here is the division of VP into exogenous and endogenous Components. The former reflect the activity of specific conductive pathways and zones, the latter reflect the activity of nonspecific associative conduction systems of the brain. The duration of both is estimated differently for different modalities. In the visual system, for example, exogenous EP components do not exceed 100 ms from the moment of stimulation. The third level of analysis is functional involves the use of EP as a tool to study the physiological mechanisms of behavior and cognitive activity of humans and animals.
VP as a unit of psychophysiological analysis. A unit of analysis is usually understood as such an object of analysis, which, unlike elements, has all the basic properties inherent in the whole, and the properties are further indecomposable parts of this unity. The unit of analysis is such a minimal formation in which the essential connections and parameters of the object that are essential for a given task are directly presented. Moreover, such a unit must itself be a single whole, a kind of system, the further decomposition of which into elements will deprive it of the possibility of representing the whole as such. A mandatory feature of the unit of analysis is also that it can be operationalized, i.e. it allows for measurement and quantification. If we consider psychophysiological analysis as a method of studying the brain mechanisms of mental activity, then EPs meet most of the requirements that can be presented to the unit of such an analysis. Firstly, EP should be qualified as a psycho-nervous reaction, i.e. one that is directly connected with the processes of mental reflection. Secondly, VP is a reaction consisting of a number of components that are continuously interconnected. Thus, it is structurally homogeneous and can be operationalized, i.e. has quantitative characteristics in the form of parameters of individual components (latencies and amplitudes). It is essential that these parameters have different functional meanings depending on the features of the experimental model. Third, the decomposition of the EP into elements (components), carried out as a method of analysis, makes it possible to characterize only individual stages of the information processing process, while the integrity of the process as such is lost. In the most convex form, the ideas about the integrity and consistency of EP as a correlate of a behavioral act are reflected in the studies of V.B. Shvyrkova. According to this logic, EPs, occupying the entire time interval between the stimulus and response, correspond to all processes leading to the emergence of a behavioral response, while the EP configuration depends on the nature of the behavioral act and the characteristics of the functional system that provides this form of behavior. At the same time, individual components of EP are considered as a reflection of the stages of afferent synthesis, decision-making, activation of executive mechanisms, and achievement of a useful result. In this interpretation, EPs act as a unit of psychophysiological analysis of behavior. However, the mainstream of the use of EP in psychophysiology is associated with the study of physiological mechanisms and correlates human cognitive activity. This direction is defined as cognitive psychophysiology. VP is used in it as a full-fledged unit of psychophysiological analysis. This is possible because, according to the figurative definition of one of the psychophysiologists, EPs have a unique dual status of their kind, acting at the same time as a "window to the brain" and a "window to cognitive processes" (see Reader 2.4).
The evoked potentials of the brain are the modern test method functions and performance of analyzers of the cerebral cortex. This method allows you to register the responses of higher analyzers to various external artificial stimuli. The most used and widely used stimuli are visual (for recording visual evoked potentials), auditory (for recording acoustic evoked potentials), and somatosensory, respectively.
process directly registration of potentials It is carried out with the help of microelectrodes, which are brought close to the nerve cells of a certain area of the cerebral cortex. Microelectrodes got their name because their size and diameter does not exceed one micron. Such small devices appear to be straight rods, which consist of high-resistance insulated wire with a sharpened recording tip. The microelectrode itself is fixed and connected to the signal amplifier. Information about the latter is received on monitor screens and recorded on magnetic tape.
However, this is considered an invasive method. There is also non-invasive. Instead of bringing microelectrodes to the cells of the cortex, the electrodes are attached to the skin of the head, neck, trunk or knees, depending on the purpose of the experiment.
The technique of evoked potentials is used to study the activity of the sensory systems of the brain; this method is also applicable in the field of cognitive (mental) processes. The essence of the technology lies in the registration of bioelectric potentials formed in the brain in response to an external artificial stimulus.
The response evoked by the brain is usually classified depending on the reaction rate of the nervous tissue:
- Short-latency - reaction speed up to 50 milliseconds.
- Medium latent - reaction speed from 50 to 100 milliseconds.
- Long-latency - a reaction of 100 milliseconds or more.
A variation of this method are motor evoked potentials. They are fixed and removed from the muscles of the body in response to the action on the nervous tissue of the motor region of the cortex of the hemispheres by electric or magnetic influence. This technique is called transcranial magnetic stimulation. This technology is applicable in the diagnosis of diseases of the cortico-spinal tract, that is, the pathways that conduct nerve impulses from the cortex to the spinal cord.
The main properties that evoked potentials have are latency, amplitude, polarity, and waveform.
Kinds
Each type implies not only a general, but also a specific approach to the study of the activity of the cortex.
Visual VP
Visual evoked potentials of the brain is a method that involves recording the responses of the cerebral cortex to the action of external stimuli, such as a light flash. The methodology is as follows:
- Active electrodes are attached to the skin of the parietal and occipital region, and the reference (relative to which the measurement is taken) electrode is attached to the skin of the forehead.
- The patient closes one eye, and directs the gaze of the other to the monitor, from where light stimulation is supplied.
- Then change eyes and carry out the same experiment.
Auditory EPs
Acoustic evoked potentials appear in response to stimulation of the auditory cortex by successive sound clicks. The patient hears the sound first in the left ear, then in the right. The signal level is displayed on the monitor and the results are interpreted.
Somatosensory EPs
This method involves the registration of peripheral nerves arising in response to bioelectrical stimulation. The implementation of the methodology consists of several stages:
- Stimulating electrodes are attached to the skin of the subject in those places where the sensory nerves pass. As a rule, such places are located in the area of the wrist, knee or ankle. The recording electrodes are attached to the scalp above the sensory area of the cerebral cortex.
- Start of nerve stimulation. Acts of irritation of the nerves should be at least 500 times.
- Computing machines average the speed indicator and display the result in the form of a graph.
Diagnostics
Somatosensory evoked potentials are used in the diagnosis of various diseases of the nervous system, including degenerative, demyelinating, and vascular pathologies of the nervous tissue. This method is also confirmatory in the diagnosis of polyneuropathy in diabetes mellitus.
Evoked Potential Monitors record the electrical activity of the nervous system in response to stimulation of certain nerve pathways. These can be somatosensory, visual, stem acoustic evoked potentials or motor evoked potentials. The recording of evoked potentials is a minimally invasive (or non-invasive) objective and reproducible research method that complements the clinical neurological examination.
With barbituric coma or drug overdose evoked potential research allows to differentiate the action of drugs from damage to the nervous system. This is possible because drugs have little effect on short latency evoked potentials, even at doses sufficient to produce an isoelectric EEG.
Indications for monitoring evoked potentials:
Monitoring the integrity of the nervous system intraoperatively, for example, in complex operations on a deformed spine.
Monitoring for TBI and coma.
Assessment of the depth of anesthesia.
Diagnosis of demyelinating diseases.
Diagnosis of neuropathies and brain tumors.
Classification of evoked potentials
summoned potentials are subdivided according to the type of stimulation, the place of stimulation and registration, the amplitude, the latent period between the stimulus and the potential, and the polarity of the potential (positive or negative).
Stimulation Options:
Electric - electrodes placed on the scalp, over the spinal column or peripheral nerves, or epidural electrodes applied intraoperatively.
Magnetic - used to study motor evoked potentials, avoiding problems with electrode contact, but inconvenient to use
Visual (checkerboard pattern reversal) or auditory (clicks).
Stimulation area:
Cortical
The vertebral column is above and below the study area.
Mixed peripheral nerves
Muscles (for motor evoked potentials).
Evoked Potential Latency:
Long-term—hundreds of milliseconds—is suppressed during anesthesia during surgery and is not useful for monitoring sedation.
Average - tens of milliseconds - are recorded against the background of anesthesia and depend on its depth.
Short - milliseconds - is usually examined during the operation, because it is the least dependent on anesthesia and sedation.
An increase in latency of more than 10% or a decrease in amplitude of >50% is a sign of an increased risk of complications.
Polarity of evoked potentials:
Each type of evoked potential has its own wave characteristics. Peculiar peaks are markers of drug effect or damage
Visual evoked potentials (VEP)
Visual evoked potentials(VEP) occur when the cerebral cortex responds to visual stimulation with flashes of light or a reverse checkerboard pattern recorded in the occipital region.
Visual evoked potentials (VEP) are recorded during operations on the optic nerve, optic chiasm, skull base, for the diagnosis of multiple sclerosis.
Visual evoked potentials (VEPs) are generally considered less reliable than other types of evoked potentials.
Stem acoustic evoked potentials
Using the stem method, auditory conduction is checked through the ear, the VIII cranial nerve to the lower parts of the bridge, and in the rostral direction along the lateral loop up the brainstem:
It is used for manipulations on the posterior cranial fossa.
Stem acoustic evoked potentials can easily be recorded in patients in a state of coma or sedation and can be useful for assessing the degree of damage to the trunk in the absence of other causes of consciousness depression.
Somatosensory evoked potentials
Somatosensory evoked potentials are recorded from the brain or spinal cord in response to stimulation of peripheral sensory nerves. The most commonly used stimulation of the median, ulnar and posterior tibial nerves during operations on the spine or brachial plexus.
All these tests must be performed by experienced technicians and their interpretation in the ICU should be combined with an underlying medical condition (eg, blindness or deafness, hypothermia, hypoxemia, hypotension, hypercapnia, and ischemic nerve changes) that may alter results.
Motor evoked potentials (electromyography, EMG)
This method allows you to measure the electrical potential of muscle cells in mowing or in a state of activity. The motor unit potential is measured by inserting a needle electrode into the part of the muscle being examined. Thus, the presence of peiropathy or myopathy is determined.
Patients in consciousness are examined muscle electrical potential at rest, with little effort and with maximum effort. It is necessary to study 20 motor unit potentials in at least 10 different areas.
Immediately after the introduction electrode there is a short period of electrical activity of less than 500 μV in amplitude, followed by a period of inactivity when examining a healthy muscle.
Background activity in the motor end plates is sometimes noted.
The presence of biphasic fibrillations usually indicates that the muscle is denervated, although fibrillations in one of the sections of the muscle can also be observed during its normal function.
Fasciculations, if not caused suxamethonium, are always a pathological symptom and usually indicate damage to the cells of the anterior horns of the spinal cord, but can sometimes occur secondary to damage to the nerve root or peripheral muscle damage.
