[Dipole models of alpha-rhythm generators]. / Dipol'nye modeli generatorov al'fa-ritma.

Different hypothesis of the alpha rhythm origin were tested by dipole simulation of the alpha rhythm sources. EEG was recorded during driving photic stimulation in order to increase the signal-to-noise ratio. Models with fixed and moving dipoles were analyzed. Dipole sources were compared with magnetic resonance imaging (MRI) scans to find the exact location of oscillations in brain anatomical structures. A two-level multiple dipole model was found to fit the EEG most adequately. The first level was represented by two oscillators localized in the thalamic reticular nuclei, and the second level is associated with two modality-specific oscillators localized in the respective cortical areas.

[Characteristics of the spatio-temporal organization of the P300 component of AEP during the "active" and "passive" stimulus perception in healthy subjects]. / Osobennosti prostranstvenno-vremennoi organizatsii komponenta P300 AVP pri "aktivnom" i "passivnom" vospriiatii stimula u zdorovykh ispytuemykh.

The concern of the work was in detection and analysis of P300 component of the acoustic evoked potential in healthy subjects in different experimental situations. During counting the rare sounds, P300 was most pronounced in the frontocentral and parietooccipital areas mainly of the left-hemisphere. The response shape was correlated with characteristics of the basic rhythm of the background EEG. Responses of simple and complex shapes were distinguished. The simplest responses were recorded in subjects with hypersynchronous alpha-rhythm. Analysis of three-dimensional dipole source localization showed that structures of the brainstem, limbic system, and frontal lobes participate in generation of the wave. In all the subjects, the decisive role in response generation was played by the brainstem structures. In persons with hypersynchronous alpha-rhythm, the contribution of the frontal lobes was less pronounced. During "passive" listening of sounds, P300 parameters significantly differed from those observed during counting only in 46% of cases (in persons having no hypersynchronous alpha-rhythm). A simplification of the response shape during "passive" listening was observed in these cases, the area of the maximal response expression was shifted to symmetrical areas of the right-hemisphere, the number of dipole sources reduced due to a decrease in the contribution of the frontal and limbic structures into the response generation.

[Change of bioelectric brain activity registered at the distance from the focus of cerebral tissue injury]. / Izmeneniia bioélektricheskoi aktivnosti mozga, registriruemye na rasstoianii ot ochaga porazheniia tserebral'noi tkani.

The focal delta-waves and the remote influences of the stroke in the form of the sinusoidal flashes of delta-waves with a maximal amplitude exceeding an amplitude of the focal delta-waves (frontal flashes--FF) in frontal-polar leads were investigated in 51 patients (22 men, 29 women) by means of mapping, dipole location (BrainLoc) and LORETA methods. The patients were examined in dynamics during the acute period of hemispheric stroke on days 1-3, 14 and 21 after the onset of the disease. The usage of the modern computer methods of EEG analysis permits to consider FF as an independent electrophysiological phenomenon and to localize a zone of the sources of such flashes, which does not coincide with the zone of generation of the focal delta-activity that corresponds to the perifocal zone of the stroke. According to the data of the three-dimensional location a probable zone of FF generation corresponds to the frontal pole and medial-basal areas of the frontal lobe. Comparison with the MRI data leads to the conclusion that a damage of anterior white substance was the most frequent cause of FF development, probably by deafferentation mechanism. FF may be an electrophysiological manifestation of the influence of the damage of the different brain structures on the frontal lobes according to diaschisis mechanism.

[Generators of the rhythmic alpha activity in the human EEG]. / Generatory ritmicheskoi al'fa-aktivnosti EEG cheloveka.

An attempt to localize brain mechanisms of the rhythmic activity in the alpha-rhythm range was made using the equivalent dipole model. It is known that light flickering stimuli with the frequency close to that of the individual alpha rhythm induce an increase in its spectral power ("photic-driving" phenomenon). It was shown that the activity of the neuronal structures generating the alpha rhythm can be identified by specific frequency of the light stimulation and localized by means of construction of dipole models. Two sources of the alpha rhythm in the narrow-frequency bands with the maximal resonance responses in the frequencies of 10.1 and 10.5 Hz were localized in the thalamic structures.

[A search for the sources of pathological alpha activity in the human EEG in limbic structure lesions]. / Poisk istochnikov patologicheskoi al'fa-aktivnosti EEG cheloveka pri porazhenii limbicheskikh struktur.

Dependence of manifestations of EEG pathological activity on the electrode montage was studied in cases of brain lesions involving limbic structures by means of the topographic mapping and 3-d dipole source localization. The original program DIPVIS on the basis of BRAINLOC software allowed us to reveal the stable sources of lateralized pathologic alpha activity and demonstrate the discrepancy between its localization and the localization of the normal human alpha rhythm. The sources of pathological activity in question were predominantly localized in the mediobasal structures of the affected hemisphere in contrast to the occipital region of a healthy human. It was established that the topography of the cortical pathological activity may be determined by peculiarities of localization and, especially, orientation of its dipole generators. Afferent stimulation produced an enhancement of the pathological alpha-activity and increase in dipole source concentration in the mediobasal portion of the temporal lobe. The findings suggest that the discovered from of the alpha-rhythm reflects the reaction of activation of the hippocampal structures involved in the pathological process.

[A dynamic assessment of the reaction of the human brain to radiation exposure (the aftermath of the accident at the Chernobyl Atomic Electric Power Station)]. / Dinamicheskaia otsenka reaktsii mozga cheloveka na vozdeistvie radiatsii (posledstviia avarii na Chernobyl'skoi AES).

The present study was aimed at the comparative assessment of electrophysiological and clinical data in persons (155 right-handed men) who took part in the Chernobyl clean-up in different periods after radiation. Dynamic evaluation of psychoneurological disorders revealed the growth of incidence and severity of cerebrovascular disturbances accompanied by the signs of organic symptoms' aggravation and encephalopathy in longer periods after radiation. The results of neuropsychological examination also showed the deterioration of patients' state manifested as growth of fatigue, cognitive defects, and emotional impairments. Analysis of the EEG parameters, including power and coherence mapping and 3-d dipole source localization analysis demonstrated the increasing number of patients with the most severe forms of EEG pathology: the "plane" type in combination with fast paroxysmal (beta-band) and slow forms of activity from 45% in 1990-92 to 63% in 1997. The "hypersynchronization" type of EEG activity was typical for the earlier period accompanied by the dominance of the pathological forms of EEG activity in mediobasal structures of the left hemisphere, and brainstem zones vs. diencephalon and the right hemisphere. The later period was characterized by decreasing coherence in symmetrical frontal and front-temporal areas of the left hemisphere, while in the early period the hypersynchronization prevailed in symmetrical central areas and in the right hemisphere. The evidence were obtained to a disconnection between the brain hemispheres. We suppose that the progressive involvement of structures of the limbic-reticular complex (especially, brainstem, mediobasal structures, and white matter) into the pathological process occurs with time in participants of clean-up of the Chernobyl disaster consequences.

[Foci of pathological activity in the human brain and their influence on the spatial-temporal relationships of the EEG]. / Ochagi patologicheskoi aktivnosti v golovnom mozge cheloveka i ikh vliianie na prostranstvenno-vremennýe otnosheniia EEG.

EEGs of 51 patients with focal brain lesions (tumor localized in brain hemispheres) were analyzed. Coherence functions were calculated between the EEG recorded from the intact and lesioned hemispheres and three-dimensional analysis of the delta-wave "source" localization was conducted. The revealed delta-wave "sources" localized in the perifocal region of the lesioned hemisphere demonstrated its dynamic properties and a tendency for fluctuations. Changes in the delta-wave "source" pattern were correlated with the EEG coherence values neither in the lesioned nor in the intact hemisphere. The focus of pathological activity produced the changes in EEG coherence and its interhemispheric asymmetry. In the majority of cases (72%), the mean level of intrahemispheric coherence at the side ipsilateral to the pathological focus was lower than the normal mean level, and the mean level of EEG coherence in the intact hemisphere was increased.

[The electroencephalographic correlates of neurological disorders in the late periods of exposure to ionizing radiation (the aftereffects of the accident at the Chernobyl Atomic Electric Power Station)]. / Elektroéntsefalograficheskie korreliaty nevrologicheskikh narushenii v otdalennye sroki vozdeistviia ioniziruiushchei radiatsii (posledstviia avarii na Chernobyl'skoi AES).

EEG mapping and three-dimensional localization of epileptic activity sources together with a neurological analysis were carried out in subjects having taken part in 1986-1987 in the liquidation of consequences of the Chernobyl accident. Experimental group included 40 right-handed 25-45 years-old men having received a radiation dose of 15-51 Ber stated officially. Control group consisted of 20 healthy men. Neurological examination of the patients revealed vegetative-vascular and endocrine dysfunctions as well as diffuse neurological symptoms. EEG of one group of patients (25 persons) was characterized by slow alpha- and theta-band foci and epileptic waves in the central-frontal regions; epileptic sources were localized at the diencephalic level mainly in the midline being shifted to the right hemisphere. In the EEG of another group (15 persons) delta-waves were recorded in the frontal regions at the background of diffuse beta-activity. The sources of epileptic activity of a diffuse character were localized at the basal level of the brain and in the cortex (predominantly) in the left hemisphere. The results obtained together with SPECT mapping and CT data permit to suppose the organic damage of different brain structures (at the cortical and the midline levels) in the patients, with participation of diencephalic structures in the pathological process hypothalamic-hypophysial system being probably connected with adaptive processes in the CNS.

[A dipole localization method in analyzing the auditory brain-stem evoked potential]. / Metod dipol'noi lokalizatsii v analize akusticheskogo stvolovogo vyzvannogo potentsiala.

The localization of generators of brainstem auditory evoked response (BAER) has been studied in one patient using the method of dipole localization based on the spatial distribution of BAER over the surface of the head. It has been found that in formation of all BAER waves the activity of several generators overlaps. It is especially marked for the peaks I', II', III and III'--their potential distribution can not be described by single-dipole model, thus preventing the defining of their generation site. Distribution of potential for other peaks corresponds to the single-dipole model. The coordinates of the equivalent sources of these peaks are in the vicinity of the auditory structures: I--near the distal part of the auditory nerve, II--near the auditory nerve in the place of its entering the brainstem, V--near contralateral auditory pontine structures, V--near lateral lemniscus while V', VI and VI'--near mesencephalic auditory structures.

[Interhemispheric correlations of electric activity of the brain in late depression]. / Mezhpolusharnye sootnosheniia élektricheskoi aktivnosti mozga pri pozdnikh depressiiakh.

A chronometric EEG analysis was made in 60 patients aged 50 to 86 years with affective psychosis in different stages of the depressive phase. It has been demonstrated that the development of depressive conditions gives rise not only to changes in the frequency and amplitude characteristics of the EEG but also in the ratios of interhemispheric asymmetry of the parameters of brain electric activity, attesting to a higher tone of the right hemisphere. This interhemispheric asymmetry was determined to a considerable measure by the presence in the reference EEG of generalized discharges of slow-wave activity. The use of the dipolar analysis of the EEG permitted detecting foci of maximum generation of discharges in the left temporal area and brain stem structures. These areas of generation were preserved after removing the depressive symptomatology with antidepressants. It is suggested that the superstem generators of discharges exert a more appreciable effect on right hemisphere function, which is specified by cortico-diencephalic relations.

[Analysis of "scalp potential fields" and the tridimensional location of the sources of epileptic activation in humans]. / Analiz "ska'lpovykh potentsia'lnykh polei" i trekhmernaia lokali- zatsiia istochnikov épilepticheskoi aktivnosti mozga cheloveka.

The method of multi-step dipole localization with potential charts preanalysis was applied to EEG spatial localization of discharge activity in 24 epileptic patients. The results were juxtaposed to the X-ray contrasting techniques and computerized tomography. Spherical symmetrical 8-layer head model was used to interprete the results. The data on three-dimensional localization of focal and generalized paroxysmal activities are presented. The role of this technique in differentiating the primarily generalized activity from secondary foci is outlined. The generation and recording sites epileptic activity could diverge largely due to the primary source orientation. For correct EEG interpretation, it is important that the primary generation zone be reconstructed. This is an additional field of application of this technique.

[Changes in evoked potentials to light in the presence of lesions of the diencephalic and brainstem regions of the human brain]. / Izmenenie vyzvannykh potentsialov na svet pri porazhenii diéntsefal'nykh i stvolovykh otdelov mozga u cheloveka.

Evoked potentials (EPs) to light were studied in groups of patients with tumours of basal-diencephalic localization (24 patients) and at the brain-stem level (24 patients), in comparison with 11 normal subjects. In the first group, a decrease of amplitude and increase of response duration were found in the frontal cortical areas, with simultaneous reduction of intrahemispheric differences due to decrease of low-frequency components in the EP spectra by amplitude in fronto-central parts, and of alpha- and beta-frequencies in the occipital parts of the hemispheres. EP changes resulting from brain-stem tumors were more expressed than in the case of cerebellar tumors. In patients with foci causing a pronounced irritation of the brain-stem, a decrease in responses amplitude was observed accompanied by an amplitude lowering of delta- and theta-frequencies in EPs-spectra in the frontal, central and temporal leads, with no changes in the occipital leads. The obtained data testify to functional inequivalence and independence of different EP frequency components suggesting that the methods of studying the EP frequency structure are informative and efficient.

[Spatial localization of sources of slow EEG activity by the equivalent dipole method]. / Prostranstvennaia lokalizatsiia istochnikov medlennoi aktivnosti EEG metodom ékvivalentnogo dipolia.

Coordinates and moments of equivalent sources of various kinds of slow activity--local, generalized and synchronous (delta- or theta-ranges) -- were estimated on a computer on the basis of a model of single equivalent dipole, by the method of the coordinates descent. Investigation of local delta-activity caused by a volume process in the brain shows that its localization coincides with the peripheral zone of the tumour. Localization of the delta-focus enables to indicate its position not only on the surface, but in the depth of the brain as well. The dipole localization method helps to specify the concept of "secondary" slow waves and to differentiate slow activity of total brain-, brain-stem and local nature.