Biomedical subjects
Publications and source records attributed to A Shimoliunas.
The potentials of EEG of tangential dipoles are estimated in the model with thin integuments of the brain. These integuments slightly shunt the currents in the shell. The EEG-potentials of the sulcus wall are half-order or even more smaller than those of the gyrus EEG for the same square of syndromized sources.
The estimaiton of ECoG-potentials is fulfilled by means of a simple model of the isolated sphere, which is exact enough, as it was proved earlier [1]. A simple limit formula for the qualitative estimation of ECoG-potentials of a source situated in the cerebral cortex is obtained. The EEG as in [2] is obtained using the transformation of the first 20 spheric harmonics. The experimental facts of registration of the evoked potentials of comparatively little subcortical sources were explained theoretically. Numerical results of the model are used to estimate the intensity of the field induced in the brain by two electrodes placed on the scalp.
The problem of finding EEG potentials is solved assuming that integuments of the brain are regarded as infinitely thin spheres. The potential is not changed along the radius in the layers with high conductivity, and the current in the resistive layers is only radial. It is shown that the EEG potentials could be described exactly enough supposing the lower bone plate to be an ideal isolator. The potentials of EEG are obtainable from ECoG by means of a comparatively simple algebraic procedure.
A model of a polylayer spheric cable for estimating EEG potentials was discussed earlier [1]. As far as the holes in the natural scull were ignored in that model, the contrary assumption is useful: the scull is regarded as an infinite plain. Among the solution of the equations of both models there exist uncomplicated relations.
In order to solve the problem of electrostimulation with the help of scalp electrodes the simplified general theory was developed. The analysis was carried out in a framework of multilayer sphere-cable. The formuli of potential on the surface of scalp and in the volume of brains were worked out. The theory was concretized for traditional bipolar and new differential ways of fixing of the stimulating electrodes. The goal is to use the obtained theory for numerical estimation of electrical parameters of brain envelopments and for calculation of the values of electrical field in the volume of brains.
The methods of estimating a momentary value of the epidural potential using several values of EEG, sampled at the same instant, are explored theoretically. The weights of the summation of these EEG-potentials for the case of simple disposition of electrodes, are single-valued by means of elementary consideration of the potential theory and symmetry. The space filters of EEG obtained in such a way, were analysed on the basis of the model of thin integuments of the brains. The systems of 4--8 electrodes on the scalp are proposed for the estimation of mono- and bipolar ECoG's, which are less sensitive to ECoG of the remote cortex than ordinary derivations of EEG.
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The proposed method is based on imitation of the integuments of man's brain by unreally thick, approximately 4 cm, layer of electrolyte. That enables us to use simple solutions of a model of the homogenous isolated sphere, describing EEG of man, if the radius of the isolating shell is rg = 12 cm and dipoles are not higher than rd = 8 cm. This approach enables us to calculate the EEG-potentials of man by means of an ordinary calculator. For instance EEG of radical cortical dipole can be calculated by the formula (3) obtained in this paper.
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Estimates of EEG-potentials obtained in the model of polylayer spherical and flat cables indicate that the geometry of the skull has only little influence. Potentials of EEG of the radial dipole and the tangential double layer are expressed by simple formules in these models. A theoretical explanation concerning great dispersion of ECoG to EEG ratio and identity of EEG-potentials, registered by two electrodes spaced within about 2 cm of one another, is presented.