Computer methods for analyzing electroencephalograms

The main methods of computer analysis of EEG used in the clinic include spectral analysis using the fast Fourier transform algorithm, instantaneous amplitude mapping, spikes, and determination of the three-dimensional localization of the equivalent dipole in brain space.

Spectral analysis is most commonly used. This method allows one to determine the absolute power expressed in μV ² for each frequency. The power spectrum diagram for a given epoch is a two-dimensional image, in which the EEG frequencies are plotted along the abscissa axis, and the powers at the corresponding frequencies are plotted along the ordinate axis. The spectral power data of the EEG presented as successive spectra give a pseudo-three-dimensional graph, where the direction along the imaginary axis into the depth of the figure represents the time dynamics of changes in the EEG. Such images are convenient for tracking EEG changes in cases of consciousness disorders or the impact of certain factors over time.

By color coding the distribution of powers or average amplitudes over the main ranges on a conventional image of the head or brain, a visual representation of their topical representation is obtained. It should be emphasized that the mapping method does not provide new information, but only presents it in a different, more visual form.

The definition of three-dimensional localization of the equivalent dipole is that, using mathematical modeling, the location of a virtual source of potential is depicted, which could presumably create a distribution of electric fields on the surface of the brain corresponding to the observed one, if we assume that they are not generated by neurons of the cortex throughout the brain, but are the result of passive propagation of the electric field from individual sources. In some particular cases, these calculated "equivalent sources" coincide with real ones, which allows, under certain physical and clinical conditions, to use this method to clarify the localization of epileptogenic foci in epilepsy.

It should be borne in mind that computer EEG maps display the distribution of electric fields on abstracted models of the head and therefore cannot be perceived as direct images, like MRI. Their intelligent interpretation by an EEG specialist in the context of the clinical picture and the data of the analysis of the "raw" EEG is necessary. Therefore, the computer topographic maps sometimes attached to the EEG report are completely useless for the neurologist, and sometimes even dangerous in his own attempts to directly interpret them. According to the recommendations of the International Federation of EEG and Clinical Neurophysiology Societies, all the necessary diagnostic information obtained mainly on the basis of direct analysis of the "raw" EEG should be presented by the EEG specialist in a language understandable to the clinician in a text report. It is unacceptable to provide texts that are automatically formulated by computer programs of some electroencephalographs as a clinical electroencephalographic report.

To obtain not only illustrative material, but also additional specific diagnostic or prognostic information, it is necessary to use more complex algorithms for research and computer processing of EEG, statistical methods for evaluating data with a set of corresponding control groups, developed to solve highly specialized problems, the presentation of which goes beyond the standard use of EEG in a neurological clinic.

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General patterns

The tasks of EEG in neurological practice are the following:

1. confirmation of brain damage,
2. determination of the nature and localization of pathological changes,
3. assessment of the dynamics of the state.

Obvious pathological activity on the EEG is reliable evidence of pathological brain functioning. Pathological fluctuations are associated with the current pathological process. In residual disorders, changes in the EEG may be absent, despite significant clinical deficit. One of the main aspects of the diagnostic use of EEG is to determine the localization of the pathological process.

• Diffuse brain damage caused by an inflammatory disease, circulatory, metabolic, toxic disorders, leads to diffuse changes in the EEG. They are manifested by polyrhythmia, disorganization and diffuse pathological activity. Polyrhythmia is the absence of a regular dominant rhythm and the prevalence of polymorphic activity. Disorganization of the EEG is the disappearance of the characteristic gradient of the amplitudes of normal rhythms, a violation of symmetry. Diffuse pathological activity is represented by delta, theta, epileptiform activity. The picture of polyrhythmia is due to a random combination of different types of normal and pathological activity. The main sign of diffuse changes, in contrast to focal ones, is the absence of constant locality and stable asymmetry of activity in the EEG.
• Damage or dysfunction of the midline structures of the cerebrum involving non-specific ascending projections are manifested by bilaterally synchronous bursts of slow waves or epileptiform activity, with the probability of occurrence and severity of slow pathological bilaterally synchronous activity being greater the higher the lesion is located along the neural axis. Thus, even with severe damage to the bulbopontine structures, the EEG in most cases remains within normal limits. In some cases, desynchronization and, accordingly, low-amplitude EEG occur due to damage to the non-specific synchronizing reticular formation at this level. Since such EEGs are observed in 5-15% of healthy adults, they should be considered conditionally pathological. Only a small number of patients with damage at the lower brainstem level exhibit bursts of bilaterally synchronous high-amplitude alpha or slow waves. In case of damage at the mesencephalic and diencephalic level, as well as higher lying midline structures of the cerebrum: the cingulate gyrus, corpus callosum, orbital cortex, bilaterally synchronous high-amplitude delta and theta waves are observed on the EEG.
• In lateralized lesions in the depth of the hemisphere, due to the wide projection of deep structures onto vast areas of the brain, pathological delta and theta activity is observed, which is distributed accordingly across the hemisphere. Due to the direct influence of the medial pathological process on the midline structures and the involvement of symmetrical structures of the healthy hemisphere, bilaterally synchronous slow oscillations also appear, which are predominant in amplitude on the side of the lesion.
• The superficial location of the lesion causes a local change in electrical activity, limited to the zone of neurons immediately adjacent to the focus of destruction. The changes are manifested by slow activity, the severity of which depends on the severity of the lesion. Epileptic excitation is manifested by local epileptiform activity.