The study of coupling rhythmic components of the EEG during the first stage of day sleep

The coupling of EEG rhythms is an important indicator of the functional state of the human brain. Currently, there are three theories explaining such interaction: a) communication of neural populations; b) neural interaction; c) interaction of generators of the studied frequencies. It is known that theta-rhythm is associated with the functioning of the cortico-hippocampal system, and alpha-thalamo-cortical, and beta-can be included in the work of both cortico— subcortical systems. The presented work can clarify the features of the interaction of the above-described cortical-subcortical systems. There are a number of works devoted to the study of the coupling of EEG rhythms in various types of mental activity. In recent years, there has been interest in the connection of rhythms at different stages of sleep. The task of our work was to study the interaction of theta-, alpha- and beta-rhythms of EEG in the first stage of sleep. The study involved 22 subjects aged 18 to 22 years. Multichannel EEG was recorded during the daytime sleep of the experiment participants. EEG segments with a well-defined theta-rhythm were selected for processing, since it is «dominant» at the first stage of sleep. Then the range filtering of the EEG signal was carried out. The following rhythms were distinguished: theta-rhythm (4—7 Hz), alpha-rhythm (8—13 Hz), beta-1- (14—19 Hz) and beta-2-rhythm (20—25 Hz). After that, for each range at each second, the average amplitude was calculated as the square root of the EEG signal dispersion. Pearson correlation coefficient was used as a measure to evaluate the interaction of EEG rhythms. As a result, it was found that the first stage of sleep was characterized by: a) lack of connections of theta-rhythm with other rhythms; b) the presence of links alpha—beta-1-, alpha—beta-2- and beta-1—beta-2-rhythms, c) the increase of the amplitude of the theta-rhythm and d) decrease the amplitude of alpha- and beta-rhythms. As noted above, theta-rhythm is associated with the functioning of the cortico-hippocampal system, and alpha — thalamo-cortical. In our work two coexisting types of functioning of these systems are shown: «independent» cortical-hippocampal circle and connected with other rhythms, in particular with beta-rhythm, thalamo-cortical. Probably, this heterogeneity is a condition that the first stage of sleep can be unstable. An increase in the amplitude of the theta-rhythm at the first stage of sleep in relation to the state of quiet wakefulness is shown. This is traditionally associated with an increase in the ascending effects of the limbic structures of the brain. The amplitudes of alpha- and beta-rhythms at first stage of sleep were significantly reduced, which indicates a weakening of the influence of prefrontal cortex on the centers of the posterior hypothalamus. Based on this, it can be assumed that the onset of first stage of sleep can be provided by the heterogeneous nature of the coupling of rhythms, and accordingly, the different functioning of the cortico-hippocampal and thalamocortical systems.

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