The Study of Rhythmic Component Coupling at the First Stage of Day Sleep

Abstract

Coupling of EEG rhythms is an important indicator of the functional state of the human brain. There currently exist three theories explaining this coupling: (1) communication of neuronal populations, (2) neuronal coupling, and (3) coupling between generators of the frequencies being studied. It is known that the theta rhythm is associated with the functioning of cortico-hippocampal and alpha-rhythm-thalamo-cortical systems, and the beta rhythm can be included in the activity of both cortico-subcortical systems. The present work may clarify the features of the above-mentioned cortico-subcortical systems. There is a number of publications devoted to the study of EEG rhythm coupling in various types of psychical activity. At the same time, a concern for coupling of the rhythms at different sleep stages appeared in recent years. The task of our work included the study of coupling between theta, alpha, and beta EEG rhythms at the first stage of sleep. The study involved 22 subjects from 18 to 22 years old. Multichannel EEG was recorded during daytime sleep of the experiment participants. EEG segments with well-expressed theta rhythm were selected for the processing since it is “dominant” at the first stage of sleep. Bandpass filtering of the EEG signal was then performed. The following rhythms were discriminated: theta rhythm (4–7 Hz), alpha rhythm (8–13 Hz), beta-1 (14–19 Hz), and beta-2 (20–25 Hz) rhythms. Afterwards, for each range at each second, the average amplitude was calculated as the square root of the EEG signal dispersion. The Pearson correlation coefficient was used as a measure to evaluate coupling of EEG rhythms. As a result, it was established that the first stage of sleep is characterized by: (1) a lack of connections between the theta rhythm and other rhythms, (2) the presence of alpha–beta-1, alpha–beta-2, and beta-1–beta-2 links, (3) the increase in theta amplitude, and (4) the decrease in the amplitudes of alpha and beta rhythms. As was noted above, the theta rhythm is associated with functioning of the cortico-hippocampal system, and the alpha rhythm is associated with the thalamo-cortical system. In our work, two coexisting types of functioning of these systems are shown: (1) the “independent” one of the cortico-hippocampal circuit and (2) the thalamo-cortical one, connected with other rhythms, particularly with the beta rhythm. This heterogeneity is probably a condition for the first stage of sleep to be potentially unstable. An increase in theta rhythm amplitude at the first stage of sleep against the state of quiet wakefulness is shown. This is traditionally associated with the increase in ascending effects of limbic structures of the brain. Amplitudes of alpha and beta rhythms at the first stage of sleep significantly decreased, which indicates an attenuation of the influence of prefrontal cortical regions on posterior hypothalamus centers. Hence, it can be assumed that the onset of the first stage of sleep can be provided by the heterogenous character of rhythm coupling, and, correspondingly, different functioning of cortico-hippocampal and thalamo-cortical systems.