Relationship between Delta, Sigma, Beta, and Gamma EEG bands at REM sleep onset and REM sleep end

Objective: The aim of the present study was to analyze in detail the relati onship of two newly introduced measures, related to the Beta and Gamma EEG bands during REM sleep, with Delta and Sigma activity at REM sleep onset an d REM sleep end, in order to understand their eventual role in the sleep mo dulation mechanism. Methods: For this purpose, power spectra of 1 EEG channel (C4, referred to Al) were obtained by means of the fast Fourier transform and the power of t he bands ranging 0.75-4.50 Hz (Delta), 4.75-7.75 (Theta), 8.00-12.25 (Alpha ), 12.50-15.00 (Sigma), 15.25-24.75 (Beta), 25.00-34.75 (Gamma 1), and 35.0 0-44.75 (Gamma 2) was calculated for the whole period of analysis (7 h). in 10 healthy subjects. Additionally, two other time series were calculated: the ratio between Beta and Gamma2, and between Gamma1 and Gamma2 (Beta and Gamma ratios). For each subject, we extracted 3 epochs of 30 min correspond ing to the 15 min preceding and the 15 min following the onset of the first 3 REM episodes. Data were then averaged in order to obtain group mean valu es and standard deviation. The same process was applied to the 30-min epoch s around REM sleep end. Results: The course of the Delta band around REM sleep onset was found to b e characterized by a first phase of slow decline lasting from the beginning of our window up to a few seconds before REM onset; this phase was followe d by a sudden, short decrease centered around REM onset. lasting for approx imately 1.5-2 min. At the end of this phase, the Delta band reached its low est values and remained stable up to the end of the time window. The Sigma band showed a similar course with stable values before and after REM sleep onset. The Beta and Gamma ratios also showed a 3-phase course; the first ph ase, in this case, was characterized by stable low values, from the beginni ng of our window up to approximately 5 min before REM onset. The following second phase was characterized by an increase which reached its maximum sho rtly after REM sleep onset (approximately I min). In the last phase, both B eta and Gamma ratios showed stable high values, up to the end of our time w indow. At REM sleep end, the Delta band only showed a very small gradual in crease, the Sigma band presented a more evident gradual increase. on the co ntrary, both Beta and Gamma ratios showed a small gradual decrease. Conclusions: The results of the present study show a different time synchro nization of the changes in the Delta band and in Beta and Gamma ratios. at around REM sleep onset, and seem to suggest that the oscillations of these parameters might be modulated by mechanisms more complex than a simple reci procity. All these considerations point to the fact that REM sleep can be c onsidered as a complex phenomenon and the analysis of high-frequency EEG ba nds and of our Beta and Gamma ratios represent an additional important elem ent to include in the study of this sleep stage. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.