Die Periodik zirkadianer und diurnaler Rhythmen im EEG

Summary

As a result of the experimental experience made over the last two decades that major phenomena of sleep-EEG underly periodic modulation a series of experiments were conceived with the aim to analysing the awake-EEG with regard to its rhythmological components. It was found in experiments on animals that so called circadian rhythms (circa-almost, dies-a day; a cycle of 24 hrs ± 4 hrs) having a large periodicity and containing wake- and sleep phases were present. In EEG-experiments on humans a persisting circadian rhythm was to be observed even after long periods without sleep. Whether or not the background activity displayed a diurnal modulation (diurnal: according to the time of day; a cycle between sunrise and sunset or some similar scheme of lighting) could not be answered satisfactorily.

11 male students (average age 26 years) had EEG recordings of 7 minute duration made every two hours between the 8.00 and 20.00 hrs. The recordings were filtered and the power of the α- and ϑ- frequencies were integrated over one-second intervals and analysed statistically. As parameters were calculated the average power, the variance and the coefficient of variation, along with the spectral parameters (tab. 1 and 2). The α-frequency band was evaluated for the parietooccipital leads on both sides, while the fronto-temperobasal leads were evaluated for the ϑ-frequency band.

In Tables 1 and 2 can be seen the results of the variance analysis. Inter-individual variations were as expected highly pronounced, while the differences with regard to which side was considered were of less consequence for the ϑ-band more than the α-band. Diurnal peakes (fig. 1 and 2) could be seen in the daily progression at 14.oo hrs. for the variance and co-efficient of variation of the α-band in the occipital region.

For the temperobasal region the average ϑ-power displayed in the evenings a highly significant maximum and variance though this was somewhat less significant. The co-efficient of variation had its highest value at 16.00 hrs., while displaying significant variation in its values over the whole day (see fig. 2).

A pronounced diurnal modulation for both sides over the occipital region was seen to underly the rhythms represented by the spectra of the lower (0.01-0.05 Hz) and upper (0.05-0.5 Hz) spectral frequencies - this being most apparent in the α-band with a day-peak around 14.00 hrs. (fig. 3). The occipital α-activity displayed, as opposed to the temperobasal ϑ-nactivity, significant changes throughout the day in its frequency components having a periodicity ranging from 2-100 s.

From our circadian experiments with the wake-EEG (23) it could be seen that the time at which the daily peaks occurred was dependant on the cerebral topography, but that the various frequency bands had their daily peaks at approximately the same time for the same regions. The highest difference accuring was up to six hours between the frontal and temperobasai regions, the time-difference here between the occipital and temperobasai regions being 2 hours (fig. 4).

If one attempts from these results to draw conclusions about the time structure of the central nervous system then it could be said that both hemispheres are co-ordinated with one another, but that the various cerebral regions have markedly different rhythmological time structures. The question remains open as to whether the sequence of daily peaks for the individual cerebral regions always follows a particular pattern.

As cause for these rhythms axe discussed both biological and social Zeitgeber. A uniform theory can however for the present not be formed. The research into electro-encephelographic rhythms gives a deep insight into the time-structure of integral functions of the central nervous system and could in future play a major part towards the understanding of the importance of rhythmological functions with regard to human behaviour.

Zusammenfassung

Anhand der im Zwei-Stunden-Abstand über den Tag von 8.00 Uhr morgens bis 20.00 Uhr abends abgeleiteten EEG bei elf männlichen Studenten Durchschnittsalter 26 Jahre - untersuchten wir diurnale periodische Einflüsse auf die Grundaktivität. Über der okzipitalen Region wurde die Ausprägung der α-Aktivität, über der Temperobasalregion die Ausprägung der ϑ-Aktivität analysiert. Die diurnalen Gipfel lagen okzipital im α-Band um 14.00 Uhr und temperobasai im ϑ-Band um 16.00 Uhr. Die von den Spektren im Frequenzbereich von 0.01-0,5 Hz repräsentierten Rhythmen zeigten nur im α- Band diurnale Tagesgipfel, d.h., daß Anteile der Grundaktivität sowohl von Rhythmen sehr kurzer (Periodenlänge 2-100 Sekunden) wie auch sehr langer Periodenlänge (12 Stunden) moduliert wurden. Die zeitliche Lage der Tagesgipfel im Wach-EEG hing, auch aus der Sicht unserer zirkadianen Untersuchungen, von der Topographie der Ableitepunkte ab. Die größten zeitlichen Differenzen traten zwischen der Frontal- und Temperobasalregion auf; die okzipitalen und temperobasalen Regionen beiderseits unterschieden sich um zwei Stunden. Die Tagesgipfel über korrespondierenden Ableitepunkten beider Hemisphären traten synchron auf. Die Befunde stützen die Hypothese, nach der die beiden Hemisphären, also bezüglich ihrer Zeitstruktur, aus elektroenzephalographischer Sicht als gleichgeschaltet betrachtet werden können. Ungeklärt ist noch, ob die zeitliche Reihenfolge der Tagesgipfel über den verschiedenen Hirnregionen nach einem festgelegten Muster erfolgt. Als Ursache dieser Rhythmen werden biologische und soziale Zeitgeber diskutiert.

Key -Words:

EEG - Orcadian rhythms - Diurnal rhythms - α-power - ϑ-power