[Ultra-slow changes in the rhythmic activity within the alpha band and their probable origin (author's transl)].

The time variation of spectral parameters of EEG data, from occipital and central regions, were analysed in normal subjects. Three-second segments, overlapping at 1-s intervals, were studied as to their peakfrequency, bandwidth and power within the alpha band using an autoregressive model of 7th order. In addition, the EKG and respiration were also analysed. Beat-to-beat intervals from the EKG were measured to quantify underlying slow rhythms. Individual EEG parameters were found to change synchronously with respiration and also with the 6/min rhythms underlying the heart rate variation. These changes occurred most frequently only for a short time and they seldom appeared in several cortical regions simultaneously. In addition, the EEG parameters also changed synchronously with the repetition of a specific task (e.g., CNV paradigm) showing dependence from stimulus modality and from task complexity. Basically a decrease in power within the alpha band, sometimes together with a small increase or decrease of peak frequency, was shown to occur predominantly in that cortical area involved with processing of task-relevant information. Ultra-slow changes of the EEG parameters can be interpreted as changes in cortical excitability and responsiveness.