There is a periodic fluctuation in amplitude of the alpha wave known as the “waxing and waning” phenomena. This periodicity was studied in human adults by means of the simplified method of frequency band analysis on the amplitude envelope of the alpha wave.
1. At least four types of fluctuations were found. Their periodic times ranged from 30 to 60 sec in type I, from 10 to 20 sec in type II, from 5 to 7 sec in type III and about 1 sec in type IV.
2. In each type of fluctuation, the phase was the same in all regions of the cortex.
3. Delivery of a steady light stimulation produced effects in only type I fluctuation.
4. The frequency of the alpha wave was also found to fluctuate at periodicities corresponding to type I and II amplitude fluctuations.
5. The amplitude of the flicker-evoked response also was noted to show periodic fluctuations. Four kinds of periodic time were found and they were almost the same as those in the spontaneous alpha wave.
6. The fluctuation of type IV had no correlation with the heart rate.
The possibility was discussed that these periodic fluctuations are dependent upon cortical excitability changes and that therefore the periodic phenomena in perception are based upon the neurophysiological mechanism.