Visual illusions and travelling alpha waves produced by flicker at alpha frequency.

The aim of the study was to obtain some experimental evidence of the 'scanning hypothesis' that links electroencephalogram (EEG) alpha-activity with rhythmically spreading waves in the visual cortex. The hypothesis was tested in experiments with 29 healthy adults. Under flicker stimulation through closed lids with the frequency of the individual alpha-rhythm, all subjects perceived illusory visual objects (a ring or a circle, a spiral or a spiral spring, or a grid). Most frequently noted was the perception of a ring or a circle; less frequently, a three-dimensional spiral; and even less frequently, a curved grid. It was found that the optimal stimulation frequency for this effect was tightly connected with the dominant alpha-rhythm frequency, with a correlation coefficient of 0.86. The probability of observing the ring and spiral illusion was highest at this frequency, while that for the grid illusion occurred at frequencies that differed by +/- 1-2 Hz. We observed 10 typical trajectories of travelling EEG alpha-waves on the scalp, and a significant interrelation between the occipital-frontal trajectory and illusions of the ring and spiral. The link between these effects and the propagation of the wave process through the visual cortex, as reflected by the EEG alpha-rhythm, is discussed. The data support the hypothesis of (Pitts, W. McCulloch, W.S., 1947), which proposes the scanning of the visual cortex by a spreading wave process operating at the frequency of the alpha-rhythm, which reads information from the visual cortex.