Association of visual illusions with frequency and phase shift of rhythmical photostimulation synchronized with EEG alpha wave

Under flicker stimulation through the closed eyes, syhchronized with different phases of the EEG alpha wave, all 18 subjects systematically perceived the illusory visual objects (ring, circle. spiral, or curvilinear grid). Most frequently, they saw a ring or a circle, a little less frequently, a three-dimensional spiral, and at last - a curvilinear grid, The incidence of the ring and spiral illusion was the highest when the stimulation frequency strictly coincided with the individual dominant frequency of the alpha-rhythm. On the contrary, the grid illusion appeared more frequently when the stimulation frequency differed from that of the alpha-rhythm by 1-2 Hz. No significant effect was found of the alpha wave phase which triggered the photo-stimulation, on the probability of the illusion perception, The revealed phenomena are probably explained by summation of the spreading alpha wave (the ''scanning'' hypothesis) with the isothythmical afferent excitation evoked by flashes due to thr quasi-stroboscopic effect.