ASSOCIATION OF VISUAL ILLUSIONS WITH FREQ UENCY AND PHASE-SHIFT OF RHYTHMICAL PHOTOSTIMULATION SYNCHRONIZED WITH EEG ALPHA-WAVE

Under flicker stimulation through the closed eyes, syhchronized with d ifferent phases of the EEG alpha wave, all 18 subjects systematically perceived the illusory visual objects (ring, circle. spiral, or curvil inear grid). Most frequently, they saw a ring or a circle, a little le ss frequently, a three-dimensional spiral, and at last - a curvilinear grid, The incidence of the ring and spiral illusion was the highest w hen the stimulation frequency strictly coincided with the individual d ominant frequency of the alpha-rhythm. On the contrary, the grid illus ion appeared more frequently when the stimulation frequency differed f rom that of the alpha-rhythm by 1-2 Hz. No significant effect was foun d of the alpha wave phase which triggered the photo-stimulation, on th e probability of the illusion perception, The revealed phenomena are p robably explained by summation of the spreading alpha wave (the ''scan ning'' hypothesis) with the isothythmical afferent excitation evoked b y flashes due to thr quasi-stroboscopic effect.