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 'scann ing hypothesis' that links electroencephalogram (EEG) alpha-activity with r hythmically spreading waves in the visual cortex. The hypothesis was tested in experiments with 29 healthy adults. Under flicker stimulation through c losed 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 freque ntly, a curved grid. It was found that the optimal stimulation frequency fo r this effect was tightly connected with the dominant alpha-rhythm frequenc y, 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 obse rved 10 typical trajectories of travelling EEG alpha-waves on the scalp, an d a significant interrelation between the occipital-frontal trajectory and illusions of the ring and spiral. The link between these effects and the pr opagation of the wave process through the visual cortex, as reflected by th e EEG alpha-rhythm, is discussed. The data support the hypothesis of (Pitts , W. McCulloch, W.S., 1947), which proposes the scanning of the visual cort ex by a spreading wave process operating at the frequency of the alpha-rhyt hm, which reads information from the visual cortex. (C) 2000 Elsevier Scien ce B.V. All rights reserved.