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).