Rhythmic photostimulation and the number of alpha-rhythm dipoles in the human brain.

The numbers of alpha-rhythm equivalent current dipoles (ECD) arising in the human brain before and during rhythmic photostimulation at the alpha-rhythm frequency was studied in six healthy adult subjects. Dipoles were calculated using a single-dipole model for the whole of the alpha-range and three subranges by solution of inverse equations in a three-layer model of the head obtained by simultaneous use of EEG data and MRI tomograms of the subjects' heads. The number of apparent ECD was significantly associated with rhythmic photostimulation and depended on the phase of the alpha-rhythm wave at which stimulation started and on the type of visual illusion (circle, spiral, grid) appearing during this time. The relationship between these data and the hypothetical wave process scanning the human visual cortex at the frequency of the alpha-rhythm is discussed.