The dynamics of phase relationships of alpha waves during photic driving.

Inter- and intrahemispheric phase relationships of alpha waves during photic driving with intermittent flash stimuli were successively calculated by our newly developed method and compared with the results obtained during the resting condition. Of 12 healthy subjects, 6 showed obvious photic driving responses in the occipital regions (driven group) but the remaining 6 did not (non-driven group). In the driven group, the interhemispheric phase differences decreased, especially when EEG frequencies coincided with stimulating frequencies. However, this was not maintained throughout the stimulation period but was accompanied by some fluctuation. The interhemispheric phase difference decreased in the posterior temporal and, in particular, in the occipital areas. The intrahemispheric phase difference for the occipito-frontal and the occipito-central pairs in each hemisphere tended to increase and remained at approximately pi radians, especially for the former pair. These findings suggest the presence of a specific response in the occipital area. Time ratios of the phase lead in longitudinal and transverse directions and the high similarity of the time courses of intrahemispheric phase differences between bilaterally homologous pairs at rest suggest that there exist phasing mechanisms which act in parallel between the left and right hemispheres with dominant antero-posterior time delay.