Half-field sinusoidally modulated light stimulation at subject's alpha frequency.

Interhemispheric responses of alpha activity were investigated by use of half-field sinusoidally modulated light (SML) stimulation at the subject's alpha frequency. The left and right visual half-fields as well as full-field were separately stimulated by the SML. The power spectrum of SML responses, averaged with Wiener filtering, was obtained at the left and the right occipital area (O1 and O2) in 11 normal subjects. Power in each hemisphere, phase difference and coherence between O1 and O2 were estimated at an alpha frequency. A laterality index of power was defined as (P(l)-P(r))/(P(l) + P(r)) where P(l) is a power value at O1, and P(r) at O2. In most of the subjects, laterality index decreased in the order: left half-field, full-field and right half-field stimulation. This relationship revealed greater SML responses on the ipsilateral occipital area. There were significant differences in laterality index among 3 visual field conditions, thus showing that full-field responses ranged in lateralization between the left and right half-field responses in most of the subjects. This hemispheric distribution suggests that half-field SML stimulation affected alpha activity selectively in each hemisphere, mostly in the ipsilateral hemisphere.