Interhemispheric asymmetry of reactions to spatial modulations of local visual indications

Visual perception starts with a parallel spatial-frequency filtering. A visual scene is presented by a number of local indications in outputs of first-order filters. Their spatial association is the next important operation. It is the grouping of local indicators that underlies the transition to spatial vision. Recent research indicates that second-order visual filters perform this grouping. It is believed that the right hemisphere plays the leading role in spatial vision. The present study puts forward a hypothesis that this dominance can be formed even at the stage of the transition from a local to a global description of visual scenes. For this purpose the authors investigated interhemispheric asymmetry of potentials caused by functioning of second-order visual filters. These elements integrate the outputs of first-order filters and respond to spatial modulations of local visual indications. To solve this problem the authors recorded visual evoked potentials to a non-modulated texture and textures sinusoidally modulated in orientation, spatial frequency, and contrast. Next, they subtracted the response to the non-modulated texture from the response to the modulated texture. In result, each derivation received three different waves (d-waves): to the modulation of contrast, orientation, or spatial frequency. The comparison of d-waves in symmetric derivations revealed that its amplitude for all the used modulations is higher in the right hemisphere. Interhemispheric asymmetry to the modulation of orientation was most pronounced; it manifested itself better in occipital regions. The findings of the study showed the leading role of the right hemisphere in the processes of spatial association of local visual indications.