HORIZONTAL OR VERTICAL OPTOKINETIC STIMULATION ACTIVATES VISUAL MOTION-SENSITIVE, OCULAR MOTOR AND VESTIBULAR CORTEX AREAS WITH RIGHT HEMISPHERIC DOMINANCE - AN FMRI STUDY

The differential effects of optokinetic stimulation with and without f ixation suppression were analysed in an fMRI study in 10 right-handed healthy subjects. Horizontal and vertical small-field optokinetic stim ulation activated the same multiple visual, ocular motor and vestibula r cortical and subcortical areas in both hemispheres. The extent of ac tivation in each hemisphere was independent of the stimulus direction. All activated areas representing cortical (occipitotemporal cortex, p osterior parietal cortex, precentral and posterior median frontal gyru s, prefrontal cortex, medial part of the superior frontal gyrus) and s ubcortical (caudate nucleus, putamen, globus pallidus and paramedian t halamus) ocular motor structures were activated during optokinetic sti mulation as well as during fixation suppression of optokinetic nystagm us. However, the activation was significantly stronger with optokinetc nystagmus compared with fixation suppression. The only relatively inc reased activity during fixation suppression was seen in the medial par t of the superior frontal gyrus (supplementary eye field) and the ante rior cingulate gyrus. The anterior insula and the posterior insula (hu man homologue of the parieto-insular vestibular cortex) were activated during optokinetic nystagmus but not during fixation suppression. A s ignificant right hemispheric predominance (regardless of stimulus dire ction) was found under both conditions in the visual motion-sensitive and ocular motor areas of the cortex, except the supplementary eye fie ld and anterior cingulate gyrus. This was most prominent in the occipi totemporal cortex, but did not occur in the primary visual cortex and in subcortical ocular motor structures (putamen, globus pallidus and c audate nucleus), Thus, cortical and subcortical activation patterns di d not differ for horizontal and vertical optokinetic stimulation, and there was distinct right-hemisphere dominance for visual motion-sensit ive and cortical ocular motor areas and the thalamus. Fixation suppres sion of optokinetic nystagmus yielded four different results: (i) incr eased activation in the supplementary eye field and anterior cingulate gyrus; (ii) unchanged activation in the visual cortex; (iii) decrease d activation in most of the ocular motor areas; and (iv) suppressed ac tivation in the anterior and posterior insula and the thalamus. Activa tion of the parieto-insular vestibular cortex may be related to ocular motor function rather than self-motion perception.