Asymmetric Hemispheric Activations during 10° Lateral Gaze (fMRI Study)

Functional MRI was used to study the differential effects of the direction of gaze on the visual and the ocular motor systems. Fixation of a target straight ahead was compared to fixation of a target 10° to the right and 10° to the left, and to eyes open in complete darkness in thirteen healthy volunteers. BOLD signal increases in fronto-parietal ocular motor and attentional structures were absent during central fixation eyes open and minimal during lateral fixation eyes open, which confirmed the earlier finding that these structures are already activated with open eyes in darkness. During lateral fixation, activations in early visual areas (calcarine sulcus) and deactivations in higher order visual areas (one ventral cluster in the lingual and fusiform gyri and one dorsal cluster in the postero-superior cuneus) were found predominantly in the hemisphere contralateral to the fixation target. Thus, the hypothesis is proposed that even during small gaze shifts into one visual hemifield, visual processing is performed predominantly in the hemisphere contralateral to the direction of gaze in head coordinates. This held true although the visual input, i.e., fixation of a single target, was the same in all fixation conditions. A further finding was an activation of the motion-sensitive area MT/V5 despite fixation of a stationary target, which is compatible with the perception of apparent target motion (autokinetic effect).