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).
