CORTICAL CONTROL OF DOUBLE-STEP SACCADES - IMPLICATIONS FOR SPATIAL ORIENTATION

To accurately localize a visual target in space despite eye movement-induced shifts of its retinal image, the brain must take into account both its retinal location and information about current eye position or at least the preceding eye displacement. We examined this ability with respect to saccadic eye movements by applying ''double-step'' stimuli, where the locations of two sequentially flashed target lights have to be fixated by two successive saccades performed after their disappearance. As the 2nd saccade will not start at the spatial location from which the 2nd target was seen, a dissonance arises between its retinal coordinates and the motor coordinates of the required 2nd saccade. Nevertheless, these saccades were performed quite accurately by 32 healthy human adults. To investigate the contribution of the cerebral cortex, we recorded horizontal double-step saccades in 35 patients with focal unilateral hemispheric lesions. Whereas frontal lesions impaired temporal properties, posterior parietal lesions caused spatial dysmetria or failure of even ipsiversive 2nd saccades following contraversive 1st saccades. This reflects an inability to compensate for retino-spatial dissonance by using nonretinal information (corollary discharge) about eye displacement associated with a previous saccade into the contralesional hemifield. In conclusion, the parietal cortex is crucial for spatial constancy across saccades.