Cortical control of double‐step saccades: Implications for spatial orientation

Abstract

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 retinospatial 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.

Intact spatial updating during locomotion after right posterior parietal lesions

“…This indicates that the right PPC is not critically involved in updating self-location along straight paths. We found this surprising, considering the aggregation of evidence for impaired eye and arm position updating after PPC lesions [15,16,29,65]. Before abandoning the notion that the PPC is a critical structure for updating during locomotion, however, other types of locomotion and more complex trajectories should be tested.…”

“…Thus, walking was always performed in the same direction, but during the preview of the workspace before walking, the path lay either on the subjects' left or right. We did this because we thought that, as is the case in updating eye movements [16,29], parietal patients may update more poorly in regions of space that lay on their contralesional side during the visual preview. During the stimulus phase, all subjects grasped the experiment's upper arm as he walked along a straight path between 2 and 6 m long.…”

“…In particular, it has been found to play a crucial role in updating extra-retinal signals related to eye position [16,29]. This evidence comes from a double-step saccade paradigm, in which two visual targets are¯ashed in sequence in an otherwise dark room [26].…”

“…When patients with unilateral PPC damage attempt to perform this task, their saccades to the second target are impaired when the ®rst saccade causes their eye to point into the contralesional half of visual space [16,29]. Thus, PPC lesions are associated with de®cient eye position updating.…”

“…The possible lateralization of this updating function has not been studied in detail, but there is some evidence that de®cits in updating eye position may be more pronounced after right PPC damage [29]; the right hemisphere dominates in other forms of spatial processing as well (for a review, see [14]). As a ®rst step toward addressing the possible laterality of self-location updating, we tested the hypothesis that the right PPC plays a crucial role in this function.…”