Right posterior parietal lesions impair spatial localization across saccadic eye movements

Objective: To investigate the influence of right posterior parietal lesions on transsaccadic spatial localization psychophysically.

Aims: Saccade studies in non-human primates and in human patients have indicated a critical role of the posterior parietal cortex around the intraparietal sulcus (IPS) for maintaining spatial constancy across saccadic eye movements whenever the use of non-visual information (efference copy) is needed. The aim of the present study was to find a perceptual correlate of this.

Methods: Postsaccadic spatial localization was investigated in 11 patients with post-ischemic right posterior parietal lesions around the IPS, compared to 12 age-matched healthy adults. During a simple visually-guided saccade of 6° or 8° to the right or left, the target was blanked for 200 ms and displaced by 2°, 1° or 0.3° to the right or left. Saccades were performed either in the right or in the left craniotopic hemi-field. Eye movements were recorded using infrared reflection oculography. Subjects had to indicate the direction of target displacement in a 2-alternative-forced-choice task.

Results: Whereas the control subjects were able to localize target displacement correctly whenever the step of displacement was 1° or 2°, parietal patients performed around chance level for all 3 step sizes. This deficit was independent of the craniotopic hemi-field and independent of saccade direction, although contralesional saccades had increased latencies.

Conclusions: We conclude that lesions around right IPS impair transsaccadic spatial localization not only with respect saccadic accuracy, but also to perception, independent of saccade direction or craniotopic hemi-fields. As the target was blanked at the end of the saccade, the deficit must be based on a deficient use of efference copy for maintaining spatial constancy.