TRANSCRANIAL MAGNETIC STIMULATION OF THE POSTERIOR PARIETAL CORTEX DEGRADES ACCURACY OF MEMORY-GUIDED SACCADES IN HUMANS

Purpose. Previous neurophysiological studies in the monkey have shown that the posterior parietal cortex (PPC) is involved in the control of memory-guided saccades. In this study, the authors investigated the effect of focal transcranial magnetic stimulation (TMS) over the posterior parietal cortex on accuracy of memory-guided saccades in humans to clarify the cortical region related to controlling memory-guided saccades. Methods. Single TMS pulses were applied systematically at various locations over the parietal cortex during memory-guided saccades in human subjects. Each subject was instructed to fixate on a central target until the target was extinguished. A lateral target was flashed for 100 msec with unpredictable direction and amplitude. The subject executed a saccade to the remembered position of the previously flashed target in total darkness when the central target was extinguished. Transcranial magnetic stimulation was applied after the extinction of the central target with various latencies. Stimulation sites were identified by three-dimensional magnetic resonance imaging (MRI). Results. Transcranial magnetic stimulation of the right PPC degraded accuracy of rightward and leftward memory-guided saccades in all subjects, when TMS was applied 100 msec after the offset of the central target. On the other hand, TMS of the left PPC showed no effects on accuracy of memory-guided saccades. Systematic error of saccade amplitude appeared as a hypometria for rightward saccades. Three-dimensional MRI revealed that the effective stimulation site was located over the posterior portion of the intraparietal sulcus and the superior part of the angular gyrus of the right hemisphere. Conclusions. These findings strongly suggest that the right PPC is involved in maintaining spatial accuracy of remembered target locations of memory-guided saccades in humans.