Abstract
Single-unit recordings in monkeys have identified effector-specific regions in posterior parietal cortex (PPC), but functional neuroimaging in the human have yielded controversial results. Here we used on-line repetitive transcranial magnetic stimulation (rTMS) to determine saccade and reach specificity in human PPC. A short train of three TMS pulses (10 Hz, 300 ms) was delivered to superior parieto-occipital cortex (SPOC), a region over the mid-posterior intraparietal sulcus (mIPS), and a site posterior to caudal IPS situated over the angular gyrus (AG) during a brief memory interval while subjects planned either a saccade or reach with the left or right hand. Behavioural measures then were compared to controls without rTMS. Stimulation of mIPS and AG produced similar patterns: increased end point variability for reaches and decreased saccade accuracy for contralateral targets. In contrast, stimulation of SPOC deviated reach end points towards visual fixation, and had no effect on saccades. Contralateral-limb specificity was highest for AG and lowest for SPOC. Visual feedback of the hand negated rTMS-induced disruptions of the reach plan for mIPS and AG, but not SPOC. These results suggest that human SPOC is specialized for encoding retinally peripheral reach goals, whereas more anterior-lateral regions (mIPS and AG) along the IPS possess overlapping maps for saccade and reach planning, and are more closely involved in motor details (i.e., planning the reach vector for a specific hand).Acknowledgments: CIHR, OGS, Canada Research Chair