Role of the posterior parietal cortex in updating reaching movements to a visual target

The exact role of posterior parietal cortex (PPC) in visually directed reac hing is unknown. We propose that, by building an internal representation of instantaneous hand location, PPC computes a dynamic motor error used by mo tor centers to correct the ongoing trajectory. With unseen right hands, fiv e subjects pointed to visual targets that either remained stationary or mov ed during saccadic eye movements. Transcranial magnetic stimulation (TMS) w as applied over the left PPC during target presentation. Stimulation disrup ted path corrections that normally occur in response to target jumps, but h ad no effect on those directed at stationary targets. Furthermore, left-han d movement corrections were not blocked, ruling out visual or oculomotor ef fects of stimulation.