TMS over posterior parietal cortex disrupts the integration of initial hand position information into the reach plan

Abstract

We previously reported that induction of a focal current using single-pulse transcranial magnetic stimulation (TMS) reveals a hemispheric asymmetry in the early stages of the putative spatial processing in the human dorsal posterior parietal cortex (PPC). Moreover, we postulated that this brief TMS pulse modifies the output of the right PPC in motor coordinates downstream from an adapted visual-motor reversal, rather than modifying the upstream visual coordinates of the memory representation (Vesia et al., J. Neurophysiol, 2006). Alternately, TMS could have altered the perception of the initial hand position. To test this hypothesis, we investigated the memory-guided pointing accuracy in six subjects during TMS of the left and right PPC while varying visual information of the effector. We tested three conditions: 1) initial and final vision of hand positions (IFV); 2) full vision of hand position (FULV) and; final vision of hand position (FV). In accordance with previous findings, subject's pointing errors/biases varied as a function of stimulation site - left parietal stimulation significantly increased endpoint variability, while right parietal stimulation produced a significantly systematic leftward directional shift in both visual fields during the FV condition. In addition, these systematic pointing error/biases significantly decreased during both the IFV and FULV conditions. These results suggest that the PPC integrates the target and effector (e.g. the hand) position in a gaze-centered representation of space to compute the movement vector at this early stage in the visuomotor transformation.