Temporal dissociation between hand shaping and grip force scaling in the anterior intraparietal area

In humans, both clinical and functional imaging studies have evidenced the critical role played by the posterior parietal cortex, and particularly by the anterior intraparietal area (AIP), in skilled hand movements. However, the exact contribution of AIP to precision grasping remains debated. Here we used transcranial magnetic stimulation (TMS) to induce virtual lesions of the left and/or right AIP in subjects performing a grip-lift task with either hand. We found that, during movement preparation, a virtual lesion of AIP had distinct consequences on precision grasping of either hand depending on its time of occurrence: TMS applied 270-220 ms before the fingers contacted the manipulandum altered specifically the hand shaping, whereas lesions induced 170-120 ms before contact time only affected the grip force scaling. The lateralization of these two processes in AIP is also strikingly different: whereas a bilateral lesion of AIP was necessary to impair hand shaping, only a unilateral lesion of the left AIP altered the grip force scaling in either hand. The present study shows that, during movement preparation, AIP is responsible for processing two distinct, temporally dissociated, precision grasping parameters, regardless of the hand in use. This indicates that the contribution of AIP to hand movements is "effector-independent," a finding that may explain the invariance of grasping movements performed with either hand.