Inhibition of the anterior intraparietal area and the dorsal premotor cortex interfere with arbitrary visuo-motor mapping.

Objective: The contribution of the human anterior intraparietal area and the dorsal premotor cortex to arbitrary visuo-motor mapping during grasping were tested.
Methods: Trained right-handed subjects reached for and pincer-grasped a cube with the right hand in the absence of visual feedback after the cube location had been displayed for 200ms. During the reaching movements, the colour of the cube changed and visual feedback about the change of colour was provided for 100ms at 500ms after movement onset (at the time of peak grasp aperture). Depending on colour, subjects were instructed to either pincer-grasp the cube in a horizontal or vertical grasp position with the latter necessitating wrist rotation (experiment 1) or to pincer-grasp and transport the cube to either a left or right target position (experiment 2). Within two consecutive 200ms time windows (TMS 1 and 2) starting 500ms and 700ms after movement onset, respectively, double pulses of supra-threshold transcranial magnetic stimulation (inter-stimulus interval: 100ms) were delivered over (i) the left primary motor cortex (90 degrees vertically angulated coil position, control stimulation), (ii) the left dorsal premotor cortex or (ii) the left anterior intraparietal area.
Results: Compared to control stimulation, stimulation of the anterior intraparietal area, but not of the dorsal premotor cortex, at TMS 1 delayed the times to wrist rotation (experiment 1) and hand transport (experiment 2). Compared to control stimulation, stimulation of the dorsal premotor cortex, but not of the anterior intraparietal area, at TMS 2 delayed both wrist rotation (experiment 1) and hand transport (experiment 2).
Conclusions: We contend that the anterior intraparietal area and the dorsal premotor cortex are both involved albeit at different phases during the mapping of arbitrary visual cues with goal directed grasp and transport movements.
Significance: These data add to the current understanding of how human cortical areas work in concert during manual activities.