Figure 1.

Experimental paradigm. A, The subject grasps the handle of the robotic manipulandum (vBOT) while seated. Visual feedback of movements is presented veridically using a top-mounted computer screen viewed via a mirror. The subject's forearm is fixed to the handle and supported by an air sled. B, Workspace layout of the experiment. There were four possible cue locations (gray circles, C1–C4), one central target (green circle; note that in the experiment this was displayed as gray), and four final targets (yellow circles, T1–T4). C, Table of the force-field directions as a function of cue and target location. During the exposure phase of the experiments, force fields were applied on the subjects' hands by the robot as the subject performed a movement between the central location and the final target. The force field applied—CW or CCW—depended on both the target direction (0°, 90°, 180°, and 270°) and the cue location (45°, 135°, 225°, and 315°), but not every combination was used. For each target location, two possible cue locations could be chosen (one for each force-field direction) corresponding to ±135° relative angle around the central target. Only the combination of the cue location and final target location specified the force-field direction. In a block of trials, all cue locations and final target locations were pseudorandomly presented such that CW and CCW trials were mixed equally. The table shows the association of force field and cue–target location used in half the subjects for a given experiment. In the other half, the association was reversed.