Figure 1. Apparatus and experimental design. A, Drawing of the experimental setup (for details, see text). B, Configuration of the start position, occluder, and the 10 training targets. Red force vectors, within the red shaded region, indicate the position-dependent force field applied to the grasped object. C, Configuration for the experimental trials with targets at −10° and 5° and the same force field as shown in B (illustrated with fewer red arrows). Shown only for the 5° target, but also present for the −10° target, are the cursor jumps (filled colored circles) and target jumps (open colored circles). Red and blue circles represent perturbations requiring a rightward and leftward movement response, respectively. D, Average object paths toward the 10 training targets for a single participant, with trajectories aligned to the time at which the cursor emerged from the occluder. Shaded regions represent ±1 SD orthogonal to the target direction. E, F, Average object paths for each target and perturbation type for the same participant as in D. Red and blue traces represent perturbation trials requiring a rightward and leftward movement, respectively. Gray traces represent unperturbed trials (with the same unperturbed trials shown in E and F. G, Procedure for calculating arm movement response latency, relative to the time of the perturbation (t = 0), illustrated for a single participant and target jumps involving the 10° target. We first determined when the p value from a running t test comparing the average object velocity perpendicular to the vector from the start position to the target for right (red trace) and left (blue trace) target jumps dropped <0.001 (vertical gray dash-dotted line), and then backtracked to the first minima in the rate of change of the p value (vertical black dashed line) .