Fig. 1.

Schematic methods for experiments 1 and 2. Left: after holding the start location for 200 ms, a fixation cross appeared. When the cross disappeared and the target appeared, the participant’s task was to “shoot through” the target in a single, fast movement, experiencing the force field (or null field in baseline). The red arrows schematically illustrate (potential) robot-generated forces. Note that the movement path on field trials could deviate substantially from the straight line illustrated. On channel trials (Scheidt et al. 2000), the robot constrained movements to the line connecting start and target. Catch channels were preceded by the fixation cross, making them indistinguishable, a priori, from field trials. On instructed channels, instead of the fixation cross, learners would see a keyword prompting them to either expect the robot not to push them off their path or to do so, and act accordingly. Note that for experiment 1 field and Catch trials, no fixation cross was shown and instead, the target appeared immediately. Right: perturbation schedules for the individual experiments, with channel trials highlighted by the colors used for results figures (green = No Push, yellow = Catch, Blue = Push.)