Force Reproduction Error Depends on Force Level, Whereas the Position Reproduction Error Does Not

When reproducing a previously perceived force or position humans make systematic errors. This study determined the effect of force level on force and position reproduction, when both target and reproduction force are self-generated with the same hand. Subjects performed force reproduction tasks at different force levels, against a fixed handle, and performed a position reproduction task against a haptic manipulator which applied constant opposing forces. Subjects performed a series of two interchanging trials: matching an onscreen force or position (reference trial), and subsequently reproducing the same force or position without visual feedback (reproduction trial). Subjects generated too high forces for low force levels (<;40 N) and too low forces for high force levels (>130 N). No effect of force level on the position reproduction error was found. Previous studies proposed that reafference feedback, i.e., the sensory prediction from self-generated forces, causes the force reproduction error. However, if the force reproduction error is exclusively caused by the reafference feedback, the force reproduction error should disappear when both target and reproduction force are self-generated. The results of this study show an effect of force level on the force reproduction error, indicating that reafference feedback is not the sole factor in force reproduction tasks.