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This study investigated force adaptation in humans during goal-directed flexion forearm motion. The ability of the motor system to adapt to changes in internal or external forces is essential for the successful control of voluntary movement. In a first experiment, we examined how under- or overdamping differentially affected the length of the adaptation and the arm kinematics between force transitions. We found that transitions diverging from a null-force produced larger transition effects than transitions converging to a null force condition, indicating that re-adaptation was less error-prone. Whether the subjects had previously experienced underdamping or the null-force had no significant impact on the spatial trajectory after switching to overdamping. That is, prior force experience had no differential effect on the spatial transition kinematics. However, the transitions underdamping-to-overdamping and underdamping-to–null force did produce differently strong transition effects. These results indicate that exposure to the new force rather than previous force-field experience is responsible for transition- and after-effects. In a second experiment, we investigated whether learning was law-like—that is, whether it generalized to unvisited workspace. Subjects were tested in new, unvisited workspaces in the null-force condition after sufficient training in either force condition. The occurrence of transferred after-effects indicated that adaptation to both positive and negative damping was mediated by rule-based rather than exclusive associative processes.
T. Kalenscher is with the Institute of Cognitive Neuroscience at the University of Bochum, 44780 Bochum, Germany. K.-T. Kalveram and T. Kalenscher are with the Department of Experimental Psychology at the University of Düsseldorf, 40225 Düsseldorf, Germany. J. Konczak is with the Human Sensorimotor Control Laboratory in the Division of Kinesiology, as well as the Program in Neuroscience, at the University of Minnesota, Minneapolis, MN 55455-0110.