Different generalization of fast and slow visuomotor adaptation across locomotion and pointing tasks.

Sensorimotor adaptation can involve multiple learning processes with different time courses, and these processes may have different patterns of transfer. In this study, we tested how slow learning and fast learning transfer across tasks, and the specificity of transfer. We tested two natural goal-directed tasks: pointing and walking toward a visible target. We also tested a novel "hand locomotion" task in which subjects used pointing movements to cause simulated self-motion in virtual reality. The hand locomotion task used the same physical movement as pointing, but performed the same function as stepping. During an experimental block, subjects performed alternating training trials with perturbed visual feedback and test trials with no feedback. The test trials were either the same task to measure adaptation, or a different task to measure transfer. Perturbations on adaptation trials varied over time as a sum of sinusoids with different frequencies. Fast learning would be expected to produce equal responses to fast and slow perturbations, while slower learning would dampen responses to higher frequency perturbations. Subjects were generally not aware of the smoothly varying perturbations, but showed detectable adaptation for all three tasks. Only pointing produced significantly different responses to high- and low-frequency perturbations, consistent with slow learning. Adaptation of pointing produced more transfer to the hand locomotion task, which shared the same effector and motor actions, than to the stepping task. The other tasks showed fast learning but little or no slow learning, and equal transfer to tasks with different effector or function. Our results suggest that the slower components of sensorimotor adaptation are more movement specific, while faster learning is more generalizable.