Intact fast learning and impaired slow learning of visuomotor adaption in Parkinson's disease

[Truncated abstract] Efficient interaction with objects in the environment relies on the ability to adapt to a change in the state of the body or the environment (motor adaptation). In motor adaptation studies, a systematic perturbation of the movement outcome evokes a discrepancy between the desired movement outcome and the actual movement outcome. Motor adaptation is widely thought to occur by updating an internal model trial by trial to produce a motor command that reduces this discrepancy. This internal model based account cannot, however, explain findings that initial adaptation learning has a persistent effect on subsequent adaptation learning, despite returning motor output to the naïve state after initial learning. The persistent effect of initial learning on subsequent learning can be shown through (1) savings, where initial learning speeds up subsequent learning to adapt to the same perturbation and (2) anterograde interference, where initial learning slows subsequent learning to adapt to an opposing perturbation. Savings and anterograde interference have been explained through a model incorporating a fast-learning, fast forgetting process and a slow-learning, slow-forgetting process (Smith et al. 2006). The slow-learning, slow-forgetting process is thought to be responsible for savings and anterograde interference. A recent reconceptualization of this model proposes that the fast-learning, fast-forgetting process occurs by updating an internal model (i.e., is modelbased) while the slow-learning, slow forgetting process occurs through model-free mechanisms of operant reinforcement learning and use-dependent plasticity (Huang et al. 2011).