Our current results suggest that subjects on second exposure recall the hand direction that was reinforced during the first exposure to the perturbation. Adp+Rep+ showed marked savings, whereas Adp+Rep− showed no savings, even though they adapted to the same mean rotation. We conclude from Experiment 2 that a reinforcement process was necessary and sufficient for savings,

and that use-dependent plasticity is not sufficient for savings. A set of previously puzzling selleck inhibitor results reported in visuomotor rotation studies may also be more easily interpreted as arising from an operant model-free mechanism. Savings for a given rotation is disrupted if subjects train with a counterrotation even at prolonged time intervals Selleck CHIR99021 after initial training and when aftereffects have decayed away (Krakauer et al., 1999 and Krakauer et al., 2005). We propose that persistent interference effects occur because successful cancellation of rotations of opposite sign is associated with different movements in hand space even if the movement of the cursor into the target is the same in visual space. That is, the corresponding motor commands to the same target are distinctly different for oppositely signed rotations. Thus, the association of the same target with different commands in a serial

manner, as is done with A-B-A paradigms, could lead to interference as is seen with other forms of paired-associative paradigms. In such paradigms, interference occurs through retrieval inhibition from (Adams and Dickinson, 1981, Anderson et al., 2000, MacLeod and Macrae, 2001 and Wixted, 2004). Complementary to this explanation for interference, we can predict that there should be facilitation,

i.e., savings, for two rotations of opposite sign if they are both associated with the same commands or movements in hand space. This was exactly what we found in Experiment 3: learning a +30° counterclockwise rotation facilitated learning of a −30° clockwise rotation when both rotations required the same directional solution in hand space. This supports the idea that an operant reinforcement process underlies savings and interference effects in adaptation experiments. Furthermore, results from Experiment 3 showed that the directional solution in hand space need not be associated with multiple targets, as in Experiments 1 and 2, for reinforcement to occur; success at a single target, as in Experiment 3 (and in most conventional error-based motor learning paradigms), is sufficient for savings. Numerous studies suggest that adaptation is dependent on the cerebellum (Martin et al., 1996a, Martin et al., 1996b, Smith and Shadmehr, 2005 and Tseng et al., 2007), a structure unaffected in Parkinson’s disease (PD), and therefore initial learning in patients with PD would be expected to proceed as in controls, as indeed was recently demonstrated (Bédard and Sanes, 2011 and Marinelli et al., 2009). Operant learning is, however, known to be impaired in PD (Avila et al., 2009, Frank et al.