Practice induces a qualitative change in the memory representation for visuomotor learning
Adaptation of our movements to changes in the environment is known to be supported by multiple learning processes which act in parallel. An implicit process recalibrates motor output to maintain alignment between intended and observed movement outcomes (\"implicit recalibration\"). In parallel, an explicit learning process drives more strategic adjustments of behavior, often by deliberately aiming movements away from an intended target (\"deliberate re-aiming\"). It has long been established that people form a memory for prior experience adapting to a perturbation through the fact that they become able to more rapidly adapt to familiar perturbations (a phenomenon known as \"savings\"). Repeated exposures to the same perturbation can further strengthen savings. It remains unclear, however, which underlying learning process is responsible for this practice-related improvement in savings. We measured the relative contributions of implicit recalibration and deliberate re-aiming to adaptation during multiple exposures to an alternating sequence of perturbations over two days. We found that the implicit recalibration followed an invariant learning curve despite prolonged practice, and thus exhibited no memory of prior experience. Instead, practice led to a qualitative change in re-aiming which, in addition to supporting savings, became able to be expressed rapidly and automatically. This qualitative change appeared to enable participants to form memories for two opposing perturbations, overcoming interference effects that typically prohibit savings when learning multiple, opposing perturbations. Our results are consistent with longstanding theories that frame skill learning as a transition from deliberate to automatic selection of actions.