Abrupt, but not gradual, motor adaptation biases saccadic target selection.

Motor costs influence movement selection. These costs could change when movements are adapted in response to errors. When the motor system attributes the encountered errors to an external cause, appropriate movement selection requires an update of the movement goal, which prompts the selection of a different control policy. However, when errors are attributed to an internal cause, the initially selected control policy could remain unchanged, but the internal forward model of the body needs to be updated, resulting in an online correction of the movement. We hypothesized that external attribution of errors leads to the selection of a different control policy, and thus to a change in the expected cost of movements. This should also affect subsequent motor decisions. Conversely, internal attribution of errors may (initially) only evoke online corrections, and thus is expected to leave the motor decision process unchanged. We tested this hypothesis using a saccadic adaptation paradigm, designed to change the relative motor cost of two targets. Motor decisions were measured using a target selection task between the two saccadic targets before and after adaptation. Adaptation was induced by either abrupt or gradual perturbation schedules, which are thought to induce more external or internal attribution of errors, respectively. By taking individual variability into account, our results show that saccadic decisions shift toward the least costly target after adaptation, but only when the perturbation is abruptly, and not gradually, introduced. We suggest that credit assignment of errors not only influences motor adaptation but also subsequent motor decisions.NEW & NOTEWORTHY Decisions between potential motor actions are influenced by their costs, but costs change when movements are adapted. Using a saccadic target selection task, we show that target preference shifts after abrupt, but not after gradual adaptation. We suggest that this difference emerges because abrupt adaptation results in target remapping, and thus directly influences cost calculations, whereas gradual adaptation is mainly driven by corrections to a forward model that is not involved in cost calculations.

An implicit memory of errors limits human sensorimotor adaptation.

During extended motor adaptation, learning appears to saturate despite persistence of residual errors. This adaptation limit is not fixed but varies with perturbation variance; when variance is high, residual errors become larger. These changes in total adaptation could relate to either implicit or explicit learning systems. Here, we found that when adaptation relied solely on the explicit system, residual errors disappeared and learning was unaltered by perturbation variability. In contrast, when learning depended entirely, or in part, on implicit learning, residual errors reappeared. Total implicit adaptation decreased in the high-variance environment due to changes in error sensitivity, not in forgetting. These observations suggest a model in which the implicit system becomes more sensitive to errors when they occur in a consistent direction. Thus, residual errors in motor adaptation are at least in part caused by an implicit learning system that modulates its error sensitivity in response to the consistency of past errors.