Large errors increase the generalization of locomotor adaptation depending on the error direction.
Generalization in motor adaptation involves the transfer of movements beyond the adaptation context. We investigated the effect of size (large vs. small) and direction (unidirectional vs. bidirectional) of performance errors during adaptation on the generalization of walking patterns from a split-belt treadmill (training context) to overground (testing context). We hypothesized that unusual errors (i.e., large unidirectional or bidirectional errors) would serve as contextual cues limiting generalization. The size of unidirectional errors was modulated either implicitly (i.e., gradual vs. semi-abrupt split-belt perturbations) or explicitly (i.e., through instructed visual feedback). Bidirectional errors were induced by a sudden removal of the split-perturbation after a long adaptation period, resulting in errors in the opposite direction to those at the start of the adaptation period. Our findings did not support our hypothesis. We found that bidirectional, but not large, performance errors limited generalization across contexts, which could be mediated by two distinct mechanisms. On the one hand, bidirectional errors upon removal of the split-perturbation are also experienced when transitioning to overground walking. Thus, bidirectional errors may facilitate switching between distinct walking patterns, thereby limiting generalization. On the other hand, large unidirectional errors induce more motor adaptation, which might lead to more generalization.