In learning and adapting movements in changing conditions, people attribute the errors they experience to a combined weighting of internal or external sources. As such, error attribution that places more weight on external sources should lead to decreased updates in our internal models for movement of the limb or estimating the position of the effector, i.e. there should be reduced implicit learning. However, measures of implicit learning are the same whether or not we induce explicit adaptation with instructions about the nature of the perturbation. Here we evoke clearly external errors by either demonstrating the rotation on every trial, or showing the hand itself throughout training. Implicit reach aftereffects persist, but are reduced in both groups. Only for the group viewing the hand, changes in hand position estimates suggest that predicted sensory consequences are not updated, but only rely on recalibrated proprioception. Our results show that estimating the position of the hand incorporates source attribution during motor learning, but recalibrated proprioception is an implicit process unaffected by external error attribution.

Motor learning and adaptation are guided by the attribution of errors to internal or external sources. When errors are clearly external, we should not update our internal models for movement or state estimation, i.e. there should be no implicit learning. However, measures of implicit learning are the same whether or not we induce explicit adaptation with instructions about the nature of the perturbation. Here we make errors even more clearly external by either demonstrating the rotation on every trial, or showing the hand itself throughout training. Implicit reach aftereffects persist, but are reduced in both groups. Only for the group viewing the hand, state estimates suggest that predicted sensory consequences are not updated, but only rely on recalibrated proprioception. Our results show that state estimation of the hand incorporates source attribution during motor learning, but recalibrated proprioception is an implicit process unaffected by external error attribution.

Both implicit (unconscious, automatic) and explicit (effortful, strategic) processes contribute to various kinds of learning, including visuomotor adaptation. Implicit adaptation may be capped at some level, regardless of the level of explicit adaptation, or implicit and explicit adaptation could be linearly added in total adaptation. Both appear well accepted, but ironically contradict each other. In the latter case, the equation used is simple: 1) adaptation ~= implicit + explicit, Which is used to "predict" that implicit adaptation is perfectly anti-correlated with explicit adaptation: 2) implicit ~= adaptation - explicit. This derived implicit adaptation is sometimes further analyzed. But the underlying additivity assumption is not always replicated and is considered the "least robust" assumption in motor adaptation. Here we directly test the additivity assumption, particularly how well it predicts implicit adaptation. We find that the relationship between explicit and implicit adaptation is not additive but more complicated and very noisy, which means that future studies should measure both implicit and explicit adaptation directly. It also highlights a challenge to the field to disentangle how various motor adaptation processes combine when producing movements.

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