Measures of Implicit and Explicit Adaptation Do Not Linearly Add

Both implicit (unconscious, automatic) and explicit (effortful, strategic) processes contribute to various kinds of learning1, including visuomotor adaptation2–4. Implicit adaptation may be capped at some level5, regardless of the level of explicit adaptation, or implicit and explicit adaptation could be linearly added in total adaptation6,7. Both appear well accepted, but ironically contradict each other. In the latter case, the equation used is simple: Which is used to “predict” that implicit adaptation is perfectly anti-correlated with explicit adaptation: This derived implicit adaptation is sometimes further analyzed8–10. But the underlying additivity assumption is not always replicated11 and is considered the “least robust” assumption in motor adaptation12. 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.