Revisiting the explicit-implicit additivity assumption in visuomotor adaptation.

Explicit aiming strategies have been shown to play an important role in visuomotor adaptation-enabling rapid improvements in performance and affording flexibility-but their interaction and downstream consequences on implicit recalibration processes remain hotly debated. Although early work assumed these processes combined additively, recent studies have challenged this view. However, these studies may have overlooked subtle spatial and temporal dynamics, which could influence how explicit aiming and implicit recalibration interact. Recent research shows that implicit recalibration anchors to where a person aims their movements, with aiming strategies directly shaping their spatial development. Moreover, implicit recalibration operates across multiple timescales, with both temporally volatile and persistent components. To examine whether these factors mask the true relationship between explicit strategies and implicit recalibration, we conducted a visuomotor rotation task while carefully accounting for the interplay of spatial and temporal dynamics. We used two complementary tests: a direct test of strict additivity examining whether implicit and explicit components sum to total adaptation, and a slope analysis examining whether the two processes exhibit a compensatory tradeoff. After controlling for spatial dynamics (plan-based generalization) and temporal dynamics (forgetting), we found that strict additivity failed in both groups-implicit and explicit measures did not perfectly sum to total adaptation. Nonetheless, both groups showed a robust inverse relationship between explicit strategies and implicit recalibration, consistent with partial additivity. This pattern of compensatory tradeoff, despite imperfect summation, may result from simple methodological imprecision, the operation of additional but unobserved processes, or more complex nonlinear interactions between processes.