Interference Between Motor Memories Arises From Implicit Recalibration

Interference between consecutively acquired motor memories is a defining feature of sensorimotor adaptation, yet its mechanistic origin remains unresolved. Because adaptation is supported by separable explicit strategies and implicit recalibration, it offers a means to identify the learning process that gives rise to interference. Across four visuomotor adaptation experiments, we examined the conditions under which the acquisition of a new, competing motor memory influences the expression of a previously acquired memory. We selectively biased new learning towards either explicit or implicit processes, and quantified its impact on the recall of the original memory 24-hours later. Under standard adaptation conditions, participants exhibited classic interference, such that re-learning was indistinguishable from naive performance. However, when new learning was driven primarily by explicit strategies induced through delayed endpoint feedback, interference was markedly attenuated and the original memory was preserved. In contrast, when the competing memory was implicitly forged under error-clamp conditions, robust interference emerged. Furthermore, disrupting posterior parietal cortex (PPC) with cathodal hd-tDCS prior to implicit learning attenuated interference, indicating that intact PPC processing is required for incorporating new learning into an existing sensorimotor representation. Taken together, these findings suggest that interference reflects the integration of new learning into a shared representational substrate via implicit recalibration, a process that limits the coexistence of competing motor memories.