Implicit adaptation to mirror-reversal is in the correct coordinate system but the wrong direction

Learning in visuomotor adaptation tasks is the result of both explicit and implicit processes. Explicit processes, operationalized as re-aiming an intended movement to a new goal, account for the lions share of learning while implicit processes, operationalized as error-dependent learning that gives rise to aftereffects, appear to be highly constrained. The limitations of implicit learning are highlighted in the mirror-reversal task, where implicit corrections act in opposition to performance. This is surprising given the mirror-reversal task has been viewed as emblematic of implicit learning. One potential confound of these studies is that both explicit and implicit processes were allowed to operate concurrently, which may interact, potentially in opposition. Therefore, we sought to further characterized implicit learning in a mirror-reversal task with a clamp design to isolate implicit learning from explicit strategies. We confirmed that implicit adaptation is in the wrong direction for mirror-reversal and operates as if the perturbation were a rotation, and only showed a moderate attenuation after three days of training. This result raised the question of whether implicit adaptation blindly operates as though perturbations were a rotation. In a separate experiment, which directly compared a mirror-reversal and a rotation, we found that implicit adaptation operates in a proper coordinate system for different perturbations: adaptation to a mirror-reversal and rotational perturbation is more consistent with Cartesian and polar coordinate systems, respectively. It remains an open question why implicit process would be flexible to the coordinate system of a perturbation but continue to be directed inappropriately. Public Significance StatementPatients with severe amnesia can improve their performance from day to day in mirror-reversal tasks. These findings led, in part, to the codification of explicit and implicit processes in classic theories regarding the taxonomy of memory systems, with motor learning resting firmly in the branch of implicit memory. However, recent evidence has shown that explicit processes also play an important role in motor learning. Whats more, these studies have found that implicit learning doesnt operate in a useful way in the mirror-reversal task. In the present study, we further examine this puzzling behavior of implicit learning in a mirror-reversal task using a design that can isolate implicit processes from explicit strategies. We clearly showed that the implicit system adapts in the wrong direction for a mirror-reversal, acting as if the perturbation were a rotation. Surprisingly, however, we found that although adaptation is in the wrong direction, the implicit system is sensitive to a particular coordinate system. These findings further challenge the flexibility of this implicit adaptation process in motor learning.