Classic taxonomies of memory distinguish explicit and implicit memory systems, placing motor skills squarely in the latter branch. This assertion is in part a consequence of foundational discoveries ...showing significant motor learning in amnesics. Those findings suggest that declarative memory processes in the medial temporal lobe (MTL) do not contribute to motor learning. Here, we revisit this issue, testing an individual (L. S. J.) with severe MTL damage on four motor learning tasks and comparing her performance to age-matched controls. Consistent with previous findings in amnesics, we observed that L. S. J. could improve motor performance despite having significantly impaired declarative memory. However, she tended to perform poorly relative to age-matched controls, with deficits apparently related to flexible action selection. Further supporting an action selection deficit, L. S. J. fully failed to learn a task that required the acquisition of arbitrary action–outcome associations. We thus propose a modest revision to the classic taxonomic model: Although MTL-dependent memory processes are not necessary for some motor learning to occur, they play a significant role in the acquisition, implementation, and retrieval of action selection strategies. These findings have implications for our understanding of the neural correlates of motor learning, the psychological mechanisms of skill, and the theory of multiple memory systems.
Abstract
Learning in sensorimotor adaptation tasks has been viewed as an implicit learning phenomenon. The implicit process affords recalibration of existing motor skills so that the system can ...adjust to changes in the body or environment without relearning from scratch. However, recent findings suggest that the implicit process is heavily constrained, calling into question its utility in motor learning and the theoretical framework of sensorimotor adaptation paradigms. These inferences have been based mainly on results from single bouts of training, where explicit compensation strategies, such as explicitly re-aiming the intended movement direction, contribute a significant proportion of adaptive learning. It is possible, however, that the implicit process supersedes explicit compensation strategies over repeated practice sessions. We tested this by dissociating the contributions of explicit re-aiming strategies and the implicit process in human participants over five consecutive days of training. Despite a substantially longer duration of training, the implicit process still plateaued at a value far short of complete learning and, as has been observed in previous studies, was inappropriate for a mirror-reversal task. Notably, we find significant between subject differences that call into question traditional interpretation of these group-level results.
The mechanisms underlying motor skill learning are at once well established and fiercely debated. Implicit recalibration, the process by which a motor skill is adjusted to account for unexpected ...feedback, has received the most attention. In fact, the processes underlying implicit recalibration was considered to account for most of skill learning. However, recent advances have shown a second process which plays a strong role in adapting an established skill, strategic compensation. It seems obvious that a strategy would be developed for dealing with unexpected perturbations to skill. However, when strategies are used, what influences their shape, and how they interact implicit recalibration is relatively unknown. We explore these adaptive processes in Chapters II – IV.While adaptation of a learned skill has preoccupied much of the motor learning literature, and indeed much of this dissertation, how we learn new is skills equally important. Termed de novo learning, the acquisition of new action-outcome mappings is essential for common skills, such as typing on a keyboard, or playing a musical instrument. In Chapter V, we describe a new paradigm for investigating de novo learning, and provide evidence that reinforcement learning is a useful model for understanding skill learning.
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