Interactions Between Explicit and Implicit Memories During Sensorimotor Adaptation and After Concussion

Sensorimotor adaptation is a form of motor learning whereby a motor skill is adjusted in the face of a disturbance to return performance to desired performance. This process uses memories of previous experiences to guide future movements. While the temporal characteristics of adaptation have been well studied, the extent to which these memories are explicit, implicit, or a mixture of both is unclear. In this Dissertation, I conduct three studies to investigate the relative contributions of sensorimotor memories when adapting to stochastic spring-like loads applied to the hand. By implementing computational modeling and system identification techniques, I fit memory models to movement errors and external disturbances to yield the relative contributions of sensorimotor memories. In Aim 1, I determined if the memories used in sensorimotor adaptation to stochastic loads are predominantly explicit or implicit. Subjects grasped a handle of the robotic device and performed out-and-back reaches while the robot imposed spring-like forces to oppose movement. Immediately after each reach, subjects reported where they thought they moved – an assay of explicit memory of performance. Model fits revealed that explicit memories of performance did not outperform models that only contained actual reach error – a proxy of implicit memory. Thus, sensorimotor adaptation predominantly used implicit memories. In Aim 2, I determined if instructing subjects to explicitly attempt to underperform during the reaching test would influence how these memories were used in adaptation. Subjects were instructed to simulate having a concussion (moving slower, less accurate, and having memory deficits). The relative contributions of sensorimotor memories did not significantly change when subjects attempted to sabotage the test, further supporting the notion of inaccessible implicit mechanisms at play. In Aim 3, recently concussed individuals participated in a longitudinal study wherein I observed changes in the relative contributions of motor memories as their injury recovered. Concussion did impact the process of sensorimotor adaptation, but practice effects obscured initial effects of the injury on sensorimotor memories. This work provides novel insights into how explicit and implicit processes interact during motor learning and adaptation, offering potential applications for assessing motor control after brain injury.