S-shaped motor learning and nonequilibrium phase transitions

S-Shaped change in performance outcome has long been considered to be a pathway of motor learning, but there is little or no evidence for it. The experiment investigated the hypothesis that S-shaped motor learning as reflected in the task outcome is a product of a transition in the movement coordination dynamics as a function of practice acting as a control parameter. Young adult participants practiced the roller ball task that required learning the transition of a coordination mode to preserve and enhance the motion of a rotating ball to transition from task failure to success. There were 50 practice trials per day for as many practice days (3-20) as required for each participant to reach the task criterion of success that was followed 1 week later by a retention test. All participants improved their task performance with practice but there were subgroups of patterns of change including S-shaped learning. The enhanced variability during the transition supported the interpretation that the S-shaped learning outcome is reflective of a saddle-node bifurcation or first-order nonequilibrium transition. The learning of a new pattern of movement coordination is a different process from learning to scale an already producible coordination mode to new task demands.