Skill learning involves optimizing the linking of action phases

Many manual tasks involve object manipulation and are achieved by an evolving series of actions, or action phases, recruited to achieve task subgoals. The ability to effectively link action phases is an important component of manual dexterity. However, our understanding of how the effective linking of sequential action phases develops with skill learning is limited. Here, we addressed this issue using a task in which participants applied forces to a handle to move a cursor on a computer screen to successively acquire visual targets. Target acquisition required actively holding the cursor within the target zone (hold phase) for a required duration before moving to the next target (transport phase). If the transport phase was initiated prematurely, before the end of the required hold duration, participants had to return to the target to acquire it. The goal was to acquire targets as quickly as possible. Distinct visual and auditory sensory events marked goal completion of each action phase. During initial task performance, the transport phase was reactively triggered by sensory events signaling hold phase completion. However, with practice, participants learned to initiate the transport phase based on a prediction of the time of hold phase completion. Simulations revealed that participants learned to near-optimally compensate for temporal uncertainty, presumably related to estimation of time intervals and execution of motor commands, so as to reduce the average latency between the end of the required hold phase duration and the start of the transport phase, while avoiding an excess of premature exits.