Natural behavior unfolds as a continuous stream of actions. Because these actions often occur in rapid succession, the brain must prepare multiple future actions while the current action is being executed—a process we refer to as online planning. We review evidence for online planning in unpredictable movement sequences and consider neural implementations that could support parallel execution and multiple planning processes. We then show how this new framework could apply to the learning of specific sequences, improving performance while retaining the ability to modify sequences online. Online planning, therefore, provides a unifying account of how both unpredictable and well-learned sequences are produced, and how training leads to skillful and coordinated performance while retaining behavioral flexibility.
