Competitive preordering during planning persists in kinematically fused sequential movements.

Abstract

Results in human and nonhuman primates have shown that elements of a movement sequence are preordered in parallel competitively before execution, a process known as competitive queueing (CQ). However, it is unclear whether the preplanning of individual movements persists in continuous skilled actions that involve greater biomechanical integration and is associated with the formation of new motor primitives (neural fusion). We investigated how kinematics impact sequence planning in a handwriting-like task asking whether fusing velocity curves between adjacent movements affects movement preparation. Participants were trained and tested for two days to perform two sequences of four sequential center-out-and-back movements from memory in a delayed sequence production task using a stylus on a Wacom tablet. To manipulate kinematic fusion between subsequent strokes, participants were assigned to one of three groups that were trained to perform the sequences either with acute, right, or obtuse angles between sequential targets. Probe trials assessed the availability of constituent movement elements for fast and accurate execution toward each target during planning. Movement elements associated with later sequence positions were less available than earlier movements, regardless of kinematic fusion, in line with CQ findings for discrete typing sequences in humans. Importantly, a more pronounced CQ gradient was associated with higher fusion, faster initiation, and greater accuracy of sequence production. These findings indicate that kinematically fused sequential actions do not result in the formation of new movement primitives (neural fusion) with a single movement plan. Instead, they continue to be planned separately and are associated with skilled performance.NEW & NOTEWORTHY When we fluidly string actions together like in cursive handwriting, does the brain fuse those movements into a single, unbreakable unit? By manipulating movement co-articulation (fusion) in a handwriting-like task, we challenge this view. Contrary to the assumption that behavioral fusion reflects the emergence of a singular, novel motor primitive, we demonstrate that even under high levels of kinematic fusion, individual sequence elements remain distinctly represented and competitively preordered based on their sequence position.