Signatures of rapid trajectory control during finger and arm movements

Normative goal-directed hand and finger movements often follow invariant kinematics, characterized by approximate straight-line trajectories. A fundamental unresolved issue is whether movement trajectories are planned or whether they are a consequence of trajectory-free online control. We addressed this question using Spearman's rank correlation, zero-crossings rate, and z-scores, and analyzed the time evolution of kinematic variability during the presence or absence of a task in finger and whole-arm reaching movements. We found that the central nervous system (CNS) implements control that is consistent with task-dependent trajectory regulation, where task-relevant movements showed enhanced control toward an average trajectory. Further, we found behavioral signatures of rapid kinematic control that might operate on these planned trajectories as early as ~ 60 ms in finger movements and ~ 70 ms in whole-arm reaching movements. Such early signatures of control in behavior suggest that the system could exploit a trajectory plan along with fast feedback to implement meaningful corrections during simple voluntary unperturbed movements. The analysis also revealed that the control peaked distinctly during the early (20%) and late (80-90%) phases, among both finger and arm movements, implying that the extent of trajectory control was not constant during the execution of movement.