Internal feedback enables rapid trajectory control during goal-directed finger and whole-arm reaching movements

Goal-directed eye, hand, and finger movements follow invariant kinematics consisting of approximate straight-line trajectories and bell-shaped velocity profiles. A fundamental unresolved issue is whether these trajectories are planned or whether they are a consequence of trajectory-free online control. We address this question using Spearmans rank correlation, zero-crossing rate, and z-scores, and analyze within-trial variability to investigate differences in the time evolution of trajectories during the presence or absence of a goal in finger and whole-arm reaching movements, along with analyzing rapid goal-directed saccadic eye movements. We found that the central nervous system (CNS) implements control to follow an average trajectory, where goal-directed movements show an enhanced degree of trajectory control. Further, we found behavioral signatures of rapid 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 suggest that the system could exploit internal feedback along with fast feedback processes to implement meaningful corrections during simple voluntary unperturbed movements. The analysis also revealed that the controller gains varied along the movement and peaked distinctly at early (20 %) and late (90 %) phases of finger and arm movements, implying that trajectory control may be accomplished through implicit way-point objectives during the execution of the movement. These later corrections were missing during the late phase of eye movements, pointing to a predominant role of delayed sensory feedback mediating corrections in the context of finger and arm movements during this period.