Signatures of internal feedback and trajectory control during goal-directed finger and whole-arm reaching movements

Abstract

AbstractGoal-directed behaviors involve the execution of a variety of eye, hand, and finger movements that follow invariant kinematics consisting of straight-line trajectories and bell-shaped velocity profiles. Despite the presence of neuromotor noise and distinct biomechanics such motions are performed with ease and reasonable accuracy. A fundamental unresolved issue in the field is to determine and delineate the extent to which these trajectories are planned or whether they are a consequence of trajectory-free online control. In this study, we address this question using Spearman’s rank correlation, zero-crossing rate andz-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. 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, by performing the analysis on the actual timing, we found behavioral signatures of rapid control that might operate on these planned trajectories as early as 30 ms in finger movements and 16.67 ms in whole-arm reaching movements which are too early for trajectory control to be derived from delayed sensory feedback. The analysis also revealed that the controller gains varied along the movement and peaked distinctly at an early (20 %) and a late (90 %) phases of movement, suggesting that trajectory control may be accomplished through virtual way-point objectives during the execution of the movement.Significance StatementThe extent to which reaching movements reflect the unfolding of a prespecified trajectory plan or whether they arise from a trajectory-free online control has remained a vexing issue for motor theorists. Using novel measures of control during the movement such as Spearman’s rank correlation, zero-crossing rate and trends inz-scores, we investigated goal-directed finger and whole-arm reaching movements and demonstrated that the CNS implements rapid control to follow a planned trajectory, especially during early and late phases of movement. Our results provide novel constraints for computational theories of motor control.