Common vs. independent limb control in sequential vertical aiming: The cost of potential errors during extensions and reversals.

The following study explored movement kinematics in two-component aiming contexts that were intended to modulate the potential cost of overshoot or undershoot errors in up and down directions by having participants perform a second extension movement (Experiment 1) or a reversal movement (Experiment 2). For both experiments, the initial movement toward a downward target took longer, and had lower peak acceleration and peak velocity than upward movements. These movement characteristics may reflect a feedback-based control strategy designed to prevent energy-consuming limb modifications against gravitational forces. The between-component correlations of displacement at kinematic landmarks (i.e., trial-by-trial correlation between the first and second components) increased as both components unfolded. However, the between-component correlations of extensions were primarily negative, while reversals were positive. Thus, movement extensions appear to be influenced by the use of continuous on-line sensory feedback to update limb position at the second component based on the position attained in the first component. In contrast, reversals seem to be driven by pre-planned feedforward procedures where the position of the first component is directly replicated in the second component. Finally, the between-component correlations for the magnitude of kinematic landmarks showed that aiming up generated stronger positive correlations during extensions, and weaker positive correlations toward the end of the first component during reversals. These latter results suggest the cost of potential errors associated with the upcoming second component directly influence the inter-dependence between components. Therefore, the cost of potential errors is not only pertinent to one-component discrete contexts, but also two-component sequence aims. Together, these findings point to an optimized movement strategy designed to minimize the cost of errors, which is specific to the two-component context.