Flexible, task-dependent use of sensory feedback to control hand movements.

We tested whether changing accuracy demands for simple pointing movements leads humans to adjust the feedback control laws that map sensory signals from the moving hand to motor commands. Subjects made repeated pointing movements in a virtual environment to touch a button whose shape varied randomly from trial to trial-between squares, rectangles oriented perpendicular to the movement path, and rectangles oriented parallel to the movement path. Subjects performed the task on a horizontal table but saw the target configuration and a virtual rendering of their pointing finger through a mirror mounted between a monitor and the table. On one-third of trials, the position of the virtual finger was perturbed by ±1 cm either in the movement direction or perpendicular to the movement direction when the finger passed behind an occluder. Subjects corrected quickly for the perturbations despite not consciously noticing them; however, they corrected almost twice as much for perturbations aligned with the narrow dimension of a target than for perturbations aligned with the long dimension. These changes in apparent feedback gain appeared in the kinematic trajectories soon after the time of the perturbations, indicating that they reflect differences in the feedback control law used throughout the duration of movements. The results indicate that the brain adjusts its feedback control law for individual movements "on demand" to fit task demands. Simulations of optimal control laws for a two-joint arm show that accuracy demands alone, coupled with signal-dependent noise, lead to qualitatively the same behavior.