Systematic scaling of target width: dynamics, planning, and feedback.

The target width of a single target in a two-target reciprocal aiming task was scaled from small (ID = 5.85) to large (ID = 2.85) and large-to-small within individual trials with movement amplitude fixed. Scaling target width produced a transition in the end-effector's dynamics and based on a measure of movement harmonicity, the transition was sensitive to the initial conditions but not to the direction of target width scaling. Hysteresis emerged in a variety of kinematic measures suggesting that the interdependency of planning and feedback control processes was sensitive to initial conditions as well as the direction of target width scaling. Practice increased the efficiency of the reciprocal movements and produced changes in movement time and the measure of harmonic motion that revealed a tuning of the end-effector's dynamics to cyclical motion over as large of range of IDs as possible. The tuning occurred through the modulation of time spent accelerating and decelerating the end-effector for IDs outside the range of 3.85-4.26. The results are discussed with reference to a critical ID boundary that separates regions of parameter space wherein the end-effector's dynamics are more cyclical (limit-cycle) or discrete (fixed-point) in nature.