Flexibility in the control of rapid aiming actions.

Across three different task conditions, the adaptability of reciprocal aiming movements was investigated. Task difficulty was manipulated by changing ID, with 9 IDs between 2.5 and 6.5 tested. Reciprocal aiming movements were performed with ID scaled (predictable) in a trial in a decreasing (high 6.5-low 2.5) or increasing manner (low 2.5-high 6.5) or with ID constant in a trial and changed randomly across trials. Movement time scaled linearly with ID in both the scaling ID and control ID presentations. A critical ID boundary (IDC) was identified, and the adaptation of aiming movements was a function of this critical boundary. For IDs < IDC, the results are interpreted as representing a predominance for pre-planned control based on a dwell time measure and a symmetry ratio measure (time spent accelerating-decelerating the limb). Within this ID range, movement harmonicity was changed to a greater extent when ID was scaled in a predictable direction as compared to being presented in a random manner. For IDs > IDC, the findings suggest a predominance for feedback control based on the dwell time and symmetry ratio measure. Within this ID range, the absolute time spent decelerating was increased, possibly to insure accuracy and minimize MT, with the predictable changes associated with an increase in ID needing less time devoted to feedback processing compared to the other ID presentations. The results are consistent with the theoretical position that aiming motions may be controlled by a limit cycle mechanism with ID < IDC, while aiming motions may be controlled by a fixed-point mechanism with ID > IDC. The results suggest that the ability of the motor system to adapt to both scaled and random changes in ID revolves around a modulation of pre-planned and feedback-based control processes.