The Reaction Time Costs Of Trajectory Planning

Issuing a goal-directed action requires specifying the goal of the action as well as planning the hand trajectory to obtain it. Accumulating results suggest that planning a straight point-to-point trajectory is more efficient and likely to involve simpler optimization process compared to the planning of trajectories with more complex shapes (e.g., curved trajectories). We sought to find evidence for the qualitative difference between the two planning modes through the investigation of reaction times (RT) in a pointing task performed with the wrist. In experiment 1, 18 subjects performed delayed straight and curved via-point reaching movements to arrays of 2 or 8 targets. Both trajectory type and number of possible targets affected RT. In experiment 2 (N=14), we demonstrate a switching cost between the issuing of the two types of trajectories, irrespective of changes in target position. Unexpectedly, trajectory type did not affect RT in experiment 2, likely due to the lack of target pre-cuing in experiment 2. Our results suggest that the planning of curved and straight trajectories differ in their memory load during pre-planning and requires a time-consuming update of the motor commands when switching between straight and curved plans.