Advance planning in sequential pick-and-place tasks

It has been suggested that the kinematics of a reach–to–grasp movement, performed within an action sequence, vary depending on the action goal and the properties of subsequent movement segments (action context effect). The aim of this study was to investigate whether the action context also affects action sequences that consist of several grasping movements directed toward different target objects. Twenty participants were asked to perform a sequence in which they grasped a cylinder, placed it into a target area, and subsequently grasped and displaced a target bar of a certain orientation. We specifically tested whether the orientation of the target bar being grasped in the last movement segment influenced the grip orientation adapted to grasp and place the cylinder in the preceding segments. When all movement segments within the sequence were easy to perform, results indeed showed that grip orientation chosen in the early movement segments depended on the forthcoming motor demands, suggesting a holistic planning process. In contrast, high accuracy demands in specifying a movement segment reduced the ability of the motor system to plan and organize the movement sequence into larger chunks, thus causing a shift toward sequential performance. Additionally, making the placing task more difficult resulted in prolonged reaction times and increased the movement times of all other movement segments.