Planning for hand shape and arm transport when reaching for objects.

Two early components of object manipulation are shaping the hand appropriately for functional interaction and transporting the arm with appropriate force and spatial precision to the target object. Three experiments addressed whether people plan these two components before the onset of reaching and if so, how the plans are coordinated. Subjects reached for and contacted a series of objects with one of four hand configurations: pinch, poke, palm, and clench. The required configuration was signaled by the object's color; in some conditions its structure provided a redundant cue. The time from object exposure to arm liftoff (reaction time: RT) and the time from liftoff to contact (movement time: MT) were recorded. In Experiment 1, a compatible stimulus-to-hand-shape mapping substantially facilitated RT but not MT, suggesting that the appropriate hand shape was planned prior to reaching. Experiment 2 showed that contact precision, as defined by the stability of the object's support plane, affected MT; a smaller RT effect also suggested some pre-movement planning of arm transport to accommodate precision demands. Experiment 3 combined compatibility and precision manipulations in a single task to test a model which proposes that planning for hand-shape and arm transport occur in parallel, with the onset of reaching deferred until the slower planning process is completed.