PLANNING FOR HAND SHAPE AND ARM TRANSPORT WHEN REACHING FOR OBJECTS

Two early components of object manipulation are shaping the hand appro priately for functional interaction and transporting the arm with appr opriate force and spatial precision to the target object. Three experi ments addressed whether people plan these two components before the on set of reaching and if so, how the plans are coordinated. Subjects rea ched for and contacted a series of objects with one of four hand confi gurations: pinch, poke, palm, and clench. The required configuration w as signaled by the object's color; in some conditions its structure pr ovided 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-sh ape mapping substantially facilitated RT but not MT, suggesting that t he appropriate hand shape was planned prior to reaching. Experiment 2 showed that contact precision, as defined by the stability of the obje ct's support plane, affected MT; a smaller RT effect also suggested so me pre-movement planning of arm transport to accommodate precision dem ands. 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.