Planning of reach-and-grasp movements: Effects of validity and type of object information

Individuals are assumed to plan reach-and-grasp movements by using two sepa rate processes. In 1 of the processes, extrinsic (direction, distance) obje ct information is used in planning the movement of the arm that transports the hand to the target location (transport planning); whereas in the other, intrinsic (shape) object information is used in planning the preshaping of the hand and the grasping of the target object (manipulation planning). In 2 experiments, the authors used primes to provide information to participa nts (N = 5, Experiment 1; N = 6, Experiment 2) about extrinsic and intrinsi c object properties. The validity of the prime information was systematical ly varied. The primes were succeeded by a cue, which always correctly ident ified the location and shape of the target object. Reaction times were reco rded. Four models of transport and manipulation planning were tested. The o nly model that was consistent with the data was I in which arm transport an d object manipulation planning were postulated to be independent processes that operate partially in parallel. The authors suggest that the processes involved in motor planning before execution are primarily concerned with th e geometric aspects of the upcoming movement but not with the temporal deta ils of its execution.