Where grasps are made reveals how grasps are planned: generation and recall of motor plans

The end-state comfort effect (Rosenbaum et al. 1990, 1992, 1993, 1996) predicts that people will grasp an object for transport in a way that allows joints to be in mid-range at the end of the transport. When participants in the present study took hold of a vertical cylinder to move it to a new position, grasp heights on the cylinder were inversely related to the height of the target position, as predicted by the end-state comfort effect. This demonstrates that where people grasp objects can give insight into the planning of movement. In the computational model of motor planning developed by Rosenbaum et al. (1995, 2001) it is assumed that goal postures are planned by a two-stage process of recall and generation. The distinction between recall and generation had not so far been tested. In the present study, the pattern of grasp heights in successive transports was consistent with the view that participants generated a plan the first time they moved the cylinder between two points, and that they subsequently recalled what they had done before, making small adjustments to that recalled plan. This outcome provides evidence for distinct effects of recall and generation on movement planning.