The effect of an external load on the force and timing components of mentally represented actions

The chronometry of real and imagined movements was investigated in a group of eight subjects under varying conditions. The visually-guided pointing ta sk was used to investigate the speed for accuracy trade-offs that occur as target size is varied for both real and imagined performance. The task was performed both with and without an external load of 2 kg. For the no-load c ondition and load conditions, the speed for accuracy trade-off for both rea l and imagined performance conformed to Fitts' law. Movement durations of r eal movements remained largely unaffected by the addition of the load, howe ver, movement durations of imagined movements increased significantly with the addition of the load. These patterns of results suggest that the weight disrupted the force calculation component of imagined movements but not th e relative timing. (C) 2000 Elsevier Science B.V. All rights reserved.