PROGRAMMING OF EXTENT AND DIRECTION IN HUMAN REACHING MOVEMENTS

The present report summarizes a series of experiments conducted to det ermine the nature of the sensorimotor transformations underlying reach ing movements. We analyzed trajectories and endpoint errors of movemen ts performed by normal human subjects. Different factors were found to affect direction and extent variability and to account for systematic errors. Anisotropy in limb inertia was found to produce systematic er rors in movement extent. Cognitive factors gave rise to systematic err ors in movement direction. Based on these results we hypothesize that extent and direction constitute the dimensions coordinate system cente red at the hand that is used to plan the trajectories of reaching move ments. It is proposed that use of this parallel system for specifying movement parameters simplifies trajectory planning but gives rise to s ystematic errors.