Task dependence of primate arm postures.

We studied the relations between arm posture and hand location as monkeys performed tasks involving three-dimensional arm movements. Two specific questions were addressed: how reliable are these relations from trial to trial and from day to day, and are the arm postures dictated only by the location of the hand or are the postures also dependent on the task being performed? Based on the variable errors in linear regressions, we found that monkeys performed the tasks in a very stereotypic fashion from trial-to-trial: for a given monkey and task, knowing the position of the hand was sufficient to determine the posture of the arm. These relations did not change from day to day; the relation between hand location and posture was stable over the course of the experiments. In some of the tasks, the postures employed by the monkeys were strikingly similar to those observed in human psychophysical studies. The relations between hand location and arm posture exhibited a large degree of task dependence. Changing the required orientation of the hand resulted in changes in the arm posture.