The co-ordination and phasing of a bilateral prehension task. The influence of Parkinson's disease.

Parkinson's disease patients and control subjects performed a simultaneous bilateral reach-to-grasp task to two different sized objects and then pulled the two objects apart. The first phase of the task (reaching-to-grasp) allowed us to examine the issue that impairments in simultaneous movements for Parkinson's disease patients are seen in some tasks but not in others. It is suggested that the reason for this selective impairment is that Parkinson's disease compromises the ability to control multiple task-level degrees of freedom independently and concurrently (task-level degrees of freedom are defined as the number of independent parameters that require specification to perform the task). The first phase was used to test the hypothesis that Parkinson's disease results in a reduction of degrees of freedom that are independently controlled. It was predicted that Parkinson's disease patients would produce similar (homologous) movements of the two limbs (a symmetrical pattern) if the target objects have different accuracy requirements when they reach bilaterally to the two objects. For bilateral reaches for two different-size objects, only the control group showed reliably different patterns in the two limbs (asymmetrical pattern), while the Parkinson's disease group displayed a symmetrical pattern. These results provide support for the hypothesis that Parkinson's disease patients have a reduced capability to control multiple task-level degrees of freedom. The second phase of the task, which involved a transition from position control (reaching-to-grasp) to force control (stabilizing and pulling) was used to examine the ability of Parkinson's disease patients to make transitions between movement tasks and force control. In contrast to control subjects, Parkinson's disease patients produced staircase patterns for grip and load forces. Furthermore, a breakdown in the parallel co-ordination between grip and load force was observed for Parkinson's disease patients. These data suggest that Parkinson's disease disrupts the normal feedforward operations responsible for the co-ordination between grip and load forces.