THE COORDINATION AND PHASING OF A BILATERAL PREHENSION TASK - THE INFLUENCE OF PARKINSONS-DISEASE

Parkinson's disease patients and control subjects performed a simultan eous bilateral reach-to-grasp task to two different sized objects and then pulled the two objects apart. The first phase of the task (reachi ng-to-grasp) allowed us to examine the issue that impairments in simul taneous movements for Parkinson's disease patients are seen in some ca sks bur not in others. It is suggested that the reason for this select ive impairment is that Parkinson's disease compromises the ability to control multiple task-level degrees of freedom independently and concu rrently (task-level degrees of freedom are defined as the number of in dependent parameters that require specification to perform the task). The first phase was used to test the hypothesis that Parkinson's disea se results in a reduction of degrees of freedom that are independently controlled It was predicted that Parkinson's disease patients would p roduce similar (homologous) movements of the two limbs (a symmetrical pattern) if the target objects have different accuracy requirements wh en they reach bilaterally to the two objects. For bilateral reaches fo r two different-size objects, only the control group showed reliably d ifferent patterns in the two limbs (asymmetrical pattern), while the P arkinson's disease group displayed a symmetrical pattern. These result s provide support for the hypothesis that Parkinson's disease patients have a reduced capability to control multiple task-level degrees of f reedom. The second phase of the task, which involved a transition from position control (reaching-to-grasp) to force control (stabilizing an d pulling) was used to examine the ability of Parkinson's disease pati ents to make transitions between movement tasks and force control. In contrast to control subjects, Parkinson's disease patients produced st aircase patterns for grip and load forces. Furthermore, a breakdown in the parallel co-ordination between grip and load force was observed f or Parkinson's disease patients. These delta suggest that Parkinson's disease disrupts the normal feedforward operations responsible for the co-ordination between grip and load forces.