Stability of reach-to-grasp movement patterns in Parkinson's disease

The performance of patients with Parkinson's disease on two reach-to-grasp tasks was compared with that of age-matched control subjects. The aim of the study was to determine whether Parkinson's disease patients have problems coordinating concurrently executed tasks within the same system of effectors in a natural context and whether such problems would be exacerbated by increases in task difficulty. We examined how subjects concurrently executed the transport and grasp components of reach-to-grasp movements in the presence of two types of change in task demands: (i) increases in demands for accurate digit pad placement and (ii) use of two reach-to-grasp tasks, i.e. the standard unimanual task and a bimanual task which increased the control and coordination demands relative to the unimanual task. If Parkinson's disease patients have coordination problems they should demonstrate increased impairment with increasing accuracy demands and in the bimanual task; any such differences should be absent or much smaller in the control group. The Parkinson's disease group showed substantial impairments in all conditions, moving about 30% slower than the control group, with much increased jerking and with signs of difficulty controlling the speed of movement. However, there were no consistent indications that the Parkinson's disease group were differentially impaired on the bimanual task nor that movement deficits increased with increasing accuracy requirements. Grasp and transport components were coordinated similarly by Parkinson's disease and control groups in both reach-to-grasp tasks, and the Parkinson's disease group co-ordinated the two limbs in the bimanual task effectively and in a fashion similar to that of the control group. These results are interpreted to mean that higher levels (effector-independent levels) of motor programming are preserved in Parkinson's disease and that execution of a motor programme need not be compromised by increasing the number of muscle-/joint-level degrees of freedom which are used.