Motor initiation versus execution in normal and Parkinson's disease subjects

We studied motor initiation and execution using wrist extension movements to changing target locations in eight normal subjects and nine Parkinson's disease (PD) patients before and after medications. Late changes resulted in double trajectories, indicating commitment to the initial target acquisition program followed by a correcting movement. There was compensation for earlier changes, even after onset of agonist muscle activity, resulting in a single trajectory, implying that the original trajectory had not yet been specified. However, movements were slowed in PD patients implying an abnormality in the content of the target acquisition program but not in the timing of its specification. In PD patients, the timing of the second movement onset correlated best with the timing of target location change and did not depend on initial movement completion. Thus, PD patients were able to program the second movement while the first movement was under way.