Movement precues in planning and execution of arm movement in Parkinson's disease

The restructuring of planning processes during movement initiation and execution in 7 persons with Parkinson's disease (PD) off medication was compared to that of 7 older controls. Participants performed a two-stroke aiming movement on a digitizer. From a start position, they passed through a center box to one of two targets located to the left and right of the center box. A precue of the target position was presented prior to the imperative stimulus. In 80% of the trials the precue was correct (valid) and in 20% of the trials the precue was incorrect (invalid). Reaction time (RT), movement time (MT) and the fluency (normalized jerk, NJ) were analyzed. In both groups, RT was significantly longer when the invalid precue was presented. However, RT showed no significant group effect or Group x Precue interaction. In contrast, MT and NJ demonstrated a PD effect. For the first stroke of the two-stroke movement, both MT and NJ showed a significant interaction between Group and Precue. The interactions indicated that in the invalid condition where participants were required to reorganize a planned action, PD patients were substantially slower and produced movements that were considerably less smooth as compared to the valid condition while older controls produced similar MT and NJ in the valid and invalid conditions. Although the execution phase is generally expected to be resistant to restructuring effects, these findings suggest that in PD, restructuring a planned action continues beyond the initiation phase into execution.