Discretely scaling a continuous movement: Parkinsons disease patients have more variable movement trajectories

Research suggests that Parkinson's Disease (PD) patients have difficulty scaling the magnitude of their movements. Longstaff et al (2001) showed that patients could continuously scale a drawing movement (spiral) up to a diameter of 5cm like elderly controls, but their trajectories were more variable and there was a smaller distance between each revolution. The present study tests the ability of patients and controls to perform a continuous drawing task with discrete scaling. Subjects were 11 idiopathic PD patients (ages 55 to 81; 8 male, 3 female; Hoehn & Yahr stages 2 and 3; off medication) and 13 elderly controls (ages 62 to 81; 5 male, 6 female). Subjects drew circles on a digitizer matching the size of target circles of diameter 1, 1.5, 2, 3 and 5cm. Five revolutions of each were drawn in the conditions accurate and fast and accurate. Both groups drew circles a similar size, increasing with target size but undershooting the larger circles. Circle size did not change with condition. Both groups drew circles at a similar speed, which was greater in the fast condition. Speed also increased with target size. Both groups increased their variability (radius SD) with increasing target size and in the fast condition, with PD patients having a greater variability than controls in all conditions. This suggests that patients can discretely scale their movements like controls, when needed, but their trajectories are more variable. It is argued that they produce smaller movements partly as a strategy to minimize variability.