Fig. 1

Three testing conditions on the graphic tablet. In S1, the subject was instructed to follow the thin line and start from the center of the spiral. The thin line was the target spiral and the thick line was the trace by the patient. In S2, the patients were instructed to draw the spiral on a blank graphic tablet without any constraints (with freehand). The thick line was the drawing. In S3, the subject was instructed to follow a thin guiding point. The guiding point moved at a constant speed. The thick line was the trace by the subject and the thin line was the trajectory of the guiding point