Fig. 2.

Reaching tasks. A) In this kind of tasks the subject is instructed to reach a target as accurately and fast as possible, by moving planarly on a table. The bottom panels illustrate a PD patient’s movement execution in progress. The top plot is a sketch depicting the corresponding velocity profile of the patient (green), in the component along the direction of movement. For comparison, the typical, bell-shaped velocity profile of a healthy subject is reported in violet. The patients usually show prolonged movement time, or bradykinesia, lower peak velocities and initial undershooting, or hypokinesia, which entails corrective submovements to reach the target. The deceleration part of the movement may be characterized by small fluctuations of velocity. B) In this version of the task, the vision of the moving hand is occluded, while the initial hand position is signaled and the target is always shown. Even though corrective submovements are still possible through proprioceptive sensory feedback, this task is generally performed ballistically in a single shot. Patients show bradykinesia and may also show hypokinesia (not illustrated here). Please, note that the actual velocity profiles, coarsely represented here and in the next figures in their main features, may differ depending on the subject and on the specific experimental conditions. MT, movement time; t₀, starting time of the task; t_onset, onset time of the movement; t_end, ending time of the movement.