Fig. 3.

Reaction time tasks. A) In a simple reaction time task, the subject knows the target position in advance, but he/she is instructed to wait for a “go” signal (imperative stimulus) to start the movement. A reaction time (RT) is measured as the temporal interval between the instant of the “go” signal appearance and the onset time of the movement. The portrayed woman with PD shows a typical delay in RT with respect to healthy controls (akinesia). B) In a choice reaction time task, some clues are given to the subject as potential responses, but the correct one is revealed only after the “go” signal appearance, so that he/she cannot fully prepare the movement in advance. The RT is generally prolonged for both healthy controls and PD patients with respect to simple RT tasks. See the text for detailed discussion. MT, movement time; RT, reaction time; t₀, starting time of the task; t_go, time of the “go” signal appearance; t_onset, onset time of the movement; t_end, ending time of the movement.