Initiation and execution of predictable and unpredictable movements in Parkinson's disease.

Nine patients with Parkinson's disease were compared with aged-matched normal controls on a tracking and a reaction time task. In the tracking task, subjects had to follow a target moving either predictably or unpredictably at various frequencies. There were no differences between the groups on this task. Both groups were able to use the predictable target movements to eliminate the lag between the target movements and their own movements, indicating that they could emit preprogrammed responses which did not depend on visual feedback. The reaction time task also contrasted predictable and unpredictable movements since the subjects were sometimes told in advance which finger they would have to lift in response to the imperative signal. The patients were significantly less able than controls to use this predictability to reduce their reaction times. It is concluded that parkinsonian patients can use prior information to control the form of a preprogrammed movement, but cannot use this information to help initiate such a movement.