Predictive responses in Parkinson's disease: manual keypresses and saccadic eye movements to regular stimulus events.

In a coincidence timing task, Parkinsonian patients and a control group were instructed to synchronize a keypress with the onset of a visual signal which had been preceded by a regular train of warning signals. Although the Parkinsonian group had previously exhibited slower reactions in a conventional simple reaction-time task, they were able to generate predictive responses that fell as close to the target onset as the controls' but showed greater variability. In a second experiment, Parkinsonian patients and controls made saccadic eye movements to a visual target that stepped at regular intervals between two fixed locations. After a few trials all the subjects tended to make predictive saccades that were initiated before the target excursion. However, the Parkinsonian group were slower to develop this strategy and when they did their saccades became considerably more hypometric than those of the controls. Both groups were able to maintain predictive responding even when the visual target disappeared and responses were paced by a buzzer. We concluded that Parkinsonian patients are capable of initiating predictive responses of the eye and the hand, at least in some circumstances, but such responses tend to be inaccurate in execution. This, in turn, may dispose the Parkinsonian patient against predictive movement.