Response slowing in Parkinson's disease: a psychophysiological analysis of premotor and motor processes.

The mechanisms responsible for reaction time slowing in Parkinson's disease were investigated using movement-related potentials in a choice reaction time task. Parkinson's disease patients and control subjects were required to respond with the left or right hand to indicate whether a visual stimulus was relatively large or small. The difficulty of the size discrimination was manipulated, as was the complexity of the manual response (single key press versus sequence of three key presses). Behavioural responses of Parkinson's disease patients were slower than those of control subjects, especially when complex responses were required. Moreover, the timing of movement-related potentials indicated that motor processes clearly required extra time, relative to control subjects, for Parkinson's disease patients making complex responses. In addition, delayed onset of the movement-related potentials indicated that one or more premotor processes are also slowed in these patients.