Responses to cued signals in Parkinson's disease. Distinguishing between disorders of cognition and of activation.
Impairment of movement execution in Parkinson's disease could be due to disorders of cognition and/or of activation. These two factors are hard to separate by measuring response times only. Therefore, in this study response force and event-related EEG potentials were measured continuously during tasks in which subjects had to respond to cued signals. Fifteen patients with Parkinson's disease and 15 healthy subjects were studied during two tasks: (i) the 'clock task', in which the signal's identity was fully precued but its presentation time was uncertain and (ii) the 'validity task' in which the cue did not always predict the response validly. Thus, the clock task required more sustained attention, and the validity task sometimes required fast switching. The patients generally responded slower than control subjects. In the clock task, the response times of both groups changed to the same extent with presentation time, whereas in the validity task the patients were additionally slower than the control group with invalidly cued signals. The patients generally had a weaker response force and a lower rate of force production. In the clock task, both force measures changed with presentation time in the control group only, whereas in the validity task, the two measures increased in both groups to the same extent with invalidly cued signals. The contingent negative variation amplitudes in the patients' event-related EEG potentials were reduced, reflecting reduced activation of movement preparation, whereas lateralization of the motor cortices (i.e. the lateralized readiness potential) did not differ significantly between groups, reflecting unimpaired response selection. Force and contingent negative variation were generally reduced in the patients showing that their general slowing is at least partially due to impaired activation. Task-specific problems added to the general activation deficit; the lack of modulation of response force by presentation time revealed pronounced deficits of activation in the monotonous clock task. The specific delay of responses with invalidly cued signals, unparalleled by activation measures, might suggest a problem of cognition. The task-specific deficits may reflect a basic dilemma for patients with Parkinson's disease: cognitive problems may arise in complex tasks but disorders of activation may become pronounced in more simple, monotonous tasks.