RESPONSES TO CUED SIGNALS IN PARKINSONS-DISEASE - DISTINGUISHING BETWEEN DISORDERS OF COGNITION AND OF ACTIVATION

Impairment of movement execution in Parkinson's disease could be due t o disorders of cognition and/or of activation. These two factors are h ard to separate by measuring response times only. Therefore, in this s tudy response force and event-related EEG potentials were measured con tinuously during tasks in which subjects had to respond to cued signal s. Fifteen patients with Parkinson's disease and 15 healthy subjects w ere studied during two tasks: (i) the 'clock task', in which the signa l's identity was fully precued bur 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 att ention, and the validity task sometimes required fast switching. The p atients generally responded slower than control subjects. In the clock task, the response times of both groups changed to the same extent wi th presentation time, whereas in the validity task the patients were a dditionally slower than the control group with invalidly cued signals. The patients generally had a weaker response force and a lower rate o f force production. In the clock task, both force measures changed wit h presentation time in the central group only, whereas in the validity task, the two measures increased in bath groups to the same extent wi th invalidly cued signals. The contingent negative variation amplitude s in the patients' event-related EEG potentials were reduced reflectin g reduced activation of movement preparation, whereas lateralization o f the motor cortices (i.e. the lateralized readiness potential) did no t differ significantly between groups, reflecting unimpaired response selection. Force and contingent negative variation were generally redu ced in the patients showing that their general slowing is at least par tially 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 pr oblem of cognition The task-specific deficits may reflect a basic dile mma for patients with Parkinson's disease; cognitive problems may aris e in complex tasks but disorders of activation may become pronounced i n more simple, monotonous tasks.