Brain, Vol 120, Issue 8 1355-1375, Copyright © 1997 by Oxford University Press
E Wascher, R Verleger, P Vieregge, P Jaskowski, S Koch and D Kompf
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.
ARTICLES
Responses to cued signals in Parkinson's disease. Distinguishing between disorders of cognition and of activation
Department of Neurology, Medical University of Lubeck, Germany.
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