Objective: Besides motor deficits, patients with Parkinson's disease can suffer from cognitive
and attentional decline, particularly from a lack of executive control functions.
This specific problem was analyzed in view of the motor and cognitive symptomatology
occurring in this condition. Method: Target detection, response inhibition and response switching were tested in outpatients
with Parkinson's disease with dementia (PDD, n=9), Parkinson's disease without dementia
(PD, n=13), Alzheimer's disease (AD, n=3) and controls (CO, n=9), using a visual Go/NoGo
paradigm. PDD and AD were matched for level of Mini-Mental State Examination. Participants
were instructed either to perform or to inhibit a button press with the right or left
index finger due to a Go- (green rectangle) or NoGo-Cue (red rectangle). At intervals
of 2 seconds these stimuli were preceded by Warning Cues (arrow), indicative for the
side of the putative motor response. In the non-switch condition the arrow was presented
only to one side. In the switch condition the upcoming arrow indicated either to the
right or left side. Results: In the non-switch condition, patients with PDD had a lower detection rate for targets
(Go-trials) than patients with AD, PD and CO. In the switch condition, patients with
PDD and AD had a comparably low detection rate for the Go-trials, whereas PD patients
performed as CO. Further, response inhibition (NoGo-trials) was more deficient in
patients with PDD than in patients with AD, PD and CO, irrespective of the condition.
Throughout all tasks, patients with PD had a similar performance as CO, independently
of their motor score in the Unified Parkinson's Disease Rating Scale. Conclusions: For patients with PDD, specific deficits of (i) target detection in the non-switch
condition and (ii) general response inhibition were identified, as compared to patients
with AD. On the other hand, target detection in the switch condition was similarly
impaired in PDD and AD patients. Interestingly, no specific deficit of PD patients
was unmasked by the tests used, if compared to controls. We conclude that PDD is a
specific condition of dementia, which can be differentiated from AD by adequate tests
on executive control functions. As executive dysfunction in Parkinson's disease seems
to affect as an inhibitory as excitatory control, putative pathophysiological parallels
to the motor deficits, impairing as movement initiation and cessation, should be addressed
in future studies.
