Deficits in Executive Control Function in Parkinson's Disease with Dementia Compared to Alzheimer's Disease and Parkinson's Disease without Dementia

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.