In this study, we investigate the interrelationship between clinical variables and working memory (WM) in Parkinson’s disease (PD). Specifically, the aim of the study was to investigate the relationship between disease duration, dopaminergic medication dosage, and motor disability (UPDRS score) with WM in individuals with PD. Accordingly, we recruited three groups of subjects: unmedicated PD patients, medicated PD patients, and healthy controls. All subjects were tested on three WM tasks: short-delay WM, long-delay WM, and the n-back task. Further, PD encompasses a spectrum that can be classified either into akinesia/rigidity or resting tremor as the predominant motor presentation of the disease. In addition to studying medication effects, we tested WM performance in tremor-dominant and akinesia-dominant patients. We further correlated WM performance with disease duration and medication dosage. We found no difference between medicated and unmedicated patients in the short-delay WM task, but medicated patients outperformed unmedicated patients in the long-delay WM and n-back tasks. Interestingly, we also found that akinesia-dominant patients were more impaired than tremor-dominant patients at various WM measures, which is in agreement with prior studies of the relationship between akinesia symptom and basal ganglia dysfunction. Moreover, the results show that disease duration inversely correlates with more demanding WM tasks (long-delay WM and n-back tasks), but medication dosage positively correlates with demanding WM performance. In sum, our results show that WM impairment in PD patients depend on cognitive domain (simple vs. demanding WM task), subtype of PD patients (tremor- vs. akinesia-dominant), as well as disease duration and medication dosage. Our results have implications for the interrelationship between motor and cognitive processes in PD, and for understanding the role of cognitive training in treating motor symptoms in PD.
Parkinson’s disease (PD) is a neurological disorder, associated with motor dysfunction (Kish, Shannak, & Hornykiewicz, 1988). The main motor symptoms in PD are rigidity, bradykinesia, and resting tremor. The primary neural dysfunction in PD is reduction of dopamine levels in the basal ganglia (Kish et al., 1988). Accordingly, PD patients are prescribed dopaminergic medications, including levodopa and dopamine agonists, to treat their motor symptoms. In addition to motor dysfunction, PD patients also show cognitive impairment (Amos, 2000, Charbonneau et al., 1996, Cooper et al., 1991, Gabrieli et al., 1996, Hodgson et al., 1999, Lees and Smith, 1983, Lewis et al., 2003, Owen, 2004, Owen et al., 1998, Partiot et al., 1996 and Taylor et al., 1986). In this study, we investigate the interrelationship between motor and cognitive variables in PD, and how these measures are affected by dopaminergic medications.
PD is a heterogeneous disorder that encompasses a spectrum of motor symptoms. Some patients present with severe resting tremor (and mild akinesia), while others mainly show severe rigidity and akinesia (Jankovic et al., 1990 and Zaidel et al., 2009). It has been argued that the different motor symptoms in PD are associated with dysfunction to dissociable neural structures. For example, akinesia and bradykinesia are arguably associated with basal ganglia (and corticostriatal circuits) dysfunction, while tremor is perhaps associated with cerebellar, thalamic, and subthalamic nucleus abnormalities (Kassubek et al., 2002, Mure et al., 2011, Probst-Cousin et al., 2003, Weinberger et al., 2009 and Zaidel et al., 2009). Importantly, studies also suggest that the severity of akinesia symptoms is a risk factor for the development of dementia and mild cognitive impairment in PD patients (Poletti and Bonuccelli, in press and Poletti et al., 2011)
In the current study, we test whether different subgroups of PD patients (based on severity of motor symptoms) might show dissociable cognitive performance. While prior research has investigated cognitive dysfunction in subtypes of PD patients (Poletti and Bonuccelli, in press, Poletti et al., 2012b, Poletti et al., 2011 and Vakil and Herishanu-Naaman, 1998), to our knowledge, no study has tested WM function in relation to motor and clinical variables in PD, including disease duration and dopaminergic medication dosage.
