The role of executive processes in working memory deficits in Parkinson’s Disease

Idiopathic Parkinson’s disease (PD) impairs working memory, but the exact nature of this deficit in terms of the underlying cognitive mechanisms is not well understood. In this study patients with mild clinical symptoms of PD were compared with matched healthy control subjects on a computerized battery of tests designed to assess spatial working memory and verbal working memory. In the spatial working memory task, subjects were required to recall a sequence of four locations. The verbal working memory task was methodologically identical except for the modality of the stimuli used, requiring subjects to orally recall a sequence of six digits. In either case, half of the sequences were structured in a way that allowed ‘chunking’, while others were unstructured. This manipulation was designed to dissociate the strategic component of task performance from the memory-load component. Mild medicated patients with PD were impaired only on the structured versions of the verbal working memory tasks. The analogous deficit in the spatial working memory was less pronounced. These findings are in agreement with the hypothesis that working memory deficits in PD reflect mainly the executive component of the tasks and that the deficits may be at least partly modality-independent.