Cognitive deficits in Parkinson's disease.

Neuropsychological investigations of patients with Parkinson's disease have shown specific impairments even in the early stages of the disease, which include deficit of behavioural regulation in sorting or planning tasks, defective use of memory stores, and impaired manipulation of internal representation of visuospatial stimuli. These deficits, reported in a disease which predominantly involves subcortical structures, have drawn attention to a potential role of the basal ganglia in cognitive processes. Given the modulatory role of the basal ganglia, these disorders might result from more fundamental deficits concerning the allocation of attentional resources, the temporal organization of behaviour, the maintenance of representations in working memory or the self-elaboration of internal strategies, all of which resemble dysfunctions of processes that are commonly considered to be controlled by the frontal lobes. This suggests a functional continuity or complementarity between the basal ganglia and association areas of the prefrontal cortex. The recent description in primates of segregated loops that interconnect discrete regions of the caudate nucleus to the dorsolateral and orbitofrontal regions of the prefrontal cortex via the thalamus may give some support to this hypothesis. Alternatively, degeneration of the ascending cholinergic and catecholaminergic neuronal systems may contribute, at least in part, to the occurrence of this frontal-lobe-like symptomatology associated with Parkinson's disease.