Motor procedural learning in Parkinson's disease

Functional neuroimaging research has repeatedly implicated the striatum in motor procedural learning, but attempts to explore this relation in patients with Parkinson's disease (PD) have yielded inconsistent results. Furthermore, the functional impact of procedural learning impairment is unknown. The present study sought to examine the effects of PD on procedural learning and to determine whether impaired procedural learning affects functional status. The performance of 95 non-demented PD patients on the serial reaction time task (SRTT) was compared with that of 44 demographically matched control subjects. The SRTT is a four-choice reaction time task in which visual stimuli are presented in six blocks of 100 trials either in a repeating sequence of 10 stimuli or randomly. Learning was inferred from the reduction of response times over five successive blocks of repeating sequence trials and from the increase in response times in the sixth random block. In addition, neuropsychological tests of declarative memory, executive and visuospatial functions were administered to all participants. Patients also received quantitative ratings of functional outcome. The two groups did not differ in the learning rate across blocks of repeating sequence trials. However, PD patients were significantly less efficient than controls in acquiring sequence-specific knowledge, although this impairment was relatively small (d= 0.38). Patients with more advanced clinical symptoms tended to show worse performance. Separate analyses of a subgroup of 24 non-medicated patients in the early stages of PD revealed no differences in SRTT performance relative to controls. Neuropsychological testing showed impairments in attention and executive functions, immediate and delayed explicit memory and visuospatial skills in the PD group, but none of the cognitive measures were related to procedural learning. Reduced motor sequence learning in PD patients did not influence their functional status. These findings indicate that procedural learning impairment is not an early feature of PD, but is likely to emerge with progression of the disease, independently of cognitive dysfunction or dopaminergic medication.