Dopaminergic modulation of the planning phase of skill acquisition in Parkinson's disease

Patients with Parkinson's disease (PD) are often impaired when performing motor acts and in the acquisition of new motor skills. However, the role of dopamine in developing plans for skill acquisition is unclear. To assess the role of dopamine on the planning of actions, we tested 12 PD and 12 matched normal participants on two skill acquisition tasks matched for motor demands, but varying in requirements for planning. The participants with PD were tested on these tasks when they were on and off dopaminergic medications. To minimize influence of movement related deficits, the subjects used a computer track-pointer that generated the required straight lines when the subjects applied a slight force and clicked the track-pointer to initiate and terminate each line segment. The amount of time the track-pointer was deflected determined the line lengths, while clicking of the mouse determined the location of the line. The simple figure replication task only required the subjects to repeatedly generate lines of two sizes, while the complex figure replication task required subjects to generate lines of different sizes. Thus, this complex task demanded more anticipatory planning. Compared to controls, the subjects with PD were slower to learn the programs needed to produce these figures and produced figures with reduced amplitudes on both the simple and complex tasks. Dopamine treatment, however, only improved the speed of figure completion on the complex task, suggesting that dopamine is important in action planning.