The effect of levodopa +/- tDCS on procedural motor learning and cortical excitability will be studied in healthy volunteers and stroke patients. Then the investigator plans to delineate the underlying mechanisms of this effect by exploring N-methyl-D-asparate (NMDA) receptor-dependency of levodopa-enhanced learning and changes in activation and connectivity (using functional magnetic resonance imaging) in the respective neural networks resulting from the interaction of learning and dopaminergic neuromodulation.