The study determines whether standard medical care (dopamine) affects learning and retention of a postural stepping task in people with Parkinson's disease (PD) and whether training on a postural stepping task generalises to performance on an untrained postural task. Half the participants will train on the stepping task after they have taken their first dose of dopamine for the day (i.e. "on" medication state) while the other half will train on the same stepping task before taking their first daily dose of dopamine (i.e. "off" medication state).