Which motor disorder in Parkinson's disease indicates the true motor function of the basal ganglia?

Parkinson's disease in its earlier stages is argued to be the best available model for human basal ganglia dysfunction. The negative motor symptoms of Parkinson's disease are considered to give the greatest clue to normal function of this region of the brain. Particular attention is given to disorders of movement. These include delayed initiation and slowed execution of simple fast movements, due to abnormal specification of initial agonist activity. This might compromise predictive motor action, but this is shown to be preserved in Parkinson's disease. Disorders of more complex movements, such as repetitive, concurrent and sequential motor actions, are also abnormal in Parkinson's disease. These various defects are discussed in terms of a motor strategy involving the selection and sequencing of motor programmes to form a motor plan, and the initiation and execution of that motor plan. On the evidence available, it is suggested that patients with Parkinson's disease are unable to automatically execute learnt motor plans.