The pathogenesis of levodopa-related motor complications (motor fluctuations and dyskinesias) is still far from clear, but there is converging evidence to suggest that presynaptic mechanisms play a major role. In fact, we have recently developed a mathematical probability model that can accommodate the whole spectrum of motor complications often seen in Parkinson disease (PD).1 The model is based on dynamic studies using positron emission tomography2,3 and other sources of research evidence.1