The strength of stimulation can be individually adjusted by changing the voltage of stimulation. A too high voltage will produce side effects (e.g. seizures) while a too low voltage will not provide pain control. The aim of this study is to analyze the optimal stimulation parameters in patients already implanted with a motor cortex stimulation who have received good pain relief. The actual voltage may vary widely between patients but the investigators feel that there may be an "ideal" voltage if it is measured as a percentage of motor threshold (PMT). If motor threshold is the stimulation voltage that can evoke a muscle contraction then a PMT = 80% would be a voltage that was eighty percent of that value. Although the actual voltage may be widely different between patients, the percentage needed may be very similar. The investigators therefore plan to measure the effect of different percentages of PMT in patients already being treated with motor cortex stimulation.