<qa>  What is electroencephalography?The study will last approximately 2 to 3 weeks for healthy volunteers and 7 to 8 weeks for patients. Participants will undergo the following baseline evaluations: electroencephalography EEG, a measure of brain wave activity; polysomnography PSG, an overnight sleep study; electromyography EMG, a measure of muscle activity; and spinal flexor reflex studies spinal cord responses to electrical stimulation. For these studies, several electrodes are attached to the scalp or to the legs and arms using a cream that helps conduct brain signals through the cables to be recorded. The nervous system activity can be assessed at rest or after stimulation with an electric current. For EMG, a small needle is inserted into a few muscles and the subject is asked to relax or to contract the muscles. The electrical activity of the muscle cells is recorded and analyzed by a computer. Spinal flexor reflex studies usually involve stimulating the nerves in the legs with very brief electric currents of mild to moderate intensity. Flexor reflex studies will be done early in the night while the subject is awake and also during sleep. Brain, muscle and breathing activities will be monitored continuously during sleep. 