 Somatosensory evoked potentials SEPs - SEP are recorded through surface electrodes to measure conduction in the peripheral nerves, cervical and lumbosacral spinal cord, deep brain structures, and sensory cortex. Surface EMG electrodes are placed over the adductor pollicis brevis and soleus muscles. Stimulation electrodes are placed over the median nerve at the wrist and tibial nerve in the popliteal fossa of the right and left upper and lower limbs. Ground straps are placed on the forearms and thighs. Repeated single-pulse electrical stimulation is delivered at intensities of 1 and 1.5 times motor threshold for the recorded muscles for each of the four nerves evaluated. Averaged responses for up to 256 stimuli will be recorded for each of these two intensities for the right and left median and tibial nerves. Simultaneous bilateral stimulation will be applied if no recognizable responses are recorded from the scalp leads. Stimulation delivery rates will not exceed 2 per second. 