Spinal cord monitoring, as currently practiced, requires a source of sensory stimulus, transmitted through filters and amplifiers, to result in an enhanced signal, which is recorded and studied. Three techniques are under investigation. In one, the stimulation and responses are recorded directly from the dura. In another, the stimulus is applied to a peripheral nerve and the response is gathered from vertebral bone. The third utilizes responses in the form of cortical evoked potentials, with the stimulus applied to peripheral nerves.