Sensations Evoked by Stimulation in the Midbrain of Man

S EVERE pain evokes in man a variety of complex physiological and psychological events that are poorly understood. Our current knowledge of the neuroanatomica1 and physiological substrates for painful experiences have been gathered from clinical observations in humans or by inference from experiments with mute animals. This report concerns observations that have been made in persons suffering from severe chronic pain who were to undergo treatment for their pain by means of stereotaxic lesions localized in the dorsolateral tegmentum that would interrupt the ascending pain pathways. Before placement of the therapeutic lesions, fine electrodes were introduced into the midbrain and thalamus and electrical stimulation of these neural regions was carried out while the patient was alert. A group of 15 persons with intractable central pains was selected for stereotaxic mesencephalic tractotomy; in 12, chronic depth electrodes were implanted in the mesencephalon and thalamus preceding therapy (Table 1). Of the 12 patients who were electrically stimulated, three had phantom arm pain and one a severe causalgic arm pain. Eight persons suffered from central pain or dysesthesia. The selection of this latter group of patients was based on Riddoch's definition of central pain as spontaneous pain and painful over-reaction to objective stimulation resulting from lesions confined to the substance of the central nervous system, including dysesthesia of any kind. ''1~ The cervical cord or the brachial plexus were the sites of pathologic involvement in the patients with phantom limb pain and the causalgia. In the eight patients with central dysesthesia, the pain was due to pathologic involvement at five sites in the central nervous system. These sites included the spinal cord, medulla, mesencephalon, thalamus,