Pain threshold alterations by brain stimulation in the monkey

✓ Experiments were carried out to indicate whether discrete brain stimulation would alter the pain threshold in awake squirrel monkeys subjected to pain by application of stepwise increments of current to the tail. It was found that concurrent brain stimulation will either lower the pain threshold (if applied to the central tegmental tract, ventral posteromedial nuclear group, or occasionally the periaqueductal gray matter); appear to elevate the threshold (if applied to the dorsomedial nucleus); transiently elevate the pain threshold followed shortly by a lowered threshold (if applied to the centromedian-parafascicular complex); or consistently elevate the threshold (if applied to the caudate nucleus, septal-preoptic region, or medial geniculate).