Self-stimulation pathways in the monkey, Macaca mulatta

Nine rhesus monkeys each had several electrodes implanted in presumed intracranial self-stimulation (ICSS) sites. For 3-4 months, only a single electrode placement supporting ICSS was used in each animal. This established a stable behavioral response to brain stimulation at one brain locus. This procedure reduced the possibility that the response made to brain stimulation at one locus would be related to brain stimulation at a second locus. Lesions were made at these stable ICSS sites and orthograde degeneration from these sites was studied using the Fink-Heimer method. Mapping of ICSS loci indicated that the medial tip of the internal capsule supported very intense levels of self-stimulation. Lesions at ICSS sites produced degeneration which followed the inferior thalamic peduncle. Because of the position of this tract in the medial tip of the internal capsule, it was strongly implicated as one major pathway involved in self-stimulation. The stria medullaris and, to a lesser extent, the stria terminalis were also involved. The major interconnecting structures implicated by silver impregnation of terminal degeneration were the dorsomedial nucleus of the thalamus and the orbitofrontal cortex. An attempt was made to relate these findings to the medial forebrain bundle ICSS system in the rodent.