Thalamic connections to insular and opercular cortex of monkey

Subpial resection of insular and opercular cortex was performed in ten hemispheres of six macaque monkeys. Retrograde degeneration was studied in myelin and cresyl violet stained serial sections of whole brain specimens follwing survival of 12 to 15 months. Lesions restricted to teh insula, effectively sparing the orelying opercula, caused no retrograde degenration in thalamus. Specifically, nucleus ventralis anterior, nucleus ventralis posteromedialis pars parvocelluliaris, and nucleus ventralis posterior inferior were normal. Lesions which involved orbitofrontal cortex produced the expected degeneration of the ventral division of paralamellar nucleus medialis dorsalis (medialis dorsalis pars multiformis). Surprisingly, minimal but unequivocal degeneration was seen in this nucleus when lesiions were restricted to the superior temporal plane; it was more marked when the parietal operculum was included in the lesion. With destruction of the white matter of the parietal or frontal operculum, degeneration was apparent surrounding the centrum medianum, most conspicuously in ventral paralamellar medialis dorsalis and parvocelular nucleus ventralis posteromedialis. Increased gliosis without nerve cell destruction in the basomedial part of nucleus ventralis anterior was present in two specimens with anterior insular ablatioins presumably as the result of interruption of cortico‐thalamic projections from orbitofrontal cortex.