The inferior olive of Saimiri sciureus: olivocerebellar projections to the anterior lobe.

Olivocerebellar projections to lobules IV and V of the anterior lobe of the squirrel monkey (Saimiri sciureus) cerebellum were studied using a wheat germ agglutinin-horseradish peroxidase conjugate (WGA-HRP). Placements of WGA-HRP in the cerebellar cortex, labeled neurons in different subgroups of the contralateral inferior olive depending on the position of the injection site. Injections in medial aspects of lobules IV and V labeled cells in subgroups a and b of the caudal medial accessory olive (MAO) and in the lateral dorsal accessory olive. Rostral parts of subgroup a of MAO and medial dorsal accessory olive contained labeled neurons when injections were made in intermediate areas of the cerebellar cortex. Subsequent to involvement of more lateral parts of lobules IV and V, labeled cells were found in the dorsal lamella and lateral bend of the principal olive. When the position and size of injection sites were compared to distribution of retrogradely labeled olivary somata, it was clear that zones A, B, C1, C2, C3 and D were present in lobules IV and V of squirrel monkey. The general pattern of olivary labeling in this study is similar to that reported for other species.