Cerebellar projections to the superior colliculus in the cat.

The projections from the cerebellar nuclei to the superior colliculus of the cat have been studied by injecting small amounts of horseradish peroxidase into the colliculus and charting the distribution of the labeled neurons in the cerebellar nuclei. The results show that the cerebellar nuclei project to the deep layers of the superior colliculus, especially contralaterally. Of the deep cerebellar nuclei, the lateral sends the most conspicuous projection to the superior colliculus. This projections is topographically organized in a rostrocaudal sense, ending principally in the intermediate layers, and is most dense in the rostral half of the contralateral superior colliculus. The lateral nucleus also projects to the rostral part of the ipsilateral superior colliculus. The nucleus interpositus anterior projects to the rostromedial part of the stratum griseum intermedium of the contralateral colliculus. The caudal pole of the fastigial nucleus projects to the strata grisea intermedium and profundum, mainly to the latter and specifically in the caudal half of both superior colliculi. These results demonstrate that, apart from the possible existence of direct cerebellar connections to the oculomotor nuclei, the visual cerebellum modulates, through monosynaptic projections, the activity of the deep layers of the superior colliculus which is related directly to the regulation of the eye- and head-orienting movements.