Visual pontocerebellar projections in the macaque.

The cerebellum plays an important role in the visual guidance of movement. In order to understand the anatomical basis of visuomotor control, we studied the projection of pontine visual cells onto the cerebellar cortex of monkeys. Wheat germ agglutinin horseradish peroxidase was injected into the dorsolateral pons two monkeys. Retrogradely labelled cells were mapped in the cerebral cortex and superior colliculus, and orthogradely labelled fibers in the cerebellar cortex. The largest number of retrogradely labelled cells in the cerebral cortex was in a group of medial extrastriate visual areas. The major cerebellar target of these dorsolateral pontine cells is the dorsal paraflocculus. There is a weaker projection to the uvula, paramedian lobe, and Crus II, and a sparse but definite projection to the ventral paraflocculus. There are virtually no projections to the flocculus. There are sparse ipsilateral pontocerebellar projections to these same regions of cerebellar cortex. In nine monkeys, we made small injections of the tracer into the cerebellar cortex and studied the location of retrogradely filled cells in the pontine nuclei and inferior olive. Injections into the dorsal paraflocculus or rostral folia of the uvula retrogradely labelled large numbers of cells in the dorsolateral region of the contralateral pontine nuclei. Labelled cells were found ipsilaterally, but in reduced numbers. Injections outside of these areas in ventral paraflocculus or paramedian lobule labelled far fewer cells in this region of the pons. We conclude that the principal source of cerebral cortical visual information arises from a medial group of extrastriate visual areas and is relayed through cells in the dorsolateral pontine nuclei. The principal target of pontine visual cells is the dorsal paraflocculus.