The lobulus petrosus of the paraflocculus relays cortical visual inputs to the posterior interposed and lateral cerebellar nuclei: an anterograde and retrograde tracing study in the monkey.

The afferent and efferent projections of the lobulus petrosus (LP) of the paraflocculus were examined by retrograde tracing with cholera toxin subunit B (CTB) and anterograde tracing with CTB or biotinylated dextran (BD) in the macaque. Following injections of the tracers into the LP, labeled neurons were seen mainly in the lateral, dorsal, paramedian and dorsolateral parts of the contralateral pontine nuclei (Pn) and in the ventromedial part of the principal olivary nucleus and the lateral pole of the medial accessory olivary nucleus at the rostral levels. Labeled terminals were seen ipsilaterally in the ventrolateral part of the posterior interposed nucleus and the basal interstitial nucleus of the cerebellum at their rostral levels, and in the ventromedial part of the lateral cerebellar nucleus. To reveal the connections between the LP-projecting pontine neurons and the corticopontine axons, BD was injected into the left prearcuate region or extrastriate visual areas, and CTB into the right LP. Labeled terminals from the prearcuate region were seen mainly in the left dorsal, medial and paramedian Pn, whereas those from the extrastriate visual areas were seen in the lateral and dorsolateral Pn. The distribution of the LP-projecting neurons overlapped that of the terminals from the extrastriate visual areas but did not overlap that of the terminals from the prearcuate region. The present study demonstrates that the LP of the monkey receives cortical visual inputs and projects to the posterior interposed and lateral cerebellar nuclei, suggesting that the LP may be involved in the control of voluntary eye movements.