The posterior lobe of the monkey cerebellum: Anatomical connections and their functional implications

The cerebellum links visual and motor structures in the brain and plays a critical role in visuo-motor co-ordination. Mossy fibre-mediated visual information is transmitted to the posterior lobe of the cerebellum via the dorsolateral pons (dlpn). Anterograde tracing studies have demonstrated that the dlpn projects heavily to the dorsal paraflocculus (dpf), and moderately to the posterior vermis and hemispheres. The projection to the flocculus is virtually non-existent, but it receives climbing fibre-mediated visual information via the dorsal cap of the inferior olive. Sixteen monkeys received injections of the bi-directional tracer WGA-HRP into cerebellar cortical areas receiving mossy or climbing-fibre mediated visual information. The following questions were addressed: i) Do retrograde studies confirm the absence of floccular projections from the dlpn? ii) How is the olivo-cortico-nuclear parasagittal zonation represented in the monkey? iii) Do cerebellar cortical projections overlap in the deep cerebellar nuclei? The results suggest that the flocculus only receives a sparse collateral input from the dlpn. The olivo-cortico-nuclear system in the monkey is shown to diverge beyond the strict parasagittal zonation seen in non-primates, allowing cross-talk between functional modules. In addition, cortico-nuclear projections from different cortical regions overlap. Indeed, hemispheric eye-movement related areas (dpf, flocculus and crus II) all project to the ventrocaudal posterior interposed nucleus, a discrete eye movement region involved in the control of vertical saccades and the far-response in vergence and accommodation (Van Kan et al., '93; Zhang and Gamlin, '94; Robinson et al., '96). Vermal lobule VII projects to the fastigial oculomotor region of Noda et al. ('88), which controls horizontal saccades and the near-response (Robinson et al., '93; Zhang and Gamlin, '96). Finally, additional observations on cortico-nuclear topography demonstrate how multiple somatotopies in the cortex (e.g. multiple eye movement areas) are reduced onto a single representation of the body in each of the deep cerebellar nuclei