Direct accessory optic projections to the vestibulo-cerebellum: a possible channel for oculomotor control systems.

The nucleus of the basal optic root (nBOR) receives direct retinal projections in all classes of vertebrates. This nucleus is also known as the medial terminal nucleus of the accessory optic tract, or as the nucleus extomamillaris. Following a series of HRP injections into the uvula and flocculonodular lobe of pigeons, both large and small cells of the nBOR were labelled bilaterally with the marker. No significant transport of HRP to nBOR was observed following injections of more rostral folia of the posterior or anterior lobes of the cerebellum. In view of the prominence of the tract from retina to nBOR and the presence of a monosynaptic pathway from nBOR directly to the vestibulo-cerebellum, we suggest this bisynaptic, "lemniscal", retino-cerebellar channel may be the substrate by which visual stimuli directly trigger oculomotor responses mediated by the vestibulo-cerebellum.