Retinal Projections in the Hairy-Nosed Wombat, Lasiorhinus latifrons (Marsupialia : Vombatidae)

Abstract

Optic nerve projections were studied in one hairy-nosed wombat. Primary optic centres were mapped by observing the distribution of autoradiographic label in the brain following injection of one eye with 3H-leucine. Retinal fibres terminate in the dorsal lateral geniculate nucleus, ventral lateral geniculate nucleus, lateral posterior nucleus, pretectum, superior colliculus, suprachiasmatic nucleus of the hypothalamus and the accessory optic system. Lasiorhinus is unusual among marsupials, because the accessory optic system receives a bilateral projection from the retina, and there is considerable overlap of the retinal fibres from the two eyes in the superior colliculus. The dorsal lateral geniculate nucleus (LGNd) has at least three cell layers and receives eight layers of retinal input. Layers 1, 3, 5, 6 and 8 receive ipsilateral retinal fibres and layers 2, 4 and 7 receive contralateral retinal fibres. The lamination of the LGNd is thus as complex as that in the possums and kangaroos, and the data does not support the hypothesis that there is a simple relationship between the lamination pattern of the LGNd and the visual behaviour of each animal. The monocular segment of the LGNd in Lasiorhinus is about twice that of the binocular segment; the large size of the monocular segment correlates well with the lateral position of the eyes, which have a large lateral monocular field of vision for each eye and a small frontal binocular field of view.