Abstract Vega-Zúñiga et al. 2008

It has been proposed that nocturnality in visually-active animals enhances binocular vision. In mammals, binocular vision is structurally associated to a region of binocular overlap containing a fovea or an area centralis, and a well developed ipsilateral projection to the dorsal-lateral geniculate nucleus (LGN). Thus, these visual traits should be enhanced in nocturnal animals when compared with closely related diurnal species. We studied the extent and position of the visual field, the distribution and densities of the retinal ganglion cells, and the characteristics of the retinal central projections in two visually-active species of the genus Octodon; Octodon degus (diurnal/crepuscular) and Octodon lunatus (nocturnal). Phylogenetic analisys shows that these species splitted from a degus-like ancestor about 2 million years ago. The shape and extension of the visual fields were determined using ophtalmoscopic techniques. Counts and mapping of cells in the ganglion cell layer (GCL) were carried out in retinal wholemounts stained with nissl, and visual central projections were revealed by means of intraocular injections of CTb. The O. lunatus visual field has a prominent (100°) frontal binocular overlap, much larger than the 50° of overlap found in O. degus. Retinal ganglion cells were 40% fewer in O. lunatus (ca. 108.00) than in O. degus (ca. 180.000). O. lunatus has a poorly developed visual streak, but a well developed area centralis, located centrally near the optic disk (peak density of 4000 cells/mm2). O. degus has a highly developed visual streak, and an area centralis located more temporally (peak density of 6000 cells/mm2). The volumes of the contralateral LGN and Superior Colliculus (SC) are about 15% larger in O. degus compared to O. lunatus (p= 0.05). However, the ipsilateral projections to LGN and SC are about 500% larger in O. lunatus than in O. degus (p=0.05). We conclude that nocturnal visual behavior enhances binocular vision at least in the case of these rodents. We discuss whether this generalization can be extended to mammals and other amniota. FINANCIAL SUPPORT: FONDECYT Nº 1030522, 1080094