The Topographic Organization of Retinal Ganglion Cell Density and Spatial Resolving Power in an Unusual Arboreal and Slow-Moving Strepsirhine Primate, the Potto (Perodicticus potto)

The potto (Perodicticus potto) is an arboreal strepsirhine found in the rainforests of central Africa. In contrast to most primates, the potto shows slow-moving locomotion over the upper surface of branches, where it forages for exudates and crawling invertebrates with its head held very close to the substrate. Here, we asked whether the retina of the potto displays topographic specializations in neuronal density that correlate with its unusual lifestyle. Using stereology and retinal wholemounts, we measured the total number and topographic distribution of retinal ganglion cells (total and presumed parasol), as well as estimating the upper limits of the spatial resolution of the potto eye. We estimated ∼210,000 retinal ganglion cells, of which ∼7% (∼14,000) comprise presumed parasol ganglion cells. The topographic distribution of both total and parasol ganglion cells reveals a concentric centroperipheral organization with a nasoventral asymmetry. Combined with the upwardly shifted orbits of the potto, this nasoventral increase in parasol ganglion cell density enhances contrast sensitivity and motion detection skywards, which potentially assists with the detection of predators in the high canopy. The central area of the potto occurs ∼2.5 mm temporal to the optic disc and contains a maximum ganglion cell density of ∼4,300 cells/mm2. We found no anatomical evidence of a fovea within this region. Using maximum ganglion cell density and eye size (∼14 mm), we estimated upper limits of spatial resolving power between 4.1 and 4.4 cycles/degree. Despite their reported reliance on olfaction to detect exudates, this level of spatial resolution potentially assists pottos with foraging for small invertebrates and in the detection of predators.