Organisation of koniocellular-projecting ganglion cells and diffuse bipolar cells in the primate fovea

The roles of the midget and parasol pathways as the anatomical foundation for high-acuity vision at the fovea are well established. There is also evidence for the presence of other (non-midget, non-parasol) ganglion cell types in the foveal retina, but it is not established whether these cells receive input from cone photoreceptors in the central few degrees of the visual field, i.e. the region most important for conscious visual perception. To address this question, we targeted injections of retrograde tracer to the koniocellular layers in the posterior aspect of the lateral geniculate nucleus, where the central visual field is represented, in marmoset monkeys (Callithrix jacchus). Labeled ganglion cells were photofilled to reveal their dendritic morphology. Potential inputs to foveal koniocellular cells from diffuse bipolar cells were investigated in vertical sections through the fovea of marmoset and macaque (Macaca fascicularis) monkey retinas using immunohistochemistry. Forty koniocellular-projecting ganglion cells were analysed. We used an established model of marmoset foveal topography to show that all these koniocellular-projecting cells receive cone inputs from the central-most 6°, with about half the cells receiving input from below 2° eccentricity, in the rod-free central bouquet of cones at the foveola. In addition, all diffuse bipolar types investigated were present in the fovea at stratification depths similar to those of their counterparts in the peripheral retina. We conclude that the diverse visual representations established for koniocellular pathways in the peripheral retina are also a feature of the fovea, suggesting that koniocellular pathways contribute to foveal vision.