Abstract
The diversity of mammalian visual pathways is well established and it is clear that over a dozen discrete ganglion cell populations project in parallel to central visual targets. Identification of diverse ganglion cell structure and function in primate is ongoing (1-6) but a complete synthesis has not yet been achieved. Our goal is to give a near complete account of primate visual pathway origins, providing an anatomical basis for 1) targeted physiological analysis (see Crook et al., this meeting), and 2) the link between primate and other mammalian species, most notably the mouse, for which transgenic technology offers increasing access to ganglion cell circuits and central projections. To characterize the morphology of macaque ganglion cells we used retrograde photostaining (Dacey et al., 2003) from central tracer injections made into either the lateral geniculate nucleus (LGN; n = 16) or the superior colliculus-pretectum (SC; n = 8), the two major retinal targets. This method has the advantage of providing complete staining of the dendritic tree for large numbers of retrogradely labeled ganglion cells, permitting a description of each population that includes an estimate of relative density derived from the mosaic organization of overlapping dendritic trees of the same type. We also used data from previously characterized midget, parasol and small bistratified cells to establish relative dendritic stratification depth in the inner plexiform layer. Beyond the well studied inner/outer midget, inner/outer parasol and small bistratified cell groups, we identified 14 additional low-density populations that project to either the LGN or colliculus or both. Density estimates derived from mosaic coverage suggest that the 19 identified populations together account for roughly 90% of all macaque ganglion cells. Given an approximate relative density of ∼ 2.0 % of the total for each of the majority low-density populations, it is likely that a few populations remain to be identified. In addition to the LGN and colliculus-pretectum, the retina also projects to the accessory optic system (AOS) at the base of the midbrain, where direction selective (DS) neurons can be recorded. It is thus possible that the remaining unidentified ganglion cell populations are DS cells that project to the AOS.