Survey of the morphology of macaque retinal ganglion cells that project to the pretectum, superior colliculus, and parvicellular laminae of the lateral geniculate nucleus.

In common with other vertebrates, the primate retina contains a number of different ganglion cell types that project to different regions in the brain. We wanted to determine how the different ganglion cell types, distinguished morphologically, mapped to these regions of the brain. We injected a fluorescent dye into one of three regions of a macaque brain: the superior colliculus (SC), the pretectal region, and the parvicellular laminae of the lateral geniculate nucleus. By means of an in vitro preparation, the retrogradely labelled ganglion cells were intracellularly injected with horseradish peroxidase, so as to reveal their dendritic morphology. When the dendritic-field diameters of the injected cells were plotted against retinal eccentricity, each of the three regions was found to receive input from a distinctive population of cells. The pretectal projection was dominated by cells with large dendritic fields. The SC projection was composed of a number of distinct types, with smaller dendritic fields. Parasol cells project to SC but are extremely rare. In addition to midget ganglion cells, the parvicellular laminae receive inputs from at least two additional groups. Parvicellular bistratified (PB) cells have bistratified dendritic fields, slightly larger than those of parasol cells. Parvicellular giant (PG) cells have dendritic-field diameters larger than that of any parasol cell, ranging from 250 microns to greater than 850 microns--the largest of any primate ganglion cells. In contrast to the retinal projections of the cat, in which a specific ganglion cell type can project to different regions of the brain, each of the regions in this survey appears to receive inputs from its own distinct group of ganglion cells.