Retinal ganglion cell inputs to the koniocellular pathway

To understand the transmission of sensory signals in visual pathways we studied the morphology and central projection of ganglion cell populations in marmoset monkeys. Retinal ganglion cells were labeled by photofilling following injections of retrograde tracer in the lateral geniculate nucleus (LGN), or by intracellular injection with neurobiotin. Ganglion cell morphology was analyzed using hierarchical cluster analysis. In addition to midget and parasol ganglion cells, this method distinguished three main clusters of wide-field cells that correspond to small bistratified, sparse, and broad thorny cells identified previously. The small bistratified and sparse cells occupy neighboring positions on the hierarchical (linkage distance) tree. These cell types are presumed to carry signals originating in short-wavelength sensitive (S or "blue") cones in the retina. The linkage distance from these putative S-cone pathway ganglion cells to other wide-field cells is similar to the linkage distance from midget cells to parasol cells, suggesting that S-cone cells form a distinct functional subgroup of ganglion cells. Small bistratified cells and large sparse cells were the most commonly labeled wide-field cells following LGN injections in koniocellular layer K3. This is consistent with physiological evidence that the role of this layer includes transmission of S-cone signals to the visual cortex. Other wide-field cell types were also labeled following injections including K3, and other koniocellular LGN layers; these cell types may correspond to "non-blue koniocellular" receptive fields recorded in physiological studies.