Abstract
Purpose :
The standard hypothesis is that human vision is served by three opponent channels with sensitivities tuned to orthogonal cardinal directions in color space. The small bistratified retinal ganglion cells (RGCs) have been proposed to be the basis for the “blue-ON” opponent channel in the visual system. For years they were believed to be the only RGCs in the primate carrying “blue-ON” signals. Presently, electrophysiology and connectomics have revealed a greater diversity of primate RGCs carrying S-cone signals than previously imagined. These challenge conventional ideas about how information in the retinal image is encoded. There are S-cone bipolar cells, amacrine cells, and horizontal cells, which carry S-cone selective signals. By identifying postsynaptic partners of these cells in serial electron microscopy, we are identifying the complete variety of S-cone ganglion cells.
Methods :
We used serial block-face scanning electron microscopy in the macaque central retina (Patterson et al., 2019, Sci Rep) to reconstruct neurons in the inner retina that receive synaptic input from S-cone selective bipolar and amacrine cells.
Results :
We identified a previously unappreciated class of S-cone selective ganglion cell. Based on their stratification within the inner plexiform layer (IPL) (~91.4% IPL Depth), thorny dendritic arbors, decentralized somas, and classic tiling these were confirmed as ON narrow thorny RGCS. The presynaptic cells to 5 narrow thornys were reconstructed and identified. The majority of excitatory bipolar cell input was from blue-cone ON bipolar cells (63%) and 27% was from diffuse bipolar type 6. The majority of inhibitory input was from S-cone amacrine cells (SCACs) whose processes co-fasciculated with the narrow thorny dendrites (Fig1).
Conclusions :
The discovery of primarily S-cone selective inputs to the narrow thornys adds them to a growing list of S-cone RGCs. In addition to the small bistratified RGCs this includes midget, M1 and M2 intrinsically photosensitive RGCs. However, unlike those cells which have color opponent spectral response properties making them detectors of changes in wavelength, narrow thorny ganglion cells are predicted to have S-ON/OFF response properties, which may be useful in mediating pursuit eye movements or other visually guided movements tracking objects against a blue sky.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.