Abstract
Presentation Description :
One promise of connectomics, the study of neural circuit motifs at synaptic resolution, is the potential to identify variation in synaptic wiring of identified microcircuits across closely related species. We hypothesize that microcircuitry is a primary target for neural system evolution leading to adaptive, species-specific, behaviors. In both the human and non-human primate retina, the circuitry arising from the sparsely distributed short-wavelength sensitive (S) cones is considered specialized primarily for color vision. In macaque monkeys, two parallel S cone pathways have been characterized previously in foveal retina by electron microscopic (EM) circuit reconstruction. S-ON bipolar cells selectively contact S cones to initiate a circuit that responds to increments in short-wavelength light intensity. Similarly, flat midget bipolar cells contact single S cones to initiate a circuit that responds to light decrement. The S-ON and S-OFF bipolar cells respectively contact S-ON and S-OFF ganglion cell types creating parallel color-coding output pathways to the brain. In contrast, in the common marmoset (Callithrix jacchus), a New World monkey species, previous light-microscopic immunolabeling studies provided evidence that the S-OFF midget bipolar pathway is absent, despite the presence of a comparable S-ON bipolar circuit. A single study of S-cone connectivity in the human retina also suggested the absence of an S-OFF-midget circuit, again despite the identification of a human S-ON pathway. We have now applied the EM approach to marmoset and human retina to directly determine how S-cone circuits vary across these three primate families. Our results confirm the complete absence of an S-OFF pathway in marmosets, but reveal its presence in the human retina, along with multiple synaptic specializations that imply fundamental differences in the contribution of S cones to chromatic pathways in the human visual system. We will argue that the macaque retina serves as an excellent model for the human retina at the level of cell types and basic retinal organization. However, at the synaptic level, a complete EM characterization of human specific wiring will be necessary to determine how retinal circuits contribute to human color vision.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.