Abstract
Purpose :
While the fovea is best known for its specialized role in spatial acuity mediated by a high density of midget retinal ganglion cells (RGCs), anatomy and transcriptomics indicate that ~15 rarer RGC types are present. However, our understanding of the visual information these RGC types convey to the brain is limited because both the fovea and rarer RGC types are difficult to address with standard physiology approaches. Here, we address these gaps in knowledge by utilizing in vivo calcium imaging to explore the diversity of foveal RGC types in the living primate eye.
Methods :
Experiments were conducted in two macaque monkeys expressing GCaMP6s in the foveal ganglion cell layer. We used a fluorescence adaptive optics scanning light ophthalmoscope to measure the calcium responses of a population of GCaMP6s-expressing foveal cells to light stimuli presented directly to the photoreceptors. With this approach, we could simultaneously survey the response properties of 171 and 177 distinct cells in each macaque. Receptive fields were characterized by their responses to achromatic and cone-isolating spatially-uniform stimuli varying in contrast. In addition, we coarsely mapped spatial extent with 4.5 µm wide bars presented in different locations in one macaque.
Results :
The vast majority of the 348 cells imaged were consistent with the well-studied L vs. M midget RGCs that dominate in the foveal output, in that they showed either ON or OFF responses, did not respond to S-cone isolating stimuli and had small receptive fields. However, 7% (26/348) had ON-OFF responses and 6% (22/348) had strong S-cone responses. We also mapped the spatial receptive fields in one macaque and found that 13% (23/177) had large receptive fields. Of those 23 cells, 11 had ON-OFF responses and 2 had S-cone responses.
Conclusions :
Here we demonstrate an underappreciated functional diversity in the visual information the fovea sends the brain. While the dominance of midget RGCs is consistent with the fovea’s specialization for spatial acuity, our results indicate that this is an incomplete picture of visual processing in the fovea. An advantage of our in vivo approach is that we can return to the same cells in subsequent experiments. As a result, the ability to functionally identify the rarest foveal RGCs established here lays the foundation for future experiments directly targeting these elusive RGC types.
This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.