RGC Dendrite Mosaics are Optimized for Efficiency

Article: Living Retina Achieves Sensitivity and Efficiency Engineers Can Only Dream About
Source: Duke University
Published: September 28, 2021

Retina cross-section showing retinal ganglion cells at top,
and the inner plexiform layer of their dendrites below

Neurobiologists studying the cellular organization of the retina found that it closely mimics a concept known as efficient coding theory, an optimization model of sensory coding in the nervous system. In two papers on retinal structure, they show that natural selection and evolution shaped the patterns of sensitivity in the retina to closely follow what efficient coding theory would predict. Specifically, these sensitivity patterns are seen in the layer of dendrites of the retinal ganglion cells (RGCs), i.e., the inner plexiform layer. Although the ganglion cells downstream only output in binary fashion, either depolarization or not, it is the three-dimensional dendrite mosaic that is sensitive to different stimuli. One of the researchers explains, “The mosaics don’t just randomly overlap, but they don’t overlap in a highly ordered way.” And those mosaics adapt to current conditions. This results in a retina that is not merely one mosaic but many stacked mosaics that each encodes something different about the visual field, parsing as many as 40 different features that together add up to form an image. Furthermore, the depth of the mosaic serves as a kind of address for the type of information that that layer encodes. For example, the deeper layers receive "off" signals, while the more shallow layers get "on" signals, meaning that even when the same (cross-section) area of the retina is stimulated, different layers of the dendrite mosaic can convey different kinds of signals.

One reason the array is so efficient is that the cells conserve energy by not responding to some stimuli. In environments that are "noisy," the receptors tune out most of the static and only respond to something that’s very bright. One of the researchers states, “The more noise there is in the world, the pickier the cell can be about what it will respond to...And when they get pickier, it turns out that there's less redundancy in them.” Moreover, the more noise, the greater the offset between on and off RGC detector pairs. In technical terms, "[I]nformation is maximized when these mosaic pairs are anti-aligned," when the distances between them are greater than average. In other words, the retina is optimized to handle high noise conditions in order to detect things that stand out, and it does so by minimizing the amount of redundant information it encodes. The researchers hope that studying how the retina is optimized for efficiency will help to design smartphone sensors; however, they acknowledge that we are a long ways away from replicating the natural arrangement in the retina.

My rating of this study: ⭐⭐⭐

Roy S, Jun NY, Davis EL, et al. "Inter-mosaic coordination of retinal receptive fields." Nature.  592:409–413. 10 March 2021. https://doi.org/10.1038/s41586-021-03317-5

Jun NY, Field GD, and Pearson J, et al. "Scene statistics and noise determine the relative arrangement of receptive field mosaics." PNAS.  118(39):e2105115118. 28 September 2021. https://doi.org/10.1073/pnas.2105115118