Scientists identify mysterious retinal cells that could alter our understanding of color perception

Researchers at the University of Rochester have made groundbreaking advancements in our understanding of color perception. In a recent study published in the Journal of Neuroscience, they identified rare retinal ganglion cells (RGCs) that could fill critical gaps in our knowledge of how we see colors. These findings indicate that, in addition to the established color detection pathways followed by most RGCs, a small number of these non-conventional cells may play an important role in color vision.

The study was driven by a fundamental question in visual neuroscience: how does the human eye process and perceive color? Scientists have long known that the retina contains three types of cone photoreceptors sensitive to short, medium, and long wavelengths of light. These cones transmit information to the brain via retinal ganglion cells.

In the 1980s, research by David Williams at the University of Rochester identified three primary pathways — known as cardinal directions — that describe basic color detection. These cones transmit information to the brain through RGCs, following three cardinal directions: luminance (combining signals from medium and long wavelengths), red-green (opposition between long and medium wavelengths), and blue-yellow (opposition between short wavelengths and the combined signals of medium and long wavelengths).

However, these pathways do not fully explain how humans perceive the richness and diversity of colors. The researchers suspected that alongside these primary pathways, there might be additional, less common RGCs that play a crucial role in color perception.

To explore this hypothesis, the researchers employed advanced imaging techniques. They used adaptive optics, a technology initially developed by astronomers to correct for distortions in telescopic images caused by the Earth’s atmosphere. This technology was adapted to correct for distortions in the eye, providing unprecedented clarity of individual photoreceptor cells.

The study involved imaging the eyes of three macaque monkeys using adaptive optics and calcium imaging. These techniques allowed the scientists to observe and measure the responses of RGCs in the fovea, the central part of the retina responsible for sharp central vision. The macaques were chosen for their similarities to human vision.

The researchers administered viral vectors to the monkeys to express a calcium indicator in the RGCs. This indicator allowed the cells to fluoresce when activated by light, enabling the scientists to track their responses to various color stimuli. The monkeys were shown a series of light patterns designed to isolate responses from different types of cones.

The study confirmed the existence of non-cardinal RGCs in the primate retina. These cells displayed unique response patterns that do not align with the previously established cardinal directions. Specifically, the researchers found RGCs that responded to combinations of red and green, and blue and yellow light in ways not predicted by existing models of color vision.

“We don’t really know anything for certain yet about these cells other than that they exist,” said Sara Patterson, a postdoctoral researcher at the Center for Visual Science, who led the study. “There’s so much more that we have to learn about how their response properties operate, but they’re a compelling option as a missing link in how our retina processes color.”

The presence of these non-cardinal cells suggests that the retina’s role in color perception is more complex than previously thought. These cells could be responsible for the nuanced way humans perceive colors beyond the primary hues dictated by the cardinal pathways. For example, they may contribute to the perception of intermediate colors and the subtle variations in color that enrich our visual experience.

But the researchers noted that their findings are based on a relatively small sample of cells from a specific region of the retina. Further research is needed to confirm these results and explore how these cells function in the broader context of the entire visual system.

Additionally, the study’s focus was on identifying these cells and their basic response properties. More work is required to understand precisely how these non-cardinal RGCs contribute to color perception and how their signals are processed by the brain.

Future research could involve more extensive imaging of the retina and the use of advanced computational models to predict how these cells might influence color vision. Studies could also explore potential clinical applications, such as developing better retinal prosthetics for people with vision loss. Understanding the full range of RGC functions could lead to improved designs that more accurately mimic natural vision.

A deeper understanding of the retina’s complex processes could pave the way for more effective methods to restore vision in people who have lost it.

“Humans have more than 20 ganglion cells and our models of human vision are only based on three,” Patterson explained. “There’s so much going on in the retina that we don’t know about. This is one of the rare areas where engineering has totally outpaced visual basic science. People are out there with retinal prosthetics in their eyes right now, but if we knew what all those cells do, we could actually have retinal prosthetics drive ganglion cells in accordance with their actual functional roles.”

The study, “Cone-Opponent Ganglion Cells in the Primate Fovea Tuned to Noncardinal Color Directions,” was authored by Tyler Godat, Kendall Kohout, Keith Parkins, Qiang Yang, Juliette E. McGregor, William H. Merigan, David R. Williams and Sara S. Patterson.