Do humans and monkeys see color in the same way?

New findings in color vision research imply that humans can perceive a wider range of blue than monkeys.

“The distinct connections found in the human retina may indicate a recent evolutionary adaptation to transmit enhanced color vision signals from the eye to the brain,” the researchers report April 25 in the scientific journal, Proceedings of the National Academy of Sciences.

Yeon Jin Kim, acting instructor, and Dennis M. Dacey, professor, both in the Department of Structure Biology at the University of Washington School of Medicine in Seattle, lead the international collaborative project.

They are joined by Orin S. Packer of Dacey’s lab; Andreas Pollreisz at the Medical University of Vienna, Austria; and Paul R. Martin, professor of experimental ophthalmology, and Ulrike Grünert, professor of ophthalmology and visual sciences, both at the University of Sydney, Australia, and Save Sight Institute.

The scientists compared the connections between the color-emitting nerve cells in the human retina with those in two monkeys, an Old World ape and a New World marmoset. The ancestors of modern humans diverged from the other two primate species about 25 million years ago.

Using fine-scale microscopic reconstruction methods, the researchers wanted to determine the neural wiring of areas associated with color vision is conserved across these three species, even though each took their own independent evolutionary path.

The scientists looked at the light wave-detecting cone cells from the retinal fovea. These tiny dimples are densely packed with cone cells. This is the part of the retina that is responsible for the sharp visual acuity needed to see important details, such as the words on the page or what’s in front of you while driving, and for color vision.

Cones come in three sensitivities: short, medium and long wavelength. Information about color comes from neural circuits that process information across different types of cones.

The researchers found that certain shortwave or blue sensitive cone circuits found in humans were absent in marmosets. This is also different from the sequence seen in ape monkeys. Another feature the scientists found in nerve cell connections in human color vision was not expected, based on previous models of nonhuman primate color vision.

Better understanding of the complex, species-specific neural circuits that encode color perception could ultimately help explain the origins of the distinct quality of color vision to humans.

The researchers also postulate the possibility that differences among mammals in their visual circuits are at least partly shaped by their behavioral adaptations to ecological niches. Marmosets live in trees while humans prefer to live on land. The ability to see ripe fruit in the changing light of a forest, for example, may offer a selective advantage for certain color visual sets. However, the actual effects of environment and behavior on color vision circuits have not been established.

More generally, comparative studies of neural circuits at the level of connection and signaling between nerve cells, the researchers note, can help answer many other questions. This includes explaining the logic underlying the design of neural circuits and providing insight into how evolution has modified the nervous system to help shape perception and behavior.

The research reported in the PNAS article “Comparative connectomics reveals a noncanonical wiring for color vision in the human foveal retina” was supported by National Institutes of Health grants EY-028282, RR-00166, P51 OD00425, and EY01730.