This finding suggests that humans may have developed distinct neural connections in the retina for enhanced color vision, which could be the result of recent evolutionary adaptations.
The international research team was led by Yeon Jin Kim, acting instructor, and Dennis M. Dacey, professor, both from the Department of Biological Structure at the University of Washington School of Medicine in Seattle.
The collaborative project also involved Orin S. Packer of the Dacey lab; Andreas Pollreisz at the Medical University of Vienna, Austria; Paul R. Martin, professor of experimental ophthalmology, and Ulrike Grünert, associate professor of ophthalmology and visual science, both at the University of Sydney, Australia, and the Save Sight Institute.
To compare the neural connections responsible for color vision between humans and monkeys, the scientists examined the retinas of humans, Old World macaques, and New World common marmosets. These two monkey species share a common ancestor with modern humans from around 25 million years ago.
The researchers sought to determine whether the neural wiring associated with color vision is conserved across these three species, despite their distinct evolutionary paths.
The team focused on the fovea, a small, cone cell-packed region of the retina responsible for sharp visual acuity and color vision. They studied the cone cells, which come in three sensitivities: short, medium, and long wavelengths. Color information is processed through neural circuits that interpret data from different cone types.
Using a fine scale microscopic reconstruction method, the researchers discovered that a specific short-wave or blue-sensitive cone circuit found in humans is absent in marmosets and differs from the circuit seen in macaque monkeys.
Additionally, the scientists identified features in the nerve cell connections of human color vision that were unexpected based on earlier nonhuman primate color vision models.
The findings contribute to a better understanding of the complex neural circuitry that codes for color perception in humans, potentially shedding light on the origins of color vision qualities that are unique to our species. The researchers also suggested that differences in visual circuitry among mammals could be partially influenced by their adaptation to specific ecological niches.
For instance, marmosets live in trees, while humans primarily dwell on land. The ability to discern ripe fruit amid shifting forest light could have provided a selective advantage for specific color vision circuitry, although the precise effects of environment and behavior on color vision have yet to be established.
According to the researchers, comparative studies of neural circuits at the level of connections and signaling between nerve cells could help answer many other questions, such as revealing the underlying logic of neural circuit design and offering insights into how evolution has modified the nervous system to shape perception and behavior.
Color vision varies significantly across the animal kingdom, with different species possessing unique visual systems and abilities to perceive colors. Here are some examples of color vision in various animal species:
Birds have a highly developed color vision system, with some species possessing up to five types of cone cells, compared to three in humans. This enables them to see a broader range of colors, including ultraviolet light. Birds use their advanced color vision for various purposes, such as finding food, recognizing mates, and navigating their environment.
Reptiles, like birds, can also see a wide range of colors, including ultraviolet light. Many reptiles, such as turtles and snakes, have four types of cone cells, allowing them to differentiate between various colors and patterns. This capability is essential for activities like foraging, avoiding predators, and identifying potential mates.
Fish exhibit a diverse range of color vision abilities. Some species, such as salmon and goldfish, have highly developed color vision systems, while others have limited color vision or none at all. Fish living in deep water, where little light penetrates, often have fewer cone cells and rely more on rod cells for low-light vision.
Insects typically have compound eyes that allow them to perceive a range of colors, including ultraviolet light. For example, bees can see ultraviolet patterns on flowers that are invisible to humans. These patterns guide them to nectar sources, facilitating pollination.
Most mammals have relatively limited color vision compared to birds, reptiles, and some fish. Many nocturnal mammals, such as rodents and bats, possess only two types of cone cells, which restricts their color vision to a limited range of hues. However, some mammals, such as primates (including humans), have trichromatic color vision, enabling them to see a broader spectrum of colors.
Cephalopods, including octopuses, squids, and cuttlefish, have a unique color vision system. Despite having only one type of photoreceptor cell, they can perceive and produce a wide range of colors by detecting the polarization of light and using specialized skin cells called chromatophores.
These examples illustrate the incredible diversity of color vision systems across the animal kingdom. Each species has evolved unique adaptations to suit its specific needs and ecological niche. Further research into these diverse visual systems may offer valuable insights into the evolution and function of color vision in various species.