Rats have dichromatic vision, meaning they have two types of cone photoreceptors in their eyes that allow them to perceive color. This is different from trichromatic vision found in humans, which uses three cone types to see the colors red, green, and blue. Rats are unable to distinguish between red and green wavelengths, but can see blue and ultraviolet light.
Rats have two cone types
The retinas of rats contain two types of cone cells that are sensitive to short (S) and medium (M) wavelengths of light. The S cones have peak sensitivity in the ultraviolet range around 350-370 nm. The M cones are maximally sensitive to wavelengths around 510 nm, which corresponds to green light. Rats lack long (L) wavelength cones that are sensitive to red wavelengths greater than 560 nm. This gives them dichromatic color vision capable of discriminating between blue and UV light from the S cones and green light from the M cones.
Rats see a limited range of colors
With only S and M cones, rats have limited color vision compared to humans. They are unable to distinguish between long wavelengths at the red end of the visible spectrum. Rat vision is similar to red-green color blindness in humans, where the L cones are missing or non-functional. However, rats can discriminate between blue and green, as well as shades like orange and yellow that stimulate both the S and M cones. Their vision is tuned more for contrast and brightness detection rather than a wide range of color perception.
Rats have high visual acuity and good motion detection because their retinas contain a high density of rods rather than a high density of cones. The rods are very sensitive even in low light conditions. The rich network of rods provides excellent night vision, but limits rats to seeing fewer colors than species like primates that have more cones.
Neural circuits process color differently
In addition to the cone photoreceptors, color vision relies on neural circuits in the retina and brain that compare and process signals from the different cone types. Research suggests rats also have differences in these retinal circuits compared to trichromats like humans.
For example, rats lack a retinal circuit called “small bistratified retinal ganglion cells” that compare L and M cone signals. They also lack opponent color cells in the lateral geniculate nucleus of the thalamus that are sensitive to red/green color opponency. So even though they have cones capable of L/M wavelength discrimination, rats lack higher visual pathways dedicated to processing this information.
Rats can’t distinguish red from green
Behavioral studies have directly tested whether rats can discriminate between red and green wavelengths. In experiments where rats are trained to press levers in response to different colors of light, they had difficulty distinguishing between wavelengths in the long (L) cone range that appears red to humans. However, they could readily learn to tell blue from green or blue from yellow light.
Other behavioral tests show that rats cannot differentiate red stimuli from neutral grey stimuli matched in brightness. Rats also generalize their conditioned response between longer wavelength lights, suggesting they are unable to discriminate between reds and greens. Collectively, these studies demonstrate rats do not have red-green color vision.
Rats respond to brightness rather than color
In cases where rats can be trained to respond differently to red and green lights, it is likely because they are responding to apparent brightness differences rather than color differences. Due to differences in luminance perception, some wavelengths are brighter and others dimmer to rats compared to humans, even when matched for physical intensity.
For example, longer red wavelengths around 650 nm actually appear darker instead of brighter to rats. Rats trained to approach the “brighter” of two lights will choose the green light over the red. But if the red light intensity is increased to match the perceived brightness, rats will no longer differentiate between the two colors.
Implications for cognition and behavior
The limited color vision of rats may have implications for their visual cognitive abilities as well as behavioral responses to colored cues in both laboratory and natural settings. For example, the use of red and green colors to signify different conditions in behavioral experiments with rats may be inappropriate if they cannot discriminate between these wavelengths.Colors that appear distinct to human experimenters may be indistinguishable to rats.
In the wild, the inability to see red light may impact how rats perceives fruits, vegetation, predators, mates and other aspects of their environment. More research is needed to understand how dichromatic vision fully shapes the way rats visually navigate their surroundings on a behavioral and cognitive level.
Comparisons with other rodents
Rats are not the only rodents with limited color vision. Mice, squirrels, guinea pigs, hamsters, and gerbils also have been shown to have dichromatic color vision similar to rats. Many mammals like dogs and cats have dichromatic vision, while primates are mostly trichromatic. Some nocturnal rodents may be monochromats and only see in shades of gray.
Among rodents, the ground squirrel and prairie dog appear to have an extension of visual range into the red spectrum, likely as an adaptation to their diurnal lifestyle. But rats along with mice and other murids have maintained dichromatic vision throughout their evolution, suggesting it is sufficient for their niches.
Spectral sensitivity
Researchers have used electroretinogram (ERG) recordings to map the spectral sensitivity of rat retinal cells. By measuring electrical responses to light of different wavelengths, they can determine the peak sensitivity of the rod and cone photoreceptors.
Rods have peak sensitivity around 498-510 nm in the green/blue region. The S cones respond maximally to ultraviolet light around 359-370 nm. The M cones have peak sensitivity around 508-510 nm overlapping somewhat with rods but corresponding to green wavelengths.
There is minimal sensitivity of any photoreceptors above 550 nm, confirming that rats lack long wavelength/red sensitivity. Additionally, the S and M cone sensitivity ranges do not overlap, supporting that they mediate dichromatic vision.
Genetic basis
The dichromatic vision of rats is genetically programmed by the expression of specific opsin photopigment genes that confer color sensitivity to the cone cells. Rats express two types of cone opsin genes:
- S opsin – codes for short wavelength sensitive opsin peak around UV light
- M opsin – codes for medium wavelength sensitive opsin peak around green light
Rats lack a third L opsin gene that would extend sensitivity to red wavelengths. The differential expression of the S and M opsin genes in distinct cone populations establishes dichromatic color vision.
Neural architecture
In addition to the cone photoreceptors, rats have unique retinal circuitry and visual pathways that underlie their limited color vision abilities:
- Lack small bistratified retinal ganglion cells that compare L and M cone signals
- Retinal W3 ganglion cells compare S cone and rod signals
- Lack red/green opponent neurons in the lateral geniculate nucleus (LGN) of the thalamus
- LGN neurons respond to S cone activation but L/M color opponency is weak
So while rats have the M cone opsins necessary for L/M discrimination, their retina and brain lack specialized circuits to actually process these subtle color differences.
Adaptations to environment
The dichromatic vision of rats is likely an adaptation to their evolutionary environment and nocturnal niche. Distinguishing red from green hues offered little advantage for rats, whereas high acuity night vision was essential.
Rats are active during dawn/dusk cycles and at night. In low light, cone color vision is less useful and rod-dominated vision prevails. Excellent night vision and motion detection from a rod-dense retina enhanced survival.
Additionally, the types of food sources rats foraged for at night had less need for red/green discrimination. Thus, there was little selective pressure to evolve additional opsin genes and neural circuits to expand color vision abilities.
Applications in research
The knowledge that rats have limited color discrimination has useful applications for vision research. Rats are a common model organism for studying the visual system. Their dichromatic vision can help answer questions about retinal processing, color perception and visual guided behaviors relevant to other mammals.
However, the red-green visual limitations of rats may impact their utility for modeling trichromatic human color vision. Primate models like monkeys may be better for understanding human trichromacy and disorders like color blindness.
Research in rats also provides insight into how the retina and brain adapt to limited visual information. This can provide general principles for how neural circuits optimize processing under constraints that are applicable to other systems like hearing or smell.
Conclusion
In summary, rats lack the photoreceptors and neural circuitry necessary to discriminate red and green wavelengths. Behavioral experiments confirm rats cannot tell apart reds from greens, even though they have limited color vision capable of distinguishing blue from green. Their dichromatic vision is likely an adaptation to their evolutionary nocturnal niche that favored scotopic vision over color discrimination. Understanding the genetic and neural basis of rat color vision continues to inform models of visual processing across species.
