Title: Can Mice See Color? The Science of Mouse Vision

Humans rely on color vision for many aspects of daily life, but the ability to perceive colors varies widely across species. Mice, commonly used in research, have a visual system that differs significantly from ours, raising questions about how they interpret their environment through color cues.

Understanding mouse color vision is important for neuroscience and behavioral studies, as it influences how these animals navigate their surroundings and interact with stimuli. Researchers have explored this topic using genetic tools, neural imaging, and behavioral experiments to determine what colors mice can see and how their brains process this information.

Distribution of Photoreceptors in Mice

The arrangement of photoreceptors in the mouse retina shapes their visual perception. Unlike humans, who have a fovea packed with cone cells for high-acuity color vision, mice have a retina dominated by rod cells, which enhance low-light vision but do not contribute to color discrimination. Cones, responsible for detecting different wavelengths of light, are present in much lower numbers and unevenly distributed, affecting how mice perceive their environment under varying lighting conditions.

A key feature of the mouse retina is the regional specialization of cone photoreceptors. The upper half, which processes light from below, is enriched with cones expressing short-wavelength-sensitive (S) opsins, making it more responsive to ultraviolet (UV) light. The lower half, capturing light from above, contains more cones with medium-wavelength-sensitive (M) opsins, attuned to greenish-yellow light. This gradient suggests an adaptation to their natural environment, where the ground reflects more UV light, while the sky emits longer wavelengths.

Many mouse cones co-express both S and M opsins, unlike primates, where each cone typically expresses a single opsin. This co-expression creates a continuum of spectral sensitivity rather than distinct color channels, influencing how mice integrate color information. Studies using immunohistochemistry and in vivo imaging confirm that opsin co-expression is highest in the dorsal retina and decreases ventrally, reinforcing the specialization of different retinal regions.

Opsin Diversity

The ability of mice to perceive color depends on the types of opsins in their cone photoreceptors. Opsins are light-sensitive proteins that determine which wavelengths a photoreceptor absorbs. Humans have three cone opsins for trichromatic vision, while mice have two: an S opsin sensitive to UV light and an M opsin responsive to greenish-yellow wavelengths. This dichromatic system limits the range of colors mice can differentiate but provides adaptations suited to their ecological niche.

The S opsin peaks around 360 nanometers, allowing mice to detect UV-reflecting surfaces, which may help identify food sources, conspecific markings, or predator cues invisible to humans. The M opsin peaks near 510 nanometers, aligning with the green portion of the spectrum. While mice cannot distinguish reds and many shades of blue as humans do, they can differentiate between UV and greenish-yellow hues, enabling functional color discrimination.

Many mouse cones simultaneously express both opsins in varying ratios, creating a spectral blending effect rather than distinct color channels. The dorsal retina favors M opsin dominance, while the ventral region exhibits stronger S opsin expression. This gradient prioritizes UV detection in ground-level vision and longer wavelengths in the upper visual field, mirroring natural light distribution in their environment.

Neural Processing of Color Cues

Once light is absorbed by cone photoreceptors, the signals are processed by the retina before transmission to the brain. Bipolar and horizontal cells refine photoreceptor inputs before relaying them to retinal ganglion cells. Unlike primates, which have specialized retinal circuits for detailed color discrimination, the mouse retina lacks dedicated color-opponent cells. Instead, color information is integrated to enhance contrast detection and environmental adaptation.

These signals travel through the optic nerve to the lateral geniculate nucleus (LGN) of the thalamus, a key relay in the visual pathway. In mice, the LGN does not segregate chromatic and luminance pathways as in species with advanced color vision. Instead, neurons respond to a combination of wavelength and brightness cues, suggesting that mouse color processing enhances contrast rather than precise hue differentiation. This adaptation benefits a nocturnal species, where distinguishing objects against different backgrounds is more useful than perceiving a broad color spectrum.

From the LGN, visual signals reach the primary visual cortex (V1), where more complex color and contrast interpretations occur. Calcium imaging studies show that while some V1 neurons selectively respond to UV or greenish-yellow light, most integrate both signals. This reinforces the idea that mouse color vision functions as a contrast-enhancing mechanism, aiding in behaviors such as foraging, predator avoidance, and social communication.

Behavioral Trials Demonstrating Color Recognition

Experiments assessing mouse color recognition use behavioral paradigms to measure their ability to distinguish between different wavelengths of light. Operant conditioning tasks train mice to associate specific colors with rewards. In these trials, mice choose between two illuminated panels—one emitting UV light and the other greenish-yellow. Correct selections linked to a food reward confirm their ability to differentiate these hues.

Maze-based studies provide additional insights into how color perception influences navigation and decision-making. In T-mazes, mice learn to follow a path marked by a particular wavelength of light to receive a reward. Their consistent choice of the correct route supports their capacity for color-based learning. These studies indicate that while mice can distinguish their two primary spectral sensitivities, they struggle with closely spaced wavelengths within the same range. Electrophysiological recordings confirm that neural responses to color cues focus on broad spectral contrasts rather than fine-grained distinctions.

Comparison with Other Mammalian Color Vision

Mice, like many placental mammals, have dichromatic vision, while other species exhibit more complex visual systems. Primates often have trichromatic vision, allowing them to distinguish a wider range of colors, particularly in the red-green spectrum, which aids in foraging. In contrast, dogs and cats also rely on two cone types, making their color discrimination more limited but sufficient for detecting environmental contrasts.

Among nocturnal mammals, rod-dominated vision reduces the need for advanced color discrimination. Bats prioritize low-light sensitivity over chromatic acuity, similar to mice. However, some marsupials, such as the fat-tailed dunnart, retain trichromatic vision, showing that color perception can persist despite nocturnal habits. Aquatic mammals, including seals and whales, have lost one or both cone types entirely, resulting in monochromatic vision. The diversity of mammalian color vision reflects adaptations to different ecological demands, with mice balancing UV sensitivity and contrast detection rather than full-spectrum color perception.

Emerging Genetic Tools in Color Vision Research

Advances in genetic technology have provided new ways to study mouse color vision. Optogenetics, which uses light-sensitive proteins to activate or silence neurons, allows researchers to manipulate specific visual pathways. Targeting retinal ganglion cells or visual cortex neurons helps determine how color information is encoded and relayed in the brain.

Gene-editing tools like CRISPR-Cas9 have enabled the creation of transgenic mice with enhanced spectral sensitivities, offering insights into how the mammalian brain adapts to new chromatic information. Additionally, calcium imaging and two-photon microscopy allow real-time observation of neuronal activity in response to color stimuli, mapping the neural circuits involved in color processing. These developments refine our understanding of mouse vision and contribute to broader research on sensory processing and neural plasticity.