Tetrachromacy

Tetrachromacy is a specialized condition of color vision. It occurs when an organism possesses four independent channels for conveying color information. This means the eye contains four distinct types of cone cells. These cone cells are high-intensity light receptors located in the retina. Most humans are trichromats, meaning we only have three types of cone cells. For a tetrachromat, the sensory color space is four-dimensional. This allows them to distinguish between many more colors than a trichromat.

The mechanism of tetrachromacy relies on the spectral sensitivity of these receptors. Each of the four cone cell classes responds to a different portion of the light spectrum. When light hits the retina, these cells trigger signals based on the specific wavelengths they detect. To achieve true tetrachromatic vision, the body also needs a post-receptoral mechanism. This system must be able to compare the signals from all four receptor classes. This process is often described by the opponent process theory. In humans, the three existing channels work through an opponent process to create color. It is still being studied if a fourth channel can function the same way.

Different species exhibit various levels of color vision complexity. Many birds, fish, and reptiles are known tetrachromats. For example, goldfish are typical tetrachromats.

Some birds, like the zebra finch, use ultraviolet wavelengths to help with foraging and mate selection. These birds can see light in the 300–400 nm range. Their vision is further enhanced by pigmented oil droplets in their photoreceptors. These droplets filter light before it reaches the visual pigment. Some animals even reach higher levels, such as pentachromacy. Pentachromacy requires at least five different classes of photoreceptors. Pigeons are believed to be pentachromats, and some lampreys may also possess this ability. Invertebrates can have even more, like the mantis shrimp with 33 different pigments.

The history of color vision shows a changing evolutionary path. The common ancestor of all vertebrates was likely a tetrachromat. However, a common ancestor of mammals lost two of its four cone cell types. Scientists believe this loss occurred during a period called the nocturnal bottleneck. Later, some primates evolved a third cone to regain more color detail. This evolutionary history explains why most mammals today are trichromats or even dichromats.

In humans, tetrachromacy is a subject of intense scientific study. Most researchers believe it may exist in a small percentage of the population. This is often linked to the X chromosome, which carries the genes for cone pigments. Females who carry recessive alleles for color vision deficiency (CVD) might actually be tetrachromats. This happens through a process called X-inactivation. One study suggested that 15% of women might have a fourth cone type. Another study estimated that up to 50% of women could have four photopigments. In 2010, neuroscientist Gabriele Jordan identified a woman who was a "true" tetrachromat. This woman could detect a much wider variety of colors than standard trichromats.

There are interesting physical limits to how we see light. While some animals see ultraviolet light, humans generally cannot. This is because the lens of the human eye blocks most light between 300 and 400 nm. The cornea also blocks shorter wavelengths. However, people with aphakia, or a lack of a lens, can see near-ultraviolet light. They often describe it as whitish blue or whitish violet. This occurs because the three types of cones are somewhat sensitive to those wavelengths.

Tetrachromacy connects to many broader biological and optical concepts. It demonstrates how the complexity of a sensory system can change through evolution. It also shows how genetic inheritance, specifically through X-linked alleles, affects how we perceive the world. The study of these pigments helps us understand the relationship between light physics and biological processing. Understanding how the brain and optic nerve handle new color channels remains a key area of research. We do not yet know if the brain can fully utilize a fourth signal if it is presented with one.