The World in Color

November 26, 2019
Written by: Sarah Reitz

 

“What’s your favorite color?” is a question you’ve almost certainly been asked at least once in your life. For humans, color is used for self-expression in everything from art to clothes and decor. Not only is color good for conveying emotion, it is also critically important in the lives of many other animals. For instance, many poisonous plants and animals are brightly colored to warn others against eating them. Other animals, like octopus and butterflies, use color to help them camouflage with their surroundings. No matter what color is used for though, have you ever stopped to wonder just how you’re able to see the colors that surround you?

The basics of vision

Before diving into the specifics of color vision, we first have to understand the basics of vision. The ability to see starts with specialized light-sensitive cells in the retina of the eye called photoreceptor cells. These cells detect light that enters the eyeball and send a signal to a group of neurons called retinal ganglion cells, which relay the message to the brain via the optic nerve¹. These neurons are sensitive to distinct kinds of visual information, such as movement, spatial contrast, or color.

The first brain region that receives this visual information from the retinal ganglion cells is the lateral geniculate nucleus of the thalamus, which processes the information and sends it on to the visual cortex¹. While you might think that the visual cortex would be near the front of the brain, close to the eyes, it’s actually located farthest away, in the back of the brain. This part of the brain consists of several regions that integrate the distinct packets of information sent by the retinal ganglion cells into the complete picture of the world around us that we ultimately perceive.

The color code

Within the retina, there are two major types of light-sensitive photoreceptor cells: rods and cones. Rods detect the presence and intensity of light, allowing our brains to construct a black and white picture of whatever we are viewing. Meanwhile, our ability to see color comes from cones².

But how do cones detect color? At its most basic, color is nothing more than specific wavelengths of light that are detected by the eye. Light exists on a spectrum based on wavelength. Many forms of light, such as ultraviolet or infrared light, aren’t visible to humans because the wavelengths are either too short or too long to be detected by our eyes. However, light with wavelengths between 380nm and 700nm fall within the “visible light” range. Light within this range can be detected by our cones, giving us the perception of color.

The human eye has three types of cones, each of which contains a specific light-sensitive pigment that only responds to a particular portion of the visible light spectrum—in our case blue, green, and red light² (Figure 1). This allows different wavelengths of light to be treated as distinct visual stimuli in the brain.

Even though we only have blue, green, and red cones, the processing power of the brain allows us to see a much wider range of colors. The wavelengths of light that each type of cone responds to overlaps slightly with the other two types of cones (Figure 1). This overlap means that a single wavelength of light actually activates each type of cones, but to different extents. For example, blue light at 420nm causes the blue cones to send a strong signal to the brain. This light also weakly activates the green and red cones in the retina, causing them to also send signals to the brain, albeit much weaker than if the light was green or red. In contrast, yellow light at 575nm causes red cones to send a strong signal, green cones to send a slightly weaker signal, and blue cones to send almost no signal to the brain.

The strength of the signal from each type of cone forms a sort of code, where each wavelength of light produces a unique pattern of activity between the three types of cones. By comparing the signal strength from each type of cone, the brain can use this information to calculate exactly which color of light was seen.

What causes color blindness?

In order for the color code to work, the brain must receive information from all three types of cones. When one or more cone types are either missing or defective, the brain cannot accurately process color, and color blindness results.

One category of color blindness is dichromacy, which occurs when one of the three cone types is missing. Because there are three types of cones, there are also three types of dichromacy that can occur depending on which cone is absent (Figure 2). The most common forms of color blindness are red-green color vision defects³. These defects, caused by a genetic loss of either the red cones (called protanopia) or green cones (called deuteranopia), are estimated to affect 8-10% of men in the United States³. For people with either of these forms of color blindness, greens, reds, oranges, and browns appear similar while blues and yellows stand out more. In contrast, loss of blue cones (called tritanopia) is much more rare, occurring in less than 1 in 10,000 people³. This type of color blindness causes greens to look blue and yellows to look more violet.

An extremely rare form of color blindness is total color blindness, or monochromacy, caused by the absence of two or more cone types. Individuals with only one type of cone remaining (cone monochromacy) can see perfectly clearly just without any color. On the other hand, people with no cones at all (rod monochromacy) oftentimes have other visual impairments in addition to complete loss of color vision, such as extreme light sensitivity and overall reduced vision³.

How do other animals see color?

While humans have three types of cones, many domestic mammals like dogs and cats only have two types, one blue and one in the green to red wavelengths^4,5. This means that your pet sees the world much like a red-green colorblind human does!

Color vision gets even more interesting as we look further into the animal kingdom. Both birds and bumblebees have cones that respond to ultraviolet light, allowing them to see a wavelength that is too short for human eyes to detect^6,7. On the opposite end of the light spectrum, reptiles such as pit vipers have cones that detect the infrared light emitted by heated objects⁸, allowing them to view temperature as a “color.”

While many animals have anywhere from 2-4 types of photoreceptors, some animals have even more. For instance, the mantis shrimp has 12 different photoreceptors⁹, while butterflies have 15⁷! While some of these photoreceptors respond to light that humans aren’t able to see, others are just tuned to a much more narrow range of the visible light spectrum than human cones⁹. For example, while humans have one type of cone that responds to green light, these animals might have 4-5 that are sensitive to various shades of green, perhaps making these shades of green look as different to them as blue and orange look to us.

One big advantage to having so many types of photoreceptors is that the brain can process color much more quickly than a human brain can. Because each photoreceptor responds to a narrower range of light, they can communicate the exact wavelength of light they detected directly to the brain. This means that the brain of a mantis shrimp does not have to wait to collect and integrate signals from all three photoreceptors like the human brain, which requires more time and energy. This allows the shrimp to quickly assess and respond to its environment, an important skill for any predator.

While we may often take it for granted, color is a critical part of how we process the world around us. It allows us to determine whether our fruit is ripe enough to eat, or quickly recognize which players on the field we should be rooting for. It even enables us to recognize things faster and remember them better^11,12. While scientists have learned so much about color vision, there is still even more left to understand. For example, why do certain colors evoke specific emotions in people? Researchers hope that by understanding how the brain processes and distinguishes various colors, they can also gain a better understanding of how it categorizes other things, such as up from down, or good from bad.

 

Image References

Cover image via Pixabay

Figure 1 from OpenStax College via Wikimedia Commons, CC-BY 3.0

Figure 2 from Johannes Ahlmann via Flickr, CC-BY 3.0

 

References:

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