Chapter 8: Color Vision

Introduction to Color Perception

Our perception of color involves complex physiological and psychological processes that enable us to differentiate between various wavelengths of light, even though light itself does not inherently possess color. In this chapter, we examine the intriguing world of color vision, exploring the major concepts, physiological mechanisms, and color mixing theories that underlie our ability to perceive and appreciate different colors.

The Nature of Color

Visible light, a small portion of the electromagnetic spectrum, is what we perceive as color. The diverse colors we encounter, such as red, green, and blue, are our psychological interpretations of different wavelengths of light. For example, when we see a 750-nanometer wavelength of light, we label it as "red," but in reality, it's simply a specific wavelength of light with no intrinsic color. Despite this, color plays a significant role in our lives, influencing various aspects, including art, design, psychology, and even tourism.

Color's Impact on Perception and Behavior

Color has a profound psychological impact on our decisions and behaviors. For instance, it affects our choices in food and beverages; we tend to be wary of consuming foods with unusual or unexpected colors. This phenomenon was notably exemplified in the 1990s when the market introduced clear drinks like "Crystal Pepsi" and clear alcoholic beverages like "Zima," both of which were met with skepticism and limited success. Even today, unusual colors in food and beverages can be off-putting to many individuals.

Color constancy illustrates our tendency to perceive the color of an object as relatively consistent, regardless of the lighting conditions. For example, a banana under green- tinted lighting will still appear yellow to us due to our brain's ability to account for the color shifts caused by the surrounding illumination.

Early Research on Light and Color Vision

Isaac Newton's groundbreaking experiments revealed that white light is composed of various wavelengths of light. This discovery challenged the prevailing notion that white light was pure and sent from the heavens. Instead, Newton demonstrated that white light could be broken down into its constituent colors when passed through a prism. He also showed that these colors could be recombined to form white light. This work laid the foundation for our understanding of color vision.

Trichromatic Theory of Color Vision

One of the key theories explaining color vision is the trichromatic theory, which suggests that we have three types of color receptors, or cones, in our eyes. These cones are sensitive to specific portions of the electromagnetic spectrum: short-wavelength cones for blue, medium-wavelength cones for green, and long-wavelength cones for red. By combining the signals from these three types of cones, our brain creates the perception of a wide range of colors.

Trichromatic theory is supported by the fact that people can match colors by adjusting the intensities of three primary colors: red, green, and blue. This concept is central to additive color mixing, where colors are created by adding different wavelengths of light together. When red, green, and blue light are combined in various proportions, we can produce a broad spectrum of colors.

Additive and Subtractive Color Mixing

Additive Color Mixing:

Additive color mixing occurs when colored lights are combined to produce new colors. It is commonly used in digital displays, such as computer screens and television monitors. In additive color mixing, different wavelengths of light are added together, and the result becomes lighter as more light is added. This process is analogous to how colors are mixed with flashlights or projectors (you might have experience with this if you’ve worked on lighting in a theater).

The primary colors used in additive color mixing are red, green, and blue (RGB). By varying the intensities of these three colors, we can create a wide range of colors, including white. Notably, when red and green light are mixed at full intensity, they produce yellow, demonstrating the additive nature of this color mixing process. You can prove this to yourself by looking at red and green dots from a distance. When the two colors are intermixed the resulting display appears yellow. Artists like Seurat have used this effect in a technique called pointillism.

Figure 8.1

Subtractive Color Mixing:

Subtractive color mixing involves mixing pigments or dyes to create colors. This method is commonly used in color printing. Unlike additive color mixing, subtractive color mixing starts with a surface that reflects all colors (usually white) and subtracts specific colors by absorbing them.

The primary colors used in subtractive color mixing are cyan, magenta, and yellow (CMY). When these pigments are mixed together in various proportions, they subtract or absorb certain wavelengths of light, resulting in different colors. For example, mixing cyan and magenta pigments subtracts red and green wavelengths, leaving blue light to be reflected, creating a blue color. When all three primary pigments are mixed together

in equal amounts, they subtract all colors, resulting in a dark or black appearance. If you have an inkjet printer then you might be more familiar with subtractive color mixing.

Figure 8.2

Opponent Process Theory

While trichromatic theory explains how we perceive a wide range of colors, another theory, known as opponent process theory, complements our understanding of color vision. Opponent process theory posits that we have specific cells, particularly ganglion cells, in our eyes that respond in an opponent manner to pairs of colors. In this theory, red and green are considered opposing colors, as are blue and yellow. This theory is supported by the fact that the receptive fields of many ganglion cells and LGN cells are opponent for color in this same arrangement. For example, blue light might increase the firing rate of a cell when presented in a central region while yellow light inhibits the activity in a surrounding region (i.e., +B/-Y). Similarly, red light might increase the firing rate of a cell when presented in a central region while green light inhibits the activity in a surrounding region (i.e., +R/-G). Opponent cells of this sort are created by the way in which a three- cone system is witred. When viewing the wiring diagram in the figure an open Y connection is facilitatory and a perpendicular T connection is inhibitory. It is also important to understand that red and green sum in an additive manner to create yellow. Here, the amicrine layer of cells (A) is hypothesized to play a critical role in summing the activity from mid and long wavelength cones (to create the perception of yellow).

Figure 8.3

If a ganglion cell is sensitive to red and green, it will respond with an increase in activity when exposed to one of these colors and a decrease in activity when exposed to the opposing color. This opposition between colors accounts for color aftereffects. Staring for a prolonged period results in perception of the opponent color when later viewing a white or achromatic surface. What do you see if you stare at a green, black, and yellow flag for 1 minute and then look at a white wall?

Figure 8.5

Color Processing in the Primary Visual Cortex

Now, let's explore how the human brain processes color information. We've discussed how ganglion cells, the lateral geniculate nucleus (LGN), and the striate cortex play essential roles in color perception. These structures transform the signals from cone cells into our conscious experience of color.

When light enters our eyes and activates the cone cells, the information is sent through the optic nerve to the LGN, where color opponency begins to take place. Ganglion cells in the LGN exhibit responses that are either excitatory or inhibitory to different colors. This opponent processing ensures that we perceive colors as distinct and not mixed.

The information is then relayed to the primary visual cortex (striate cortex), where complex neural processing occurs. Within the striate cortex, we find the so-called "blobs" of cells. These blobs are specialized regions where color information is processed. Within the blob regions two classes of cells have been identified that respond to colors.

The first class, denoted as Type I cells, exhibit response patterns that closely resemble those observed in ganglion cells and cells within the lateral geniculate nucleus (LGN). Type I cells exhibit a center-surround arrangement, where certain color pairs are in opposition. For example, in one scenario, the presence of red in a central region is excitatory and green in a surrounding region is inhibitory (+R/-G).

In contrast, Type II cells exhibit a slightly different response profile and there are two types of Type II cells. For one of these, a specific color, such as red, may evoke excitatory responses in a particular region, while the opposing color, green, elicits inhibitory responses within the same spatial region, rather than in the surrounding area. For the other type, known as "double opponent" cells, the central region responds in an excitatory manner to one color while inhibiting responses to its opposing color (e.g., +R-G). Then, in the surrounding region, double opponent cells respond to light in the opposite manner (e.g., +G-R).

Figure 8.6

Color Vision Deficiencies

Color vision deficiencies, commonly known as color blindness, can result from various genetic factors. The most common types are red-green deficiencies, which affect the ability to distinguish between these two colors accurately. These deficiencies are more prevalent in males because they involve genes located on the X chromosome.

Color vision deficiencies, often referred to as color blindness, are usually inherited genetic conditions. These conditions stem from anomalies in the X chromosome. Because males have one X chromosome (XY) and females have two (XX), the way color vision deficiency is inherited and expressed differs between genders.

For a male (XY) to exhibit color vision deficiency, the single X chromosome he possesses must carry the gene responsible for the condition. Since males have only one X chromosome, there's no second X chromosome to provide a normal gene that could override the deficient one.

For a female (XX) to show color vision deficiency, both of her X chromosomes need to carry the gene for the condition. If only one of the X chromosomes has the gene, the other X chromosome can compensate, and she typically will not exhibit a color vision deficiency. This scenario makes color vision deficiencies less common in females than in males, as a female would need to inherit a color vision deficiency from both parents, while a male only needs to inherit this from his mother.

Punnett Square Example: A Punnett square is a diagram that is used to predict the genetic makeup of the offspring from two parents.

Let's consider a scenario to illustrate this: The gene for normal color vision on the X chromosome will be represented as "XN" (where "N" denotes not problematic for color vision). The gene for color vision deficiency on the X chromosome will be represented as "Xp" (where "p" denotes problematic for color vision deficiency). The Y chromosome will be represented as "Y" since it does not carry a gene for this trait.

Scenario: A woman who is a carrier for a color vision deficiency ("XNXp") has children with a man who has normal color vision (XNY).

Mother's genotype: XNXp Father's genotype: XNY

To predict the possible genetic outcomes for their children, we can set up a Punnett square:

From this Punnett square, we can infer:

There's a 25% chance (1 out of 4) the child will be a girl with normal color vision (top left of table). There's a 25% chance (1 out of 4) the child will be a girl who is a carrier like her mother (bottom left of table). There's a 25% chance (1 out of 4) the child will be a boy with normal color vision (top right of table). There's a 25% chance (1 out of 4) the child will be a boy with a color vision deficiency (bottom right of table).

The most common types of red-green color vision deficiencies are deuteranopia (lack of functional green cones) and protanopia (lack of functional red cones). Individuals with these deficiencies see the world in a limited color palette compared to those with normal color vision. For them, red and green objects may appear similar or indistinguishable.

Figure 8.7

Our perception of color is influenced not only by the physical properties of objects but also by contextual factors. Some of these phenomena include:

1. Simultaneous Color Contrast: Colors can appear differently depending on the surrounding colors, showcasing the role of context in color perception.
2. Color Constancy: Our brain adjusts for changes in lighting conditions to maintain a consistent perception of an object's color.
3. Dress Illusion: The infamous dress photograph (see Figure 8.8) highlights how different lighting conditions can lead to variations in color perception among individuals. If participants perceive the background illumination as golden then they will perceive the dress as having blue/black. However, if you perceive the background as having a blue illumination then you will perceive the dress as white/gold. You can see this same effect in Kahan's Bates Bobcat Illusion (see Figure 8.9). To try the demonstration, click the link to open a Google document. Then go to 'File' and select 'Make a copy.' Once you've opened your own copy in your Google Drive, you'll be able to interact with the image and see for yourself how changes in background lighting can make an image appear blue and black or white and gold.
4. Benham's Top: A rotating monochromatic pattern (see Figure 8.10) can create the illusion of colors. The most common explanation for this is that the rotation stimulates the three types of cones to fire at slightly different rates.
5. Illusory Color Contours: Specific visual patterns can create the perception of colors where none exist, illustrating the complexity of our visual processing.

Figure 8.8

Figure 8.10

Emerging research strongly suggests that the V4 region of the brain plays a pivotal role in color perception. Studies utilizing neuroimaging techniques, such as functional magnetic resonance imaging (fMRI) and electrophysiological recordings, have consistently shown heightened activity in V4 when individuals are exposed to different colors, indicating its involvement in processing and discriminating various hues. Furthermore, experiments involving selective V4 lesions in animal models have demonstrated significant impairments in color discrimination tasks, providing compelling evidence for the critical role of V4 in the neural underpinnings of color perception.

Figure 8.11

Conclusions

Color vision is a complex interplay between the eye's cone cells, the brain's processing, and external environmental factors. While most people experience color vision in a similar way, variations in cone sensitivities and visual context can lead to diverse and sometimes surprising color perceptions.