PART ONE OF TWO
By: Lara Boyle
February 26th marks a strange and special date in the history of the Internet. On a musician’s fan page, a photo of a dress surfaced along with a plea: “Guys, please help me. Is this dress white or gold, or blue and black?” Over the next week, the picture surged across social media. In its wake it left divided friendships, unresolved quarrels, and a general state of confusion.
Three years have passed since “the dress” took over the Internet, and the photo still seems to straddle two realities. In one reality, the dress is definitively black and blue, while in the other the dress is without question white and gold. The confusion arises because brains, not eyes, decide what color people see.
Last month I delved into the biology of cones, which respond to different colors of light by “firing” an electrochemical signal towards the brain. In some theoretical world, this information could pass unchanged and unfiltered on its journey, and in the process create vision that better resembles objective reality than the world humans actually see. However, the brain is not interested in generating a completely accurate representation of reality. Rather, it aims to present a reality that is useful for survival.
Color constancy, the ability to recognize an object’s “true” color even when the environment changes, is one of the most important devices in the brain’s toolkit. For example, there are a host of red poisonous berries across the United States. To not to eat those berries, one needs to recognize their tell-tale color on a bright sunny day, in a blustering dark storm, and in the red and golden hues of a soft sunset. It is this phenomenon that explains “the dress” – the brain sees different colors based the ambient light level in which it thinks the picture was taken. Gold and white appears if the brain interprets the dress cast in a shadow, while blue and black becomes prominent if the brain interprets the dress as overexposed in a flash of light.
Color processing starts with cones, but it doesn’t end there. Each cone sends information about only a particular point that the eye can see, sort of like a single pixel in a camera. Cones makes synapses, or connections, with other cells called bipolar cells. The bipolar cells tinker with the signal, then pass information onto the next layer of cells called ganglion cells. In this way, brain cells form a circuit where cells talk to one another, with each layer of cells changing the information it receives.
A small number of cones and bipolar cells talk to each ganglion cell. Because each cone corresponds to a single point in the visual space, the ganglion cells map out a part of space and process information about that area’s color and brightness.
Below is a visual representation. An observer, who we will call Fludd (after Robert Fludd, who drew this figure as a representation of consciousness), is checking out his favorite painting of a red poisonous berry in a green field. In his retina, red and green cones fire based on the wavelength of light that they receive from the painting. A group of cones communicate with a single ganglion cell through bipolar cells. Here, I simplified the number of cones to just nine, but, in reality, more cells would be part of the conversation.
By gathering information from a group of rods or cones, ganglion cells can figure out all sorts of interesting data about an area of space. Are there contrasting colors or brightness in the area? Do the colors and brightness change over time? One type of ganglion cell is the single-opponent cell, which asks whether there is a color difference within a population of firing cones.
Single-opponent cells come in two flavors – those that process red-green light and those that process yellow-blue light. Below is an example of a cell that responds to red-green light. Specifically, scientists call it a “red-on, green-off” cell because it increases its firing if the central point in space (for instance, the point that corresponds to the berry) registers red light but decreases its firing if the surrounding area is green. These cells will fire a lot when someone is looking at a square that is completely red, but not fire at all if someone is looking at a square that is completely green. The opposite would be true for the “green-on, red-off” ganglion cell.
These cells present colors as dualities, and as such limit the color spectrum we can see. For instance, it is easy to imagine a greenish blue or a yellowish red, but very hard to image a yellowish blue or a greenish red.
Apart from single-opponent cells, there are also double-opponent cells in the brain. Finally, we have reached the cells responsible for color constancy. These cells take more than one color into account when monitoring the central and surrounding regions of an area in the visual space. An example can be seen below. This cell will increase its firing if cones detect red light in the central region or green light in the surrounding region. They will decrease firing if cones detect green light in the central region or red light in the surrounding region.
This concept is tricky, so here’s an example. Take our friend Fludd. He looks at his favorite painting, with a focus on that murderous berry. The red-green double-opponent cell registers red in the central position and green in the surrounding position. This combination is the magic mixture that causes the cell to fire at a high rate.
Fludd isn’t happy, though. Something is missing from his favorite painting. He buys a blue lightbulb. Turning it on, Fludd could not be happier as the room is bathed in a bright aquamarine light. The red-green double-opponent cell tries to sort out the signal in the new light by determining the ratios of red and green light. Since the ratio of the red/green light has not changed, the cell will continue to fire at the same frequency as it did under white light. The berry still looks red and deadly.
This post covered a lot of science, and not as much art, so as I wrap up I’d like to quickly mention an important tool artists use: the complementary colors.
If you are like me, you learned about the complementary colors during an art class in elementary or middle school. Complementary colors were originally defined as colors on the opposite ends of the color wheel, determined by mixing red, yellow, and blue paints. The Impressionists prominently used complementary colors after they recognized that colors change their appearance depending on their neighbors. Monet remarked in 1888, “…Color makes its impact from contrasts rather than from its inherent qualities….the primary colors seem more brilliant when they are in contrast with their complementary colors.”  Today, artists continue to use this principal to emphasize or mute aspects of their paintings.
However, as we have discussed, all colors are combinations of red, green, and blue light – not red, yellow, and blue paint. Thus, the opposite of red is a mix of green and blue light, or cyan. The opposite of green is magenta, and the opposite of blue is yellow. Compare the images below, with the traditional complementary colors on the left and the updated complementary colors on the right. Which pops from the screen more?
It may be all the better that artists primarily use the system on the left to add color emphasis. The colors on the right are so vibrant that they are almost painful to observe. There might be much to learn from them, however. An object’s shadow often carries a hint of the object’s complementary color. For instance, a red poisonous berry will have a shadow with notes of blue and green. Painters who seek realism can use this information to add lumosity and depth to their shadows.
The single-opponent cells also represent a type of color complementation, since they define colors that can or cannot exist together. Green cannot exist in red, while yellow cannot exist in blue. As this knowledge gains more traction in the art world, I look forward to seeing the kind of art it inspires.
Next month, I will finish this two-part series on color constancy by turning my lens (I love a good pun) onto the ways this principle changed the world of photography and art conservation.
- Ball, Philip. Histoire vivante des couleurs: 5000 ans de peinture racontée par les pigments. Hazan, 2010. pg 260.