Color Vision

The earliest theories of vision were egocentric. They thought rays came out of the eyes, reflected off an object (or captured it) and returned it the eye. The images were conveyed to the soul. Thought was believed to be the manipulation of images. There was no imageless thought.

Issac Newton (1643-1727) is well known for discovering calculus, articulating 3 laws of motion, and for studying and naming the phenomenon of gravity. He also was quite interested in optics. Nearly 65 years after Galileo made his first telescope, Newton created his own. But Newton is better known for his experiments with prisms.

In the mid 160os, Newton conducted a series of experiments on color. He put a pinhole in his window shade, making a sharp, narrow beam of light, which he aimed at a prism. The prism decomposed the incoming light into its elemental components.

For example, sunlight, white light, is not a color. It is not a “pure” light. it is the sum of all colors. Black is the absence of light.

When sunlight goes through a prism, it comes out as a spectrum, an array of colors. In particular, a ray of white light might enter the prism as a circle but it exited as a band of colors. It was always the same colors, and always in the same order: ROYGBIV. That is, red, orange, yellow, green, blue, indigo and violet.

This was not a surprise. Anyone who played a prism or glass of water knows a rainbow appears. Newton took it another step. He took some of the colored light and tried to recombine them. He found that combining red, green and blue together resulted in white light, sp he called these three primary colors. With more experimentation, he identified three secondary colors (yellow, cyan and magenta)..

Tertiary colors are formed by mixing one primary and one secondary color together. The six combinations are yellow-orange, red-orange, red-purple, blue-purple, blue-green, and yellow-green.

Newton described the interrelationships of colors as being a wheel. To create your own, place red, green and blue evenly spaced on the wheel. Then fill the remaining space with color combinations.

Color wheel

Newton’s wheel helped in the understanding of color relationships. Analogous colors are any 3 colors which are side by side, such as yellow-green, yellow, and yellow-orange. Usually one of the 3 colors is predominate.

Complementary colors are any 2 colors directly opposite each other, such as red and green.

Color harmony can be discovered by placing a geometric shape on the wheel. It is any pleasing combination.

With light, adding one color to another works well. If you are lighting a theater stage, mixing shades of light follow Newton’s wheel because he was studying light.

Mixing different colored paints, as any kindergartner will tell you, will produce brown. Paint is a reflected color. Light hits it, energy is absorbed, and the residual is seen by the eye. Reflected light is not the same as emitted light. Mixing colored light will approximate white light.

The color of a light is its hue. A light could then be described as having a red hue, or a green hue. Consequently, mixing colored lights is the mixing of hues. The brightness is different. It is the intensity of a color. It would be the difference between bright and dark red. Using different hues and different levels of brightness makes mixing more difficult. 9n addition, there is saturation to think about. This is the strength of a color, how much white it contains. Pink, for example, is desaturated red. Mixing is quite complex.

Functions of Color

Color can function in several ways.most of these you have encountered yourself.

First, it can facilitate perceptual organization. Color helps highlight form, gives depth cues, and can indicate motion.

Second, color can help separate figure from ground.

Third, color helps shift attention. They say to look here or notice this.

Airport signs

Fourth, color can make life easier. You can color code your files, only buy black socks, or highlight the coffee pot so you can find it in the morning. You can choose a color to stand out or fit in.

Fifth, you can use color to impact mood. You can select a color palette you find pleasing, or use color to try to influence others.

In a controversial series of studies, Alexander Schauss tried hundreds of shades of pink to find one that is maximally soothing. He concluded that the RGB combination of R:255, G:145, and B:175 produces the best results. This is remarkably similar to the color of Pepto-Bismol. Painting the walls of a jail cell with this pink paint reportedly reduced hyperexcitability.

But there are problems. Research on mood is extremely difficult because moods change frequently. There is no good operational definition of hyperexcitability. The studies were not double blind, judging of behavior was not objective, and the results haven’t been replicated.

Sixth, color adds beauty to life. Think of all the sunsets you’ve seen.

Seventh, color can improve the chance of survival. Colors that don’t stand out but work to camouflage from predators. The “walking stick” is an insect which, by a combination of the shape of its body and its color, looks like a twig on a bush.

Polar bears blend in with the snow. Leopards blend into the grass. Most birds have some form of camouflage. Wild turkeys, owls, and the great potoo are hard to see in their natural habitat.u Abbott Thayer’s 1907 painting Peacock in the Woods is a good depiction of actual camouflage.

camouflage

Theories of Color Vision

There are 4 parts to the process of seeing. There is the source, the object, the reflection, and receptive eye.

The source can be artificial but often is the sun. In te rms of color, the sun radiates white light, incandescent lights are fairly yellow, and other sources are typically between those two.

The object is the item we are viewing. In the case of an Apple, the object is the Apple itself.

The reflection is what bounces off the object. When rays from the sun hits it, the apple absorbs some of the radiation. What is not absorbed is reflected, and that’s what we see.

Absorption is not always uniform. Light spots on the Apple indicate little or no absorption. The dark stem has absorbed nearly all the light. The general redness of the Apple indicate that everything but red has been absorbed.

The eye of the receiver transducers the reflected light waves into neural patterns. And we perceive color.

Objects don’t actually have color. There have absorption. Color occurs in the mind.

Trichromatic Theory

Newton maintained that color was an inherent property of light. It is not an add on. Color is in light itself. He believed light is composed of particles. This agrees with Einstein’s 1905 proposal that there are discrete energy packets emitted. These packets are called photons.

Photons vary in size, and are measured in nanometer (nm), a billionth of a meter. We can detect photons in the 400 nm to 700 nm in range. They can’t be reduced to anything else but are elementary particles.

It took awhile to convince people but light also comes in waves.

Cones
In 1802, Thomas Young proposed that light is composed of waves, and we have have small cells in our eyes to detect those waves. In 1850, Hermann von Helmholtz expanded on Young’s work, and said the three types of cells were long-,medium- and short-wave detectors. It would be nearly 100 years before cones were physically discovered.

The Young-Helmholtz trichromatic theory is based on the idea that any color can be made by mixing 3 lights. If any color can be found by mixing red, green, and blue lights, then there must be three receptors in the eye to match.

It took many years to discover there are three types of cone pigments, each with peak sensitivity in different regions of the visible spectrum. Rather than red, green and blue, they are receptive to a range of values but are particularly sensitive to short, medium and long wavelengths.

The actual number of each cone type varies greatly between people but the relative relationship is the same. The largest number of cones are L cones, which report well to red but even better to green. This is 60-75% of the cones. The M cones are in second place. They easily respond to red and other long wavelengths. They also respond well to green.

The smallest category is the S cones, which respond best to blue and other short wavelengths. Much of our light is bluish, so we compensate for this by having less cones that are highly sensitive to blue. They represent about 10% of the cones. They are not in the fovea. We see blue but not as sharply as green images. The greatest density of S cones is just outside the foveal center.

Cones work together to represent a specific color. Obviously, activating S cones and not the other two types produces blue. Green is the result of little or no input from S cones, a lot from M, and a little from L. M alone gives green-blue. Add some L cone input to get yellow. Add more L to get orange. Color, then, is the result of firing patterns.

Newton, Einstein, Young and Helmholtz were all correct. It turns out light can be a particle or a wave at the same time, maybe. Quantum mechanics says they can occur at the same time but experiments show one or the other. So it is theoretically possible for light to be both particle or eave but in practice it is better to use one model or the other.

Helmholtz was wrong about one thing. The theory didn’t explain everything about color vision. In particular, it doesn’t explain re-green color blindness.

Opponent Process Theory

Ewald Hering had the audacity, from Helmholtz point of view, to question the trichromatic theory. Hering argued that color information was coded in pairs: red-green, blue-yellow and black-white. Like Helmholtz, there was no physical evidence. No one had seen the inside of the eye in enough detail to know what was there, or what they did. It was all theoretical conjecture.

What Hering could show is that red-green color blindness can’t be explained by the trichromatic theory. If you can’t see red or green, all you could see, according to the Young-Helmholtz model, would be blue. That would mean two cones weren’t working; only S cones would respond to stimuli.

Even in people with perfect vision, afterimages always are of an opposite color. Stare a green image for a long time, turn to a white wall, and you’ll see the image in red. It also works in reverse. But a red image doesn’t produce an afterimage in blue or orange.

Start with a red light. The only color light you can add to get white is green. Similarly, the only light you can add to blue to get white is blue.

As it turns out, the ganglion cells which take the outputs of the cones, code color in opposite pairs. it is opponent processing because the processing occurs in opposite pairs.

Ganglion

The processing in question occurs in the ganglion cells. Ganglion neurons have a spontaneous firing rate. It is a base rate while resting. When the ganglions are stimulated, the firing rate increases. When they are inhibited, the firing rate decreases.

Inside the eye, unmyelinated ganglions receive input from rods and cones. The overall ratio is 100:1, photoreceptors to ganglions. But that is misleading. On the edges of the eye, it is hundreds of rods to 1 ganglion. In the fovea, it is 1:1.

There are two major types of ganglion cells in the retina: And one other we should consider.

Midget retinal ganglion cells are small. Their dendrite trees are small. Their cell bodies are small. Their processing power is small. But there are a lot of them. Approximately 80% of all retinal ganglion cells are midget cells.

Midget cells receive input from relatively few rods and cones, and project to parvocellular layers of LGN. Sometimes they are called P cells or parvocellular cells because of where they are headed. P cells respond to changes in color but have slow conduction velocity. They respond to changes in contrast but only if the change is substantial. They Have a simple center-surround receptive field, meaning the center is ON for one cone but the surround (off center) is coded OFF for another cone.

Parasol retinal ganglion cells have a similar center-surround receptive field but have large receptive fields, large dendritic trees, and large cell bodies. These giant ganglion cells respond to low-contrast stimuli, and have fast conduction velocity. They are not very sensitive to changes in color but are great for carrying data for black-white images. They synapse with many rods.

Parasol retinal ganglion cells project to the magnocellular layers of LGN, so they are sometimes called M cells. They only account for about 10% of retinal ganglion cells.

There is a third type of ganglion cell in the retina, called giant retinal ganglion cells. There are only about 3000 in each eye, and they seem to have little to do with forming images. They contain a visual pigment (melanesian), and respond directly to light. They also receive input from rods and cones. They seem to encode luminance levels, and perhaps encode color and spatial information.

M cells (rods) and P cells (cones) go on to the LGN, which encodes their data into different layers. This presorted but raw data is then passed on to the occipital lobe.

Retinex Theory

Edwin Lard points out that neither the trichromatic or opponent process theory explains color constancy. When you select a blue sweater from your closet, it still looks blue in the kitchen, and still looks blue in the sunshine. The actual color changes with illumination but we automatically adjust to lighting conditions.

Retinex is a combination of retina and cortex. It suggests that photoreceptors and the brain adapt to various levels of light. Our perception remains relatively constant under changing circumstances. The theory proposes that illumination and reflectance must be included.