by Leo Anghart
In the late 1950s Leo Hurvich and Dorothea Jameson provided quantitative data in support of the notion that color opponency plays an important role in processing color information. They used a hue cancellation procedure (hue is added to the stimulus until it turns white) to determine spectral sensitivities of the red–green and blue–yellow opponent channels.
Nerves leading to the brain
It is believed that the red–green and blue–yellow channels only code hue information. An independent brightness channel presumably codes the brightness. If the frequency of action potential is plotted as a function of wavelength then we see that a low wavelength stimulus (below 550 nm) causes inhibition, or a decreased firing rate from this cell. In contrast, a long-wavelength stimulus (longer than 550 nm) produces excitation, or an increased rate of neural firing. When a photosensitive cell responds to one portion of the spectrum with excitation and another portion with inhibition, it is referred to as a color opponent neuron.
The discovery of color opponent neurons in the visual system tells us that the receptoral information (trichomacy) is coded in an opponent fashion at post-receptoral levels. In other words, the three classes of color sensitive cone cells are “wired” together in such a way that they are spectrally antagonistic. This opponent processing occurs very early in the visual system, at the level of horizontal cells. It is scientifically established that hue information is encoded by red–green and blue–yellow neurons which are also involved in the red–green color perception deficiency. It is not clear whether brightness information is encoded by these neurons or by a separate class of non-color opponent cells.
Hue discrimination
The hue defines the smallest wavelength of light with the smallest difference that it is possible to differentiate with the human eye. We notice that there is a dramatic increase in the red–green sensitivity, producing exceptional hue discrimination ability in the green–yellow–orange–red region. This is a very useful feature because various foods and fruits display their readiness to be eaten by their color change. You can instantly see whether or not a strawberry is ripe by its color.
Color film is a complex arrangement of the three primary color filters. Kodachrome slides are able to reproduce all the complexity of color seen in nature.
Color vision is very complex and cannot be reduced to simple theories. It depends not only on wavelengths and intensities, but also on differences in intensity between regions and whether the patterns are accepted as representing known objects. This involves high level processing in the brain which is extremely difficult to investigate.
The eye tends to accept white not as a particular mixture of colors, but rather the general illumination. Thus you see a car’s headlights as white while on a country drive, but in town where there are bright white lights for comparison, they look quite yellow, and the same is true of candlelight. This means that what you take as a reference for white can shift. Expectation and knowledge of normal color is an important factor in color perception.
Color perception deficiency
Surprisingly, the common confusion of red with green was not discovered before the late eighteenth century, when chemist John Dalton found that he could not distinguish certain substances by their color although others could do so easily. Color vision tests depend on being able to isolate color as the sole identifying characteristic. Once this is established, it is easy to show whether a person has normal ability to distinguish between colors, or whether he or she sees as a single color that which others perceive as different colors. It is much more common to find a reduced sensitivity to certain colors than a complete absence of color.
The properties of red and green light required to match a monochromatic yellow is the most important measure of color deficiency. Lord Rayleigh discovered in 1881 that people who confuse red with green require a greater intensity of either red or green to match yellow. A special instrument, called an anomaloscope, has been developed for testing this color deficiency. The instrument takes advantage of the fact that yellow is always seen as a mixture of red and green.
The reason for red–green color perception deficiencies is not clear. However, experiments with the anomaloscope show that color anomaly cannot be due to color adaptation. The general belief is that red–green color perception deficiency is due to a reduction in the sensitivity of one or more color receptors (cone cells) in the retina, perhaps through partial loss of photo-pigment. There may be many causes, but it is not due to shortage of photo-pigment, otherwise the anomaloscope would not work. The common red–green color perception deficiency is more likely to be an interpretation of sensory data provided to the visual cortex that processes color vision.
The typical test used for defective red–green color perception is the Ishihara Color Test. This consists of scattered dots of different colors where the hues that are difficult for red–green deficiency people to see are used to form numbers. If these images are scanned into graphic software and the hue variable changed by about +70, a red–green color deficient person will clearly see the obscured numbers. Indeed, it may be possible to train the eye to see the colors that were confusing and then notice how they begin to be more distinct with less hue distortion.
The ability to label colors is, to a certain extent, a learned ability. As we grow up we learn to identify colors in the same way that we learn to recognize time on a watch. For some this learning is not quite complete, so color perception training may be necessary in order to improve the situation.
Wavelength (mm)
The graph above shows how people with normal color perception label colors. Each vertical bar indicates a successful identification. You will notice that there are many lines between blue and green and between yellow and orange. The blue–green spectrum is the one that causes the most trouble for people with the typical red–green color perception deficiency. Turquoise colors are confused with gray, especially if the intensity of the color is the same.
The other end of the spectrum also causes problems. Red-violet or scarlet colors are often confused with brown. As you can see, those with protanomaly and deuteranomaly can distinguish all the colors but with fewer distinctions between them.
It is possible to improve this discrimination by training your color perception and practicing the labeling skills. Most color perception deficiency is a red–green problem. This type of color vision is one of two main types related to the wavelength of the light. Long-wavelength red light (protanomaly) may be complete protanopia where blue is recognized but the green spectrum is perceived as white or gray and anything between green and red is distinguished by brightness or context. Each area appears to those with protanopia as one system of color with different brightness and saturation within each area.
With the more common protanomaly most colors are recognized. The part of the spectrum that normally appears blue-green is perceived by the person with protanomaly as a grayish or indistinct color. The complement to blue-green, which is red-violet, will also appear indistinct.
Middle wavelength green light may be complete deuteranopia where the part of the spectrum that is normally perceived as green, appears gray. The complementary color to green – violet-red – also appears gray or indistinct.
The less severe deficiency is deuteranomaly. In this case, there is no part of the spectrum that appears to be gray, but the green part of the spectrum (which appears to the deuteranope as gray) appears indistinct and close to gray.
Protanomaly and deuteranomaly are easier to improve than the more severe protanopia and deuteranopia. The latter have fewer distinctions to begin with so there is much more work involved.
Counting colors
Some people have difficulty in distinguishing variations of a color. This exercise develops your ability to see and label colors in different materials and in different lights.
1. For a period of a day, start counting all the examples of a color you already know. For example, one of the primary colors, red, blue or yellow.
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2. The next day count all the different shades of green you can find.
3. On another day count all the examples of other secondary colors you can find (the secondary colors are orange, green and violet).
4. Next, begin to identify and count all the gray shades you can find.
5. Progress to browns and earth colors like ochre, sienna and burnt umber.
Notice what the colors look like in different lights. How does brown look in the morning compared to how it looks at sunset? How does it look on a rainy day? Counting and identifying colors develops and refines your ability to see them. A large part of color perception difficulty has to do with the number of colors you can distinguish. Developing more reference experiences with progressively finer hue distinctions will eventually lead to better color perception.
Matching colors
When you develop your ability to distinguish more colors with the working with colors exercise below then you can begin to match colors. Perhaps you can even begin to learn more about the different ways of organizing colors.
1. Practice naming colors and have a friend verify them for you. You need feedback to keep your progress on the right track.
2. Arrange colors in order of intensity, so you have the lighter shade at one end and getting progressively darker and darker. Try to make the difference in hue as small as possible. Play especially with the blue-green and red-violet colors, as they are the ones you are likely to have the most difficulty distinguishing.
3. Progress to matching different materials and arrange them in order of intensity and hue. Try working with pieces of cloth and match sewing threads to the colors of the cloth.
Be creative and match and order things. The more you play this game the better.
Working with colors
In order to build broader color distinction, it is very valuable to experiment with either watercolors or pastels. The objective is to understand how colors work and especially what they look like. Color pigments behave differently according to the light, but in practice you are mostly asked to recognize the colors reflected off printed cards.
Play with the primary colors
All possible variations of color can be mixed from just the three primary colors – red, yellow and blue – so the first step is to play with these colors. Find the brightest yellow possible. It is probably called something like chromium yellow or bright yellow. Paint samples on a piece of white paper. Experiment with different intensities of yellow from 100 percent to just 1 percent – a very pale yellow that is almost imperceptible. Now look at these color samples in different lights and notice how they change.
Try painting the same yellow on neutral gray paper and notice how the colors change compared to the same color on a white background. When you are using pastels, try painting different intensities of yellow on black or brown paper. Notice how this changes the appearance of the colors. Next try to find similarities between all the test colors you have made so far.
Do the same with the blue and red paints. Some of the colors will be easy for you to notice, while others might be a stretch. In either case you are enriching your sense of color.
Play with the secondary colors
The secondary colors are made from mixing equal amounts of the three primary colors. First mix red and yellow to produce a bright orange. Next mix yellow and blue to produce green. Finally mix red and blue to produce violet. You now have samples of the three secondary colors and can make a six color spectrum. Take each of the 12 colors and make different densities of each of them. Notice what they look like.
The next project is to add one more color on each side of the six colors you already have. This will give you a 12 color circle. You can now begin to see how the colors blend together. If you like, you can add one more color between each of the 12 colors you have already. This will show an even finer blend between the colors.
Playing with gray and black
Next make a series of samples of gray and black, so you can get an idea of how black pigment behaves. Notice that pigments may look different depending on their purity. Begin to match gray shades and colored shades according to their intensity.
Playing with brown and earth colors
Browns are pigment mixtures of orange and black. Try mixing orange and black in equal quantities and then make this color into a series of lighter and lighter shades. Play with burnt umber, sienna, burnt sienna and ochre. Notice the different intensities and compare and match brown shades with the other color samples you have in your collection.
Experiment with adding slight amounts of violet and red to the browns and notice how this affects the colors. By now you already have a good understanding of how colors appear and how they work together. You could also read about how colors have been organized into different color systems to help you further.
Blending colors
The problematic area for you is likely to be in identifying the blends that lie between blue and green. This is the most difficult area for protanomaly eyes. To develop your ability to identify these colors, take an A3 sheet of paper and draw a large box filling most of the paper. Next divide this box into 10 levels from top to bottom. Then make 10 vertical columns so you end up with a grid of 100 squares.
Label the upper right hand corner 0 percent and the next lower box 10 percent, 20 percent and so on. Do the same horizontally. You will end up with a sheet that has progressive intensity from the right corner to the left corner and from top to bottom.
Next make a gradation of gray along the side. This will serve as a comparison of intensity. Now fill in the colors column by column. You will end up with a color chart that shows the gradual blend of green and blue. The diagonal from right top to left bottom includes all the colors you need to practice identifying.
Make another chart using red and violet. This is the complementary color of blue-green, another area where you need to develop finer hue distinction.
Arranging colors
Here is another exercise that is useful for arranging color samples in order of intensity. This exercise will develop your ability to label colors correctly. Initially you may need help from someone with good color perception to verify your arrangement.
Cut out samples of the colors you have made during the previous exercises. Alternatively you could use a collection of paint swatches. The purpose of this exercise is to arrange samples according to their intensity. Pay special attention to colors that have the same intensity, but a different hue. You are extending the limits of your ability to distinguish colors.
Now you can progress to matching and arranging different kinds of material. These could be fabric samples, pieces of thread and so on. Matching sewing thread to the color of fabrics is a great way of training your color perception. It is a game you can play whenever you like.
23. The Visually Impaired
Visual impairment varies a great deal with some individuals having absolutely no visual stimulus whatsoever. However, many people who are considered legally blind actually have some light perception and can benefit from Vision Training exercises. Many of those who have some quality of light perception – that is they can distinguish day and night and can recognize bright light – can improve their vision and possibly achieve object recognition. Clara Hackett, a Vision Trainer working during the 1940s and 1950s, reported that of the eight people she worked with that had only light perception, one man was able to resume his regular work and four attained object perception. Only three of the people she worked with showed no improvement (Hackett, 1955: 7).
People with object perception – who can identify pieces of furniture or people moving about – can often attain improvement of vision ability. Hackett reported that of 14 people with some object perception, eight gained useful vision and were no longer considered “blindish.” Of the 34 people who were considered occupationally blind, 16 were able to work again, while eight others showed improvement in visual acuity. Ten had no appreciable improvement.
This
is wonderful news; at least there is the possibility of improvement. The possibly traumatic events that lead to a diagnosis of visual impairment can leave a person without hope. While there is no guarantee that Vision Training exercises will work, there is the possibility that vision can stabilize and even improve. Whatever degree of vision you have now, why not try the following techniques for a month? If after a month you are not sure if you are really improving or only imagining it, continue for another month, and by that time you will have no doubts. It will be clear by then whether or not there are signs of definite improvement.
Gaining light perception
If you have no light perception at all, then your first efforts should be directed towards achieving this. The techniques are very simple. When the sun shines, turn your closed eyelids towards the sun for a minute or two about ten times a day. When you turn your head towards the sun you will feel its warmth. Move your head slowly from side to side always keeping your closed eyes towards the sun. The energy from the sun is pure and will help to energize your eyes. It is safe as long as you keep your eyes closed.
Relaxing your eyes
Relaxation is an important aspect of your Vision Training. To palm your eyes, rub your hands together vigorously so that they get warm. Place the palms of your hands over your closed eyelids without touching the eyes, with your fingers folded over your forehead. While palming try to imagine what it would be like to actually see some light. Think about things you remember with pleasure. Visualization is known to activate the mind–body connection. The basic principle is that energy follows thought, so the more things you can imagine the better.