distinguish six colors in the spectrum, or in the rainbow, red, orange, yellow, green, blue, and violet, while a com- 422 DIAGNOSIS FROM OCULAR SYMPTOMS paratively few can distinctly see a seventh color, indigo, between the blue and the violet. No one has yet been known to be able to recog- nize an eighth color, but if any individual were to claim that he could do so we might be obliged to admit such a thing to be possible, particularly if he had excellent vision of the other seven, because it has been demonstrated that persons whose color vision is supposed to be normal differ enormously in their powers of color perception. We have no more right to assume the extreme limit of the color perceiving center to be fully known, than the most of us have to deny the existence of those who can see indigo as a separate color because to us it is simply a deep shade of blue. Tests calculated to reveal slight differences show that the gradations in the perception of colors cover a very large range, and that distance, as well as at- mospheric conditions, affects in different ways the visibility of each. Occasionally a person will confound blue and green at first glance, but distinguish them after a moment. Any one will have difficulty in distinguishing a blue from a violet, or a yellow from an orange light, when it is seen from a distance, or in a fog, even though a red can be distinguished easily from a green one under the same circum- stances. Then there is a minority of people who can perceive only five, four, three, or two colors of the spectrum, and a few who are totally color blind.
<Callout type="important" title="Important">Color blindness can significantly impact survival tasks that rely on accurate color recognition.</Callout>
Color Blindness We say that a person is color blind when he is unable to perceive certain colors. If we use the term strictly in this sense it may per- haps be held to apply to nearly every human being, so ordinarily we use it to indicate a defect of the color sense that is sufficiently marked to unfit the person for certain callings. The most common form with which we have to deal is that in which red cannot be distinguished from green, and it is at the same time one of the most important, because it unfits the individual for service on railroads, or at sea, where red and green lights are used as signals. Equally im- portant, though much rarer, is total color blindness, in which no color at all can be perceived, but the whole world is supposed to appear in varying shades of gray.
Red is used as a signal light because the red rays are able to penetrate a fog to a greater dis- tance than any of the others, green because it is complementary to AFFECTIONS OF THE COLOR SENSE 423 red. If a man is blind to red it makes little difference from a prac- tical standpoint whether he can perceive green or not, that is whether he is a protanope or a deuteranope, he must be placed in the dan- gerous class so far as signals are concerned.
<Callout type="risk" title="Risk">Misidentification of red and green signal lights could lead to dangerous situations.</Callout>
Less important are the rare cases in which blue and yellow look alike, and those in which orange cannot be distinguished from yellow, or blue from green, because such defects are not likely to become sources of danger, although they render the persons incapable of following certain callings in which good color vision is required.
Color blindness may be either congenital or acquired. A child who is congenitally unable to distinguish red from green is apt to become aware of his defect very early, and to conceal it with great ingenuity. A gentleman who was thus afflicted said that when he was a boy he was obliged to take lessons in art, and to conceal his defect he labelled his paints, laboriously studied the proportions in which they should be mixed to produce certain shades of green and red, ascertained what colors were needed, and followed directions so carefully that his teacher never expressed any suspicion that green and red looked alike to him. He tried to explain how be was able to recognize colors under ordinary circumstances, which seemed to be largely through an elimination of possibilities or probabilities, but had learned that his method was unreliable through the purchase of a gaudy red necktie under the impression that it was a sober green.
These patients study every detail of appearance that can guide them in detecting the presence of a color to which they are blind, and sometimes they become remarkably expert, but it is as hard for them to tell how they do it as it is for anyone to convey an impression of any other sensation. It may be that their ability to sort yarns, to read party-colored letters, and to recognize colored lights, depends upon the nature of the training to which they have subjected their eyes, at least to some extent.
Some can pass either one, or both of the first two tests, or pass one to fail in the other, but probably few succeed in passing the lantern test with its mutations of brightness and distinctness, and the recognition of color developed in this way is not reliable. In other cases the defect is not so marked, and the patient may be wholly unaware of its presence until he enters some business in which good color vision is necessary, when he learns of its existence through his mistakes.
Such patients are apt to be worried and annoyed by the discovery, and the demonstration of the char- acter of the defect seems to be easy, as a rule. 424 DIAGNOSIS FROM OCULAR SYMPTOMS Acquired color blindness is apt to appear first as a minute cen- tral scotoma, to which the patient unconsciously adapts himself by utilizing his parafoveal retina for the recognition of color. As such a scotoma is due almost invariably to disease of the retina, or of the papillomacular bundle of optic nerve fibers, it tends to become larger and to produce a serious impairment of the central vision.
<Callout type="warning" title="Warning">Acquired color blindness can be an early sign of retinal disease.</Callout>
It is very important that we should be able to detect it, not only because it is a symptom of a disease which may not yet have mani- fested itself in any other way, but also because it is making the pa- tient dangerously color blind. It would be of interest to know what proportion of the persons who pass the lantern test and afterward are discovered to be color blind belong to this class, for, as long as the scotoma is small and the color sense remains unimpaired in the retina about the fovea, it is readily conceivable how the patient might succeed in passing all of the ordinary tests with perfect honesty on his part, and how the defect would become apparent both to himself and to others as the scotoma grew larger.
In all cases in which ac- quired color blindness is demonstrated or suspected the fundus should be examined carefully, for it is not unlikely that we shall be able to find there signs that help to guide us to a general disease, or to a toxic condition of the organism. Detection of Color Blindness It has long been claimed that in testing the color vision the pa- tient should not be required to name the colors, because theoretically he might be abie to distinguish them clearly without knowing their names, but Marshall makes the point that color ignorance does not exist apart from color blindness, that a person of average intelli- gence who calls primary colors by the wrong names does not see the difference between them, and goes so far as to say that a test cannot be reliable unless the colors are named. It may be well not to insist on the names when we are dealing with a foreigner who is not thoroughly conversant with our language, and possibly with a young child, but aside from these exceptions, Marshall’s point seems to be well taken.
Another point in favor of the naming of the colors is that this is absolutely necessary for the detection of amnesic color blindness, in which colors can be seen, recognized, and matched, but cannot be named. Amnesic color blindness is almost if not invari- ably associated with a right homonymous hemianopsia and other AFFECTIONS OF THE COLOR SENSE 425 symptoms of intracranial trouble, which lead us to think that the lesion is probably in such a place in the occipital lobe as to interrupt the tracts connecting the visual and the speech centers.
The commonest test is to match skeins of colored yarns in good daylight. A sample skein is handed the patient and he is told to se- lect the different shades of the same color from a considerable num- ber of skeins of various other colors. This test is known as Holm- gren’s, and has been modified in various ways by Oliver, Thomson, Edridge-Green, and others. It enables us to detect defects well enough for the ordinary purposes of life, so it is very useful in gen- eral clinical work, but it is insufficient when the question of com- petence to recognize railway signals is raised, because many persons can pass it who are absolutely red blind.
<Callout type="tip" title="Tip">When testing for color blindness, ensure that the patient names the colors to accurately assess their ability.</Callout>
In most cases we eliminate the probability of color blindness if the patient matches the colors quickly and correctly, and know him to be deficient if he hesitates and makes errors, but if he is slow in his choice, even though he is correct, he may have to regard certain colors for a moment to iden- tify them, or he may be habitually slow in all of his movements. E'dridge-Green devised another simple test in which the patient is given a tray containing a large number of beads of various sizes and colors, along with a pair of forceps and four boxes labeled red, yellow, blue, and green, each with a hole large enough to admit a bead, and is instructed to pick out the beads of these colors and drop each into its proper box. When he has finished the boxes are opened and the result is apparent.
Stilling invented another test in which letters are formed by spots of colors so that a person who is color blind will read them incor- rectly, but this seems to be not much more efficient than matching yarns. The best test yet devised to detect color blindness, which does not consist of a very small color scotoma, is a lantern in which colored lenses are placed before a light seen through apertures of varying sizes, and with the additional interposition of smoked, ground, ribbed and otherwise modifying glasses, so as to reproduce as nearly as possible the various conditions in which signal lights are seen in actual service on the railroad and at sea. Quite a number of de- signs of these lanterns have been invented.
A person fails to pass this test if he calls a color by the wrong name, and if he does not see a red, green, or white light that is visible to people with normal sight. Even this has not proved to be perfect, as individuals have 426 DIAGNOSIS FROM OCULAR SYMPTOMS been known to pass it who were discovered, either then or later, to have faulty color vision.
To detect a central color scotoma we have the patient fix his eye on a small spot or object which is made to change its color; if he does not recognize the color when he is looking directly at the spot, but does when it is moved a little to one side, or he moves his eye a trifle, he has a scotoma for that color. The test object must be at least equal in size to the minimum required for perception by normal eyes, which varies for different colors and proportionately to the distance. Commonly we use on the perimeter a disk about a tenth of an inch in diameter, which is close to the minimum for violet, and ten times the minimum for red.
<Callout type="important" title="Important">Central color scotomas can indicate underlying retinal diseases.</Callout>
We can measure the size of the scotoma, if it is large enough, by moving the test object in various directions until the color is perceived and marking the boundaries thus obtained on a chart. DEFECTS IN THE FIELD OF VISION THE NORMAL FIELD OF VISION The field of vision is the extent of space in which objects can be distinguished by an eye while it remains fixed on one point. Its peripheral limits for light and white are greatest, over 90°, to the temporal side, as rays of light which come from that direction in the plane of the pupil strike the cornea and are refracted into the eye, but they are circumscribed elsewhere by the tissues that surround the organ.
Tests of the Visual Field A rough test of the limits of the field of vision may be made by having the surgeon and the patient sit or stand face to face a short distance apart with their eyes about on a level. To test the patient’s right field his left eye and the surgeon’s right are covered, and, while each looks steadily into the exposed eye of the other, the surgeon brings his hand, or better a white object on a black rod, slowly in, on a level with the eyes, midway between him and the patient. As- suming that the surgeon’s field is normal, the object approaching from the temporal side should be perceived at the same moment by both him and the patient, but if the latter is unable to see it until it has been brought nearer to the median line, his field is contracted in DEFECTS IN THE FIELD OF VISION 427 that direction. The limit to the nasal side, above, and below is learned in the same manner, but it is not apt to agree exactly with that of the examiner, because of differences in the prominence of the features of the faces. This test is made easily and quickly, and by it we can detect any considerable contraction of the field, but it is not able to furnish us much of the information that we need when an abnormality is present.
Accuracy in measurement, and in the location of a de- fect in the field is secured by means of a perimeter, which consists of a metal arc marked off in degrees, which rotates about its central point, where it is attached to an upright bar secured firmly to a base or standard. Attached to the same standard with the upright is a head rest which can be so adjusted as to place the patient’s eye in the center of the curvature of the arc, his other eye being covered. The central point of the rotation of the arc is marked by an aperture, or a white spot, which serves as a point of fixation for the eye. While the patient’s eye remains fixed on this central point a white test object, placed in a carrier, is moved from without inward along the inner surface of the arc until it is perceived, when the degree on the are at which it stopped is noted and marked upon a chart. The arc is then rotated a certain number of degrees, fifteen, forty-five, or ninety, as desired, and the observation is repeated.
After the limit in each meridian has been marked on the chart we connect the points by a curved line, which marks the boundaries of the field of vision for white. ‘The boundaries of the color fields are determined in the same way, after we have substituted red, green, blue, or other colored test objects for the white ones.
Scotomata are detected by moving the test object along the arc from the periphery of the field toward the center, when the patient may notice that it disappears at a certain point to reappear at another. By changing the inclina- tion of the arc a few degrees, repeating the test, and noting on the chart the point at which the test object disappears and reappears in each position of the arc, the shape, size, and location of a scotoma can be mapped out accurately.
Measurements of the Field of Vision As the form and size of the field are influenced by the sunken con- dition or the prominence of the eye, by the lids, by the upper and lower margins of the orbit, by the shape and size of the nose, and to a slight extent by the size of the pupil, the condition of accommoda- tion, and the length of the eyeball itself, it is not surprising that the average figures given in various text books do not agree very closely. We may accept those given by Berry to be probably as nearly cor- rect as any. These are:—
Upward, 45°; upward and outward, 50° to 55°; outward, 90° (often slightly more) ; outward and downward, 80° to 85°; down- ward, 70°; downward and inward, 60° (variable on account of the nose); inward, 55° to 60°; inward and upward, 55°. The extent of the field upward and upward and inward is found to be 5° to 15° greater when the point of fixation is situated 20° or 80° from the center of the perimeter in the opposite direction.
Charts abound in the market in which an approximately normal field has been marked off, but it is evident that the boundaries in any individ- ual case are not likely to agree exactly with those indicated, except that outward the bounding line should always be at 90° or more. The limits of the color fields, likewise are subject to variation, but approximate fairly well with these figures, which
Key Takeaways
- Color blindness can significantly impact survival tasks that rely on accurate color recognition.
- Acquired color blindness is often an early sign of retinal disease and should be carefully monitored.
- Testing for color blindness using the naming method ensures more reliable results, especially in critical situations like signal recognition.
- The field of vision varies based on individual factors such as eye prominence and nose shape.
Practical Tips
- Always ensure that individuals with potential color blindness are tested before assigning them tasks requiring accurate color perception, particularly in emergency response scenarios.
- Use alternative methods or equipment to compensate for color blindness when necessary, such as using different colored filters or labels on objects.
- Educate team members about the signs and risks of acquired color blindness so they can recognize early symptoms and seek medical attention.
Warnings & Risks
- Misidentification of red and green signal lights due to color blindness could lead to dangerous situations in emergency response scenarios.
- Acquired color blindness may indicate an underlying retinal disease, which requires immediate medical evaluation.
- Color perception tests should be conducted by trained professionals to ensure accurate diagnosis and appropriate action.
Modern Application
While the historical techniques for detecting color blindness and visual field defects are still relevant in understanding human vision, modern technology has improved diagnostic tools and methods. For instance, digital retinal imaging can provide more detailed information about potential retinal diseases than manual testing. However, the importance of recognizing these conditions remains crucial for ensuring safety and effective emergency response.
Frequently Asked Questions
Q: What are the most common forms of color blindness mentioned in this chapter?
The most common form of color blindness discussed is the inability to distinguish red from green. This condition unfits individuals for service on railroads or at sea, where red and green lights are used as signals.
Q: How can acquired color blindness be detected according to this chapter?
Acquired color blindness can be detected by the appearance of a minute central scotoma. The patient unconsciously adapts to it by utilizing their parafoveal retina for recognizing colors, but as the scotoma grows larger, it leads to a serious impairment in central vision.
Q: What is the significance of color blindness in emergency response scenarios?
Color blindness can significantly impact survival tasks that rely on accurate color recognition. For example, misidentification of red and green signal lights could lead to dangerous situations during emergency responses.