SHIFTING AS AN AID TO VISION.
By W. H. Bates, M. D.,
When the eye regards a letter with normal vision either at a near point or at a distance, the letters appear to pulsate, or move in various directions, from side to side, up and down, or obliquely. When it looks from one letter to another, not only the letters, but the whole line of letters and the whole card, appear to move from side to side. This apparent movement is due to the shifting of the eye and is always in a direction contrary to its movement. If one looks at the top of a letter, the letter is below the line of vision, and, therefore, appears to move downward. If one looks at the bottom, the letter is above the line of vision and appears to move upward. If one looks to the left of the letter, it is to the right of the line of vision and appears to move to the right. If one looks to the right, it is to the left of the line of vision and appears to move to the left. Persons with normal vision are rarely conscious of this illusion, and may have difficulty in demonstrating it; but in every case that has come under my observation the patients have always, in a longer or shorter time, become able to do so. When the sight is imperfect the letters may remain stationary, or even move in the same direction as the eye.
It is impossible for the eye to fix a point longer than a fraction of a second. If it tries to do so, it begins to strain and the vision is lowered. This can readily be demonstrated by trying to hold one part of a letter for an appreciable length of time. No matter how good the sight, it will begin to blur, or even disappear, very quickly, and sometimes the effort to hold it will produce pain. In the case of a few exceptional people a point may appear to be held for a considerable length of time; the subjects themselves may think that they are holding it; but this is only because the eye shifts unconsciously, the movements being so rapid that objects seem to be seen all alike simultaneously, just as the parts of a moving picture appear to be seen as one.
The shifting of the eye with normal vision is usually not conspicuous, but by direction examination with the ophthalmoscope, it can always be demonstrated. If one eye is examined with this instrument while the other is regarding a small area straight ahead, the eye being examined, which follows the movements of the other, is seen to move in various directions, from side to side, up and down, in an orbit which is usually variable. If the vision is normal, these movements are extremely rapid and unaccompanied by an appearance of effort. The shifting of the eye with imperfect sight, on the contrary, is slower, its excursions are wider and the movements are jerky and made with apparent effort.
It can also be demonstrated that the eye is capable of shifting with a rapidity which the ophthalmoscope cannot measure. The normal eye can read fourteen letters on the bottom line of a Snellen test card, at a distance of ten or fifteen feet, in a dim light, so rapidly that they seem to be seen all at once. Yet it can be demonstrated that in order to recognize the letters under these conditions it is necessary to make about four shifts to each one. At the near point, even though one part of the letter is seen best, the rest may be seen well enough to be recognized; but at the distance, in a dim light, it is impossible to recognize the letters unless one shifts from the top to the bottom and from side to side. One must also shift from one letter to another, making about seventy shifts in a fraction of a second. A line of small letters on the Snellen test card may be less than a foot long by a quarter of an inch wide, and if it requires seventy shifts to a fraction of a second to see it apparently all at once, it must require many thousands to see an area of the size of the screen of a moving picture, with all its detail of people, animals, houses, or trees, and to see sixteen such areas to a second, as is done in viewing moving pictures, must require a rapidity of shifting that can scarcely be realized. Yet it is admitted that the present rate of taking and projecting moving pictures is too slow. The results would be more satisfactory, authorities say, if the rate were raised to twenty, twenty-two, or twenty-four a second.
The human eye and mind are not only capable of this rapidity of action, but it is only when the eye is able to shift thus rapidly that the eye and mind are at rest and the efficiency of both at their maximum. It is true that every motion of the eye produces and error of refraction; but when the movement is short this is very slight, and usually the shifts are so rapid that the error does not last long enough to be detected by the retinoscope, its existence being demonstrable only by reducing the rapidity of the movements to less than four or five a second. Hence, when the eye shifts normally no error of refraction is manifest. The more rapid the unconscious shifting of the eye the better the vision, but if one tries to be conscious of a too rapid shift a strain will be produced.
Perfect sight is impossible without continual shifting, and such shifting is a striking illustration of the mental control necessary for normal vision. It requires perfect mental control to think of thousands of things in a fraction of a second, and each point of fixation has to be thought of separately, because it is impossible to think of two things, or two parts of one thing, perfectly at the same time. The eye with imperfect sight tries to accomplish the impossible by looking fixedly at one point for an appreciable length of time, that is, by staring. When it looks at a strange letter, and does not see it, it keeps on looking at it, in an effort to see it better. Such efforts always fail, and are an important factor in the production of imperfect sight.
One of the best methods of improving the sight, therefore, is to imitate consciously the unconscious shifting of normal vision, and to realize that apparent motion produced by shifting. Whether one has imperfect or normal sight, conscious shifting and swinging are a great help and advantage to the eye; for not only may imperfect sight be improved in this way, but normal sight may also be improved. The eye with normal sight never attempts to hold a point more than a fraction of a second, and when it shifts it always sees the previous point of fixation worse (1). When it ceases to shift rapidly, and fails to see the point shifted from worse, the sight ceases to be normal and the swing is either prevented or lengthened; occasionally it is revered. These facts are the keynote of the treatment by shifting.
In order to see the previous point of fixation worse, the eye with imperfect sight has to look farther away from it than does the eye with normal sight. If it shifts only a quarter of an inch, for instance, it may see the previous point of fixation as well or better than before; and instead of being rested by such a shift, its strain will be increased, there will be no swing and the vision will be lowered. At a couple of inches it may be able to let go of the first point; and if neither point is held more than a fraction of a second, it will be rested by such a shift, and the illusion of swinging may be produced. The shorter the shift, the greater the benefit; but even a very long shift—as much as three feet or more—is a help to those who cannot accomplish a shorter one. When the patient is capable of a short shift, on the contrary, the long shift lowers the vision. The swing is an evidence that the shifting is being done properly; and when it occurs the vision is always improved. It is possible to shift without improvement, but it is impossible to produce the illusion of a swing without improvement, and when this can be done with a long shift the distance can be gradually reduced till the patient can shift from the top to the bottom of the smallest letter on the Snellen test card, or elsewhere, and maintain the swing. Later he may be able to be conscious of the swinging of the letters without conscious shifting.
No matter how imperfect the sight, it is always possible to shift and produce a swing, so long as the previous point of fixation is seen worse. Even diplopia and polyopia do not prevent swinging with some improvement of vision. Usually the eye with imperfect vision is able to shift from one side of the card to the other, or from a point above the large letter to a point below it, and observe that in the first case the card appears to move from side to side, while in the second the letter and the card appear to move up and down.
In some cases the eyes are under such a strain that no matter how far a patient looks away from a letter he sees it just as well, so long as he sees it at all, as if he were looking directly at it. In these extreme cases of eccentric fixation considerable ingenuity is sometimes required, first to demonstrate to the patient that he does not see best where he is looking, and then to help him to see an object worse when he looks away from it than when he looks directly at it. The use of a strong light as one of the points of fixation, or of two lights five or ten feet apart, has been found helpful. In such cases the patient, when he looks way from the light, is able to see it less bright more readily than he can see a black letter worse when he looks away from it. It then becomes easier for him to do the same thing with the letter. The highest degrees of eccentric fixation occur in the high degrees of myopia, and in these cases, sine the sight is best at the near point, the patient is benefited by practicing seeing worse and producing the illusion of a swing at this point. The distance can then be gradually extended until it becomes possible to do the same thing at twenty feet. Usually such patients can begin shifting at the near point with the letters of the Snellen test card, but occasionally it is necessary to use a light, or lights. In hypermetropia, too, the sight is often best at the near point, when the same methods can be used as in myopia.
After resting the eyes by closing, or by covering with the palms of the hands in such a way as to exclude all the light, shifting and swinging are often more successful. By this method of alternately resting the eyes and then shifting persons with imperfect eyesight have sometimes obtained a temporary or permanent cure in a few weeks.
Shifting may be done slowly or rapidly, according to the state of vision. At the beginning the patient will be likely to strain if he shifts too rapidly, and then the point shifted from will not be seen worse, and there will be no swing. As improvement is move the speed can be increased. It is usually impossible, however, to realize the swing if the shifting is more rapid than two or three times a second.
A mental picture of a letter can be made to swing precisely as can a letter on the test card. For most patients mental swinging is easier at first than visual swinging, and when they become able to swing in this way it becomes easier for them to swing the letters on the test card. By alternating mental with visual swinging and shifting rapid progress is sometimes made. As relaxation becomes more perfect the swing can be shortened, until it becomes possible to conceive and swing a letter of the size of a period in a newspaper. This is easier, when it can be done, than swinging a larger letter, and many patients have derived great benefit from it.
All persons, no matter how great their error of refraction, when they shift and swing successfully, correct their error of refraction partially or completely, as demonstrated by the retinoscope, for at least a short fraction of a second. This time may be so short that the patient is not conscious of improved vision, but it is possible for him to imagine it, and then it becomes easier to maintain the relaxation long enough to become conscious of improved sight. For instance, the patient, after looking away from the card, may look back to the large letter at the top, and for a fraction of a second the error of refraction may be lessened or corrected, as demonstrated by the retinoscope. Yet he may not be conscious of improved vision. By imagining that the C is seen better, however, the moment of relaxation may be sufficiently prolonged to be realized.
When swinging, either mental or visual, is successful, the patient may become conscious of a feeling of relaxation which is manifested as a sensation of universal swinging. This sensation communicates itself to any object of which the patient is conscious. The motion may be imagined in any part of the body to which attention is directed. It may be communicated to the chair in which the patient is sitting, or to any object in the room, or elsewhere, which is remembered. The building, the city, the whole world, in fact, may appear to be swinging. When the patient becomes conscious of this universal swinging he loses the memory of the object with which it started, but so long as he is able to maintain the movement in a direction contrary to the original movement of the eyes, or the movement imagined by the mind, relaxation is maintained. If the direction is changed, however, strain results. To imagine the universal swing with the eyes closed is easy, and some patients soon become able to do it with the eyes open. Later the feeling of relaxation which accompanies the swing may be realized without consciousness of the latter, just as the letters may swing without consciousness of the fact, but the swing can always be imagined when the patient thinks of it.
Associated with all failures to produce a swing is strain. Some people try to make the letters swing by effort. Such efforts always fail. The eyes and mind do not swing the letters; they swing of themselves. The eye can shift voluntarily. This is a muscular act resulting from motor impulse. But the swing comes of its own accord when the shifting is normal.
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