LEGRAND 03606 PDF

Show more. (96)Get rights and content P. Tougne, H. Hommel, A.P. Legrand, N. Herlin, M. Luce, M. Cauchetier. DOI: /(96) Cite this publication + 2. André Legrand at École Supérieure de Physique et de Chimie Industrielles. André Legrand. (Pease et al., ; Leon and Legrand, ). Human-. derived al., ; Léon and Legrand, ). Research Letters 32(3):L

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YOUNG Development of Optical Characteristics for Seeing Although it is not yet possible to describe completely the development of the optical characteristics of the human eye, it is possible, by relating results of published and unpublished studies with some amount of con- jecture, to put together a likely description.

Much of the available information concerning the effects of heredity and environment on the development of the optical characteristics of the eye is based on studies of subhuman primates, primarily chimpanzees and monkeys.

However, a recent study of the Eskimo population at Barrow, Alaska, which supports the results of studies of subhuman pri- mates, suggests strongly that the early visual environment and early visual experience play an important role in developing and modifying the opti- cal characteristics of the eye, that the reaction of the eye to its visual environment plays a determining role in the development of the optical characteristics necessary for seeing and reading, that the mechanism of this role should be investigated in all children as they approach the read- ing age, and that the mechanism cannot be effectively assessed by deter- mining the Snellen visual acuity at 20 ft or at 20 in.

YOUNG about how he accomplishes this resolution. For practical purposes, the primary value of the Snellen acuity at either far or near distance would be to demonstrate whether a person has some type of visual refractive error large enough to prevent compensating for it through the use of accommodation, head tilt, squint- ing, or other techniques. The refractive characteristics of the eye may be readily determined by retinoscopy, either with or without a cycloplegic drug, such as atropine or Cyclogyl cyclopentolate.

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This technique provides an adequate de- scription of the gross refractive characteristics of the eye and indicates whether the person has any degree of astigmatism in conjunction with a measurable degree of myopia or hyperopia. This approach, which re- quires some clinical skill, permits the clinician to determine which lens or combination of lenses will bring about the neutralization of his ret- inoscopic shadow in a given eye under a given set of conditions.

Because he is examining under a state of maximally relaxed accommodation, not an ordinary state, he cannot accurately predict what will happen when the eye is functioning without the effect of the cycloplegic.

If he per- forms retinoscopy without the use of a cycloplegic drug, but attempts to induce relaxed accommodation through the use of a plus lens, he can describe more accurately what the subject can accomplish visually under more nearly normal conditions. In that case, the clinician may be de- ceived effectively by the subject, and conclude that he has achieved a rather basic measure of refractive characteristics, whereas his results may actually be affected by a considerable degree of accommodation within the eye; he is not on much safer ground if he depends on a subjective refraction, calling for the patient’s cooperation when he places lens com- binations in front of the patient’s eye until the level of visual perfor- mance is satisfactory.

In any determination of the refractive characteristics of the eye, one Development of Optical Characteristics for Seeing can discuss the findings in terms of the types of lenses required to neu- tralize the movement of the retinoscope shadow or to achieve maximal acuity. If the person requires a minus, diverging lens to neutralize the movement or to achieve maximal acuity, he may be said to have myopia.

If he requires a plus, converging lens to accomplish the same ends, he may be said to have hyperopia. If he requires no lens, he may be said to have emmetropia.

If he requires a plus or minus cylinder to correct astig- matism, he may be said to have hyperopic or myopic astigmatism. The rest of this discussion will deal with the lens characteristics required to neutralize the movement of the retinoscope shadow or to achieve the best subjective acuity. An eye that requires a minus lens will be a “myopic eye”; no lens, an “emmetropic eye”; and a plus lens, a “hyperopic eye.

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The refrac- tive characteristics depend on the eye taken as a whole. Thus, an emmetropic person may show a considerable change in axial length, which is com- pensated by a change in corneal curvature, and still have no refractive error. Retinoscopy or subjective refraction is insensitive to changes in the optical components, unless the changes themselves result in an im- balance of the optical legranr.

Consequently, retinoscopy must be supplemented with some techniques that will provide a more accurate measure of the optical characteristics of the eye. Only by combining such techniques with the measurement of refractive error is it possible to describe what legrnad taking place over time and then to determine the re- lative importance of changes that occur with growth and with the devel- opment of visual ability.

The three techniques levrand commonly used to supplement retinoscopy and other measures of refractive characteristics are the determination of YOUNG the corneal curvature by means of keratometry; the measurement of distances within the eye, such as the depth of the anterior chamber, the thickness of the lens, the depth of the vitreous body, and the overall axial length of the eye by ultrasonography ; and the determination of the curvature of the front and rear surfaces of the lens by ophthalmo- phacometry.

Keratometry permits a highly accurate measurement of the surface curvature of a ball bearing but a somewhat less accurate measurement of the curvature of the cornea, which is not completely uniform but has more than one radius of curvature.

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However, if the same instrument is used consistently on the same eye, it is possible to develop reasonably accurate measures of changes within the cornea. Ophthalmophacometry is based on the demonstration of Purkinje-Sanson images, or the reflec- tion of light from the surfaces of the cornea and lens.

Essentially, it in- volves directing two points of light into the eye and measuring the legrandd ration of their reflections from the back surface of the cornea, the front surface of the lens, and the back legrrand of the lens the second, third, and fourth Purkinje-Sanson images.

The combination of measures of refractive error, corneal curvature, depth of anterior chamber, front lens surface curvature, thickness of lens, rear lens surface curvature, depth of vitreous, and total axial length provides an accurate description of the optical characteristics of the eye; anything less than this combination does not.

Although we have added considerably to our knowledge of the basic characteristics of the eye, we are still unable to describe exactly what will happen when the eye operates normally, because some of these measurements must be made while the eye is under cycloplegia. Without such a description and ac- curate measurements from birth onward in the same persons, it is not possible to characterize completely the development of the optical char- acteristics of the eye.

Fortunately, techniques are being developed that will permit the description of ocular performance under dynamic condi- tions, and it may be hoped that a complete description of the eye, in- cluding its static and dynamic characteristics, may be developed within the next decade. Size There is no direct evidence dealing with the size of the eye in a living human at birth or during leyrand first 3 years of life.

The reasons for the Development of Optical Characteristics for Seeing lack of information are related to the difficulty of applying the methods outlined to neonates and very young children. Consequently, most of the information available concerning the overall diameters of the eye has been obtained by measuring eyes post mortem.

However, as soon as blood pressure drops, the intraocular pressure drops and the eye becomes quite flaccid; furthermore, most in vitro measurements, even when the eye is perfused to reinstate a probably normal intraocular pressure, can vary widely from the measurements that would have been obtained in vivo.

Sorsby and Sheridan17 have provided possibly the best measurements available on the sagittal or anterior-posterior axial length of the eye of the newborn and of children days old. There is no significant dif- ference between these two groups, and the mean sagittal legramd is ap- proximately The sagittal diameters are smaller in premature infants, and follow closely the body weight at birth.

In the full-term baby of some 3.

Therefore, the sagittal diameter increases by around mm during growth, if the adult value is taken to be mm. It is likely that the usual increase is about 6 mm. There is apparently little growth during the first 2 weeks of life. Growth The most complete information available on the growth of the eye in the living human has been provided by Sorsby and co-workers4′ in a series of studies; they used refraction techniques, photographic ophthalmophacometry, x-ray, and ultrasonic measurements to study the growth and development of the human eye.

A series of investigations by van Alphen21 supplied some of the missing links in our understanding of factors that contribute to changes in ocular size. When the results of these studies are combined with those obtained by me and my co- workers, on humans and other primates, a more complete descrip- tion of the growth characteristics of the optical components is possible.

The sclera, choroid, and retina of the eye of a primate are closely ad- herent layers with various degrees of elasticity that enclose more or less viscid liquids to form a nearly spherical globe.

This globe continues to increase in size after birth. According to van Alphen,21 the human eye at birth is three fourths of its adult size, and all the ocular structures are probably still growing at birth. The adult size of the cornea is reached YOUNG between the first and second years, at which time the eye has not yet attained its adult size. Whether the sclera continues to grow after the cornea reaches its adult size is unknown, but intraocular pressure must be important in stretching the sclera.

The adult size of the eye is related to the genetic growth component, the elasticity of the sclera, the intraocular pressure, and a number of other variables yet to be described. In cases of congenital glaucoma, large eyes with large, flat corneas develop as a result of high intraocular pressure and scleral elasticity. In cases of experimentally induced low intraocular pressure, the eye remains small microphthalmia.

But even in these extremes, as well as in most cases between them, the eye remains nearly spherical. In the myopic eye, also, we find that the only significant deviation from sphericity occurs in the axial diameter.

Furthermore, if the same significance level is used, there is no difference in diameter between the hyperopic and emmetropic eyes, and the myopic eye is significantly longer than the emmetropic eye in the axial diameter and significantly longer in every diameter than the hyperopic eye. These comparisons suggest that the eye is normally a sphere and that the shape is determined by the variables of genetics, scleral elasticity, and intraocular pressure.

A developing eye that is characterized by a low intraocular pressure and a relatively high scleral rigidity may remain small in diameter even if the genetic component is directed toward greater size. Although one cannot evaluate the genetic component directly, it is possible to measure scleral rigidity and intraocular pressure independently and to estimate the contribution of the genetic compo- nent from those measurements. Moreover, it is likely that the cornea has reached its adult size by the Development of Optical Characteristics for Seeing end of the first year.

Inasmuch as the globe is about 18 mm long at birth and some 5 mm longer by the age of 3 years and there is no drastic change in the refraction of the eye in the first 3 years, compensatory reduction in the powers of the cornea and the lens by as much as 20 diopters must occur during that period. The hyperopic eye apparently never experiences this later growth, but remains arrested at the infantile phase.

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The emmetropic eye should also be included in this category, even though it is “compensated,” be- cause it does not undergo the later growth changes. These growth changes are based on changes in axial length, which can be measured with phacometry and ultrasonography. It is not possible to say whether the total size of the eye or only the axial length is increased during the later growth period. Comparisons based on the x-ray measure- ment of adult eyes suggest strongly that only the anterior-posterior axial length or sagittal diameter increases during this later growth period; it thus causes a change in shape, as well as size, of the globe, which is probably not determined by genetic aspects of the eye itself.

The fact that the dimensions of the eye remain relatively stable between the ages of 3 and 11 or 12 supports the concept that new factors contribute to the growth of the eye in the juvenile phase. InJames Ware22 presented a paper to the Royal Society of London in which he described his investigation on nearsighted- ness. He found, for example, that among 10, footguards in the British Service not even a half-dozen men were known to be nearsighted.

He pursued his inquiry at a military school at Chelsea where there were 1, boys; he found that the complaint of nearsightedness had never YOUNG been made among them until he mentioned it, and even then only three experienced any inconvenience from it.

He then inquired at several col- leges in Oxford and Cambridge and, although there was great diversity in the number of students who used glasses in the various colleges, glasses were used by a considerable portion of the total number of stu- dents in both universities. In one college in Oxford, he accumulated a list of names of no fewer than 32 of students who used either a hand glass or spectacles, between and Ware described the effects of fitting concave or minus lenses to nearsighted persons as follows: It should be remembered, that, for common purposes, every near sighted eye can see with nearly equal accuracy through two glasses, one of which is one number deeper than the other; and though the sight be in a slight degree more assisted by the deepest of these than by the other, yet on its being first used, the deepest num- ber always occasions an uneasy sensation, as if the eye was strained.

If, therefore, the glass that is most concave be at first employed, the eye, in a little time, will be accommodated to it, and then a glass one number deeper may be used with similar advantage to the sight; and if the wish for enjoying the most perfect vision be in- dulged, this glass may soon be changed for one that is a number still deeper, and so in succession, until at length it will be difficult to obtain a glass sufficiently concave to afford the assistance that the eye requires.

In an appendix to Ware’s paper, Sir Charles Blagden2 gave the follow- ing comments. Ware states in his Paper, that near sightedness comes on most frequently at an early age; that it is more common in the higher than in the lower ranks of life; and that particularly at the universities, and various colleges, a large proportion of the students make use of concave glasses.

All this is exactly true, and to be accounted for by one single circumstance; namely, the habit of looking at near objects. Chil- dren born with eyes which are capable of adjusting themselves to the most distant objects, gradually lose that power soon after they begin to read and write; those who are most addicted to study become near legraand more pegrand and, if no means are used to counteract the habit, their eyes at length lose irrecoverably the faculty of being brought to the adjustment for 033606 rays.

The statements appear to be as valid as they were inand this concept, that the use of the eyes for near-work is responsible for the development of myopia, has a long history in ophthalmology and op- tometry, but there is inadequate evidence for supporting or rejecting it.

Myopia lgerand develops between 10 and 14 years of age and usually tends legranx increase with time but to stabilize around 18 years of age. Development of Optical Characteristics for Seeing ever, there appear to be some persons who do not develop myopia until legrandd age 18; they tend to stabilize around the age of It should even be possible to estimate intelligence by determining the refractive char- acteristics of the eye.

Most of the studies that have attempted to dem- onstrate a relationship between intelligence and refractive error have found none, except when intelligence was measured by written tests. However, when reading ability is statistically adjusted for, the correla- tion of refractive error and intelligence approximates zero. The myopic person is a substantially better reader than the nonmyopic.

Myopia and Personality Studies of the personality characteristics ,egrand myopic and nonmyopic per- sons indicate that there are consistent personality characteristics asso- ciated with myopia. Several investigators have found that myopic persons on the average make significantly better grades in college than emmetropic or hyperopic students, tend to be more introverted in thinking and in social behavior than emmetropes, and are more emotionally inhibited and less inclined to motor activity than nonmyopes.

YOUNG characteristics are present before the myopia develops and lead to the development of myopia, or whether the myopia causes the development of the personality characteristics.