Effects of Retinal Image Degradation on Ocular Growth in Cats

Effects of Retinal Image Degradation on Ocular Growth in Cats J. Nathan, 5. G. Crewrher,* D. P. Crewrher,* and P. M. Kielyf High-powered negative and positive contact lenses have been used to produce a state of continuous retinal defocus in the eyes of 11 kittens in an attempt to induce a predictable amount of axial lengthening and myopia. Another group of six kittens had one eye continuously atropinized and a third group of four animals had the lids of one eye sutured. The axial dimensions and refraction were measured using ultrasonography and retinoscopy respectively. Although the treated eyes of eight kittens tested behaviorally were shown to be amblyopic, no myopia appeared in any animal at any stage during development and only three cats showed a small axial length difference between the two eyes. These results differ from other retinal deprivation studies reported on kittens and no satisfactory explanation for this discrepancy can be offered. However, it is suggested that the lack of change may be associated with the gross anatomy of the cat's eye, and it is concluded that myopia cannot be induced reliably in kittens by retinal image degradation. Invest Ophthalmol Vis Sci 25:1300-1306, 1984 It has been noted clinically that human infants deprived of vision as a result of disease tend to develop myopia. 1 " 4 There also have been numerous reports of axial elongation and myopia development in the eyes of infant experimental animals in response to monocular deprivation through lid suture, 5 "" corneal scarring, 12 total and sectorial occlusion, 13 and optical defocus. 14 However, the experimental results have varied between species and between individual animals within a species. Chickens became fairly reliably myopic and showed axial elongation in response to both lid closure" and lateral or total field occlusion. 13 On the other hand, suturing the eyelids of kittens has been reported to result in some eyes showing axial elongation and others showing axial shortening in relation to the fellow control eye. 9 The eyes of rhesus monkeys also consistently became elongated and myopic in response to lid suturing in one study 5 but reacted more variably in another. 7 However, rearing of monkeys in darkness together with monocular lid suture has resulted in no changes in ocular dimension or refraction. 15 These findings have led to the view that while the From the National Vision Research Institute, 386 Cardigan Street, Carlton, 3053, Australia. * Present address: School of Optometry, University of New South Wales, P.O. Box 1, Kensington NSW 2033, Australia. t Present address: Department of Optometry, Melbourne University, Parkville Vic 3052, Australia. Submitted for publication: August 4, 1983. Reprint requests: Dr. S. G. Crewther, School of Optometry, University of New South Wales, P.O. Box 1, Kensington NSW 2033, Australia. primate eye may develop to its normal dimensions in the absence of light, the presence of a degraded retinal image causes an abnormal growth pattern to occur. In most instances the abnormal growth pattern leads to a disproportionate axial elongation and myopia. The issue of environmentally induced myopia initially received experimental support from studies by Young 16 showing that rhesus monkeys reared experimentally in an environment of restricted visual space of 14 inches develop myopia of low degree. Laboratory-reared cats also have been reported to be on average more myopic than street-reared cats though the axial length did not differ significantly between the two groups. 17 Our own extensive observations of more than 40 laboratory-reared cats do not support any trend away from emmetropia. While environmentally induced myopia can be established in rhesus monkeys, Young 18 also has reported that the concurrent use of atropine, which paralyzes accommodation, arrested the change in refraction. This report implicates accommodation as a mediating factor in laboratory-induced myopia. In the present study, retinal image degradation has been imposed on three groups of kittens in an attempt to induce myopia environmentally. With the first group, the authors have sought to induce a predictable degree of myopia by using contact lenses to create a state of continuous optical defocus of known degree in the eyes of developing cats. In a second group of kittens, atropine was instilled daily to eliminate the accommodation of that eye, thereby producing hypermetropic blur. The availability of a third group of 1300

No. 11 RETINAL IMAGE DEGRADATION AND OCULAR GROWTH / Norhon er ol. 1301 animals monocularly deprived through lid suture from soon after birth to adulthood provided an opportunity to compare the results of these animals raised with the most extreme type of defocus with those of the two groups of cats with optically induced defocus and with a group of normal animals, while utilizing the same methods of measurements. Animals Materials and Methods Twenty-four cats were used in this study of which two were normal controls. All animals lived in the main colony (12 hr light, 12 hr dark) in cages with their mothers and litter mates until 12 weeks of age (unless otherwise stated). After 12 weeks, animals were free to roam the colony. Three types of experimental rearing procedures were used. Eight kittens were raised from 3 weeks of age to 16 weeks wearing a gas-permeable hard contact lens in one eye, and three were raised wearing two such contact lenses. All these kittens were given approximately 8-hr visual experience each day while wearing their contact lens and then returned to a darkroom where they were housed with their mothers and litter mates. This group of animals is referred to as the lens group "L." Three kittens were raised from natural eye opening at 7-10 days until 9 or 12 months with the lids of one eye sutured, while one further animal, whose left eyelids also were sutured at 10 days only had its eyelids reopened on the day of first measurement after 21 months of suture ("MD" group). Six kittens were raised from 3 weeks to 6 months with daily monocular administration of 1% atropine sulphate, and one kitten was given atropine binocularly ("ATR" group). Of this last group, all were regularly refracted while awake using retinoscopy and ophthalmoscopy with cycloplegia, but only one animal was anesthetized for ultrasonography. Table 1 contains all details of treatment and ages of all animals at the time of the final ultrasonographic measurements. The visual acuities of some of these animals were assessed on a Mitchell jumping stand 19 (indicated by an asterisk in the table) as part of another experiment, and where the acuity of one eye was significantly worse (one to two octaves) than the binocular performance or that for the other eye, it was concluded that the rearing procedure had induced a severe amblyopia. Surgery The only surgical procedure involved was monocular lid suture of four kittens. These animals were sedated initially with xylazine (0.5 mg/kg) and subsequently anesthetized with 1-2% halothane in a gaseous mixture of 2:1 nitrous oxidexarbogen. The lid margins were trimmed and the opposing edges of the skin sutured together after the conjunctiva had been pulled away. Topical ophthalmic solutions of anesthetic (Ophthetic) and antibiotic (Neosporin) were applied to the wound area. Lids were checked daily for any small openings, and these were repaired immediately. These procedures conform to the ARVO Resolution on the Use of Animals in Research. Lens Wear Central corneal radius of curvature was measured with a modified keratometer to determine the variation in contact lens parameters required over the first 16 weeks of a kitten's life. Both soft extended-wear (75% water) contact lenses and hard gas-permeable lenses were tried. The hard lenses proved more successful. Kittens were introduced to contact lenses gradually. At three weeks of age after experiencing a normal visual environment, the hard gas permeable lenses were placed on the kittens' eyes following the application of local anesthetic solution (Ophthetic). The animals were observed for a period of time to check for any apparent irritation. The exposure periods were increased gradually as the animals became used to wearing the lenses. Within a week, a wearing time of 8 hr per day was achieved. After the first few days, the use of Ophthetic was discontinued as an uncomfortable lens fit could be identified quickly by the animal's immediate response to its insertion. The animals were observed during the exposure sessions and the fit of the lenses was checked regularly using fluorescein and ultraviolet illumination. Over the period of ocular growth, several lens sizes (back surface radius of curvature 5.8- mm) were required. Throughout development and until the times shown in Table 1, the animals wearing contact lenses and those raised with daily atropine were regularly refracted by retinoscopy using 0.5% tropicamide (Mydriacyl) or 1% atropine as the cycloplegic (Fig. 1). Periodically, each animal was anesthetized with intramuscular ketamine chloride and xylazine (15 mg/kg and 0.15 mg/kg, respectively), and its eyes were refracted by ophthalmoscopy and axial lengths measured by ultrasonography. The anesthetic mixture of ketamine and xylazine caused rapid corneal drying, and, despite the constant use of ophthalmic lubricants, retinoscopy was difficult. Thus, greater credibility is placed on the ophthalmoscopic refractions made under anesthesia. Neither atropine nor mydriacyl was administered to anesthetized animals, as in this state adequate cycloplegia is induced. Repeated measure-

1302 INVESTIGATIVE OPHTHALMOLOGY G VISUAL SCIENCE / November 1984 Vol. 25 Table 1. Information on each cat and its treatment, age when final ultrasonographic measurements were taken, and end-point (cycloplegic) refraction of the animals while alert Cat Age final meas. Treatment Anterior chamber Lens Vitreous chamber Axial length Refraction (cycloplegic) (diopters) LI* L2* L3* L4 L5* L6 LI* L8* L9 L10* Lll* MD1 MD2* MD3* MD4* ATR1 Cl C2 19 months 9 months 9 months 7 months 17 months 13 months 17 months 18 months 2 months 5 months 4 months 4 years 3 years 4 years 21 months 10 months 6 months 6 months RE lenst LE -6.0 Df RE Normal LE-/-11.0Dt LE-11 D RE lens LE-11 Df RE lens LE+6D LE +6 Dt RE-11 Dt LE-11 Dt t LE+6 D LE -3 D lenst LE-16 D lenst LE MD until 9 mot RE rotate 90 deg LE MD until 9 mot LE MD until 12 mot REMD until 21 mot RE MD (open 1 wk) RE MD (open 3 wk) RE atropine 5.2 4.8 4.9 4.6 4.7 5.3 5.6 2.8 3.1 4.3 4.2 4.8 4.9 5.6 5.6 5.8 6.0 4.2 4.2 7.5 7.5 7.5 6.3 6.4 6.5 6.6 7.7 9.6 9.0 8.9 8.4 8.6 7.1 6.9 6.9 6.8 8.2 8.6 8.2 8.3 7.1 8.7 6.9 7.7 7.8 6.7 8.4 8.2 21.1 20.4 20.7 20.3 20.4 20.3 19.6 20.5 20.8 19.4 19.6 21.6 21.7 20.3 20.4 15.2 1 18.4 18.4 1 18.4 20.5 20.7 20.6 20.6 21.4 21.1 21.1 21.4 20.8 20.9 19.0 19.1 19.0 19.2 Pi. Pi. Pi- -1.0-1.0 +0.25-1.25-1.25 +0.75 +0.75 +0.50 +0.50 * Cats tested behaviorally; teats shown to be amblyopic in one or both eyes. The ultrasonograph readings are shown in terms of anterior chamber depth, lens thickness, vitreous chamber depth and total axial length of the eyeball. In the right hand column, the cycloplegic refractions of the awake animals made with retinoscopy prior to anesthesia are shown. The cats that were raised with lenses (L) in one or both eyes were given normal binocular experience to 3 weeks, then at least 8-hr binocular visual experience each day through 16 weeks when the wearing of the lenses was discontinued. The monocularly deprived group (MD) had the lids of one eye sutured at 7-10 days and maintained until the age shown. To the third group of experimental animals (ATR), a drop of atropine sulphate 1% was administered daily to one or both eyes from 3 weeks to 6 months of age. ments indicated that comparisons of refractions with and without cycloplegia did not differ significantly. When retinoscopy was used, the best-fit sphere was recorded; only low degrees of astigmatism being found. The cats that were deprived monocularly as neonates were already full-grown when the axiallength study was initiated, so all readings were made as adults, as indicated in the "Age" column of Table 1. In all cases, the period of monocular deprivation is shown in the "Treatment" column. A small artificial pupil was used initially, but was not found to aid retinoscopic examination significantly, and so its use was discontinued. At each examination, animals were refracted by at least two experienced clinicians, and their findings were averaged. All ultrasonography measurements were made on anesthetized animals with a clinical A-scan ultrasonograph (Xenotecresolution 0.1 mm). This machine gives a digital read-out of all aspects of axial dimensions, using an average sound velocity through the optic media for this calculation. The possibility that

»- - " " ' No. 11 RETINAL IMAGE DEGRADATION AND OCULAR GROWTH / Norhon er ol. 1303 Fig. 1. A, The refractive states of both eyes of five animals raised (from 3-16 weeks of age) with either unilateral or bilateral contact lenses. In both groups of animals (Figs. 1A, B), treatment was initiated at three weeks and is indicated by. Right eye and left eye refractions are indicated by * and, respectively. The course of refractions for a treated eye is shown as a broken line, whereas the course of refraction for the normal eye is shown as a continuous line. The lens powers worn by the cats represented in Figure 2A can be ascertained from Table 1. In all cases, the refractions of the two eyes were very similar. L2 L3 L5 L7 V i A \ r V \ * * ' - ". -, I ^ " - * - ^ - -.. 15 20 25 30 35 45 50 55 AGE (WEEKS) 75 variations in velocity through the different media might obscure a real change in axial length was avoided procedurally by measuring the anterior chamber depth, lens thickness, vitreous chamber depth as well as the total axial length for both eyes. At each session, four series of readings were obtained for each eye after independent placements of the probe. The most difficult part of the procedure is aligning the probe along the visual axis of the eye. An eyecup filled with aqueous gel was interposed between the probe and the cornea. We have assumed, as have others, 14 that the visual axis is that alignment giving equal reflections of the front and back of the crystalline lens of the eye (Fig. 2). A drop of 10% phenylephrine hydrochloride (Neosynephrine) was administered to retract the nictitating membrane when necessary. eye show a significant change in axial length relative to the normal eye. The defocussed eye was shorter in axial length by 0.7 mm and was slightly hypermetropic o 5-2 ATR1 ATR2 ATR3 ATR4 Results It has been shown clearly with ultrasonography and retinoscopy that no predictable nor significant changes in ocular dimensions were caused either by prolonged optical defocus or by lid suturing or by paralysis of accommodation in the 21 animals examined from these three groups. No significant differences in axial length were seen between the two eyes of each of the two normal control animals (Table 1). In only one cat (L4) did the optically defocussed ATR 5 15 20 25 30 AGE (WEEKS) Fig. 1. B, The refractive states of both eyes of five animals raised from 3 weeks to 6 months with either daily unilateral or bilateral administration of 1% atropine sulphate. For description of T, *, and, see legend to Figure 1A. 35

1304 INVESTIGATIVE OPHTHALMOLOGY G VISUAL SCIENCE / November 1984 Vol. 25 Fig. 2. A-scan ultrasonograph of a cat eye. Ultrasound is transmitted through the various media of the eye. Each media boundary causes a reflection represented by a peak in the ultrasonograph. The initial two peaks on the left are artifacts caused by the probe membrane and the aqueous gel on the surface of the cornea. The instrument is equipped with a gating device, which enables accurate measurement of the distance between peaks (shown here is the measurement of the anterior chamberthe distance between the front surface of the cornea and the front surface of the lens. The two peaks to the right are the back lens surface and the retinal reflection). The scale shown is millimeter and half-millimeter. (1-2 D) under anesthesia. However, when refracted without anesthesia, both eyes were emmetropic. One other animal (Lll) showed a relative increase in axial length of 0.4 mm and another (L2) of 0.3 mm. No other animal showed any significant change in axial length or refraction. Although ultrasonography was carried out on MD4 on the day the lid-suture was terminated, and over the next few weeks, no change in axial length was observed. In two cats (MD2, MD4), the lens thickness was observed to increase gradually over a couple of hours. The vitreous chamber length correspondingly decreased. This surprising phenomenon is presumably an effect of long-term anesthesia. While retinoscopy was found to be a satisfactory technique for measuring the refraction of most alert cats, it was found to be far less reliable when the ketamine-xylazine anesthetic mixture had been administered. The enlarged pupil combined with a cornea prone to extreme drying despite the repeated instillation of lubricant eye drops, rendered the retinoscopic reflex too uncertain to be reliably interpreted. Direct ophthalmoscopy as a tool for measuring spherical refraction was found to be more useful under these conditions. Variations between observers were both small and consistent (less than 1,0 D) and were possibly a reflection of the differences between their amplitudes of accommodation or between their criteria for focus. Findings using this method suggested that over the developmental period, there was no apparent difference in refractive error between the control and treated eyes, in any of the animals from the L and ATR groups, nor in the MD group (as adults). The repeated refractions of the awake contact lens reared cats and the atropinized cats were extremely consistent as shown in Figures 1A and B. No significant differences were seen between the treated and untreated eyes. Since eight of the anisometropic animals have been part of a concurrent experiment, which assesses behavioral acuity and all are known to have developed amblyopia, there can be no doubt that the lenses were causing optical defocus. Similar experiments examining recovery of vision after prolonged monocular deprivation indicate that the suturing was also adequate. Even LI, the cat with a zero power lens in one eye became amblyopic in that eye. It was observed on two occasions that this kitten possessed 5 D of myopia when wearing the lens, indicating that the lens fitting was too "steep," inducing a defocussing convex tear film between the cornea and contact lens. Subsequently, a new piano lens with a greater radius of curvature was fitted. Two of the cats treated monocularly with atropine were also tested behaviorally as adults and were shown to be amblyopic through the penalized eye. Discussion The failure to induce myopia or increased axial length either as a result of lid-suture or optical defocus does not support a previously stated view

No. 11 RETINAL IMAGE DEGRADATION AND OCULAR GROWTH / Norhon er ol. 1305 that the unpredictable response of the cat's eye to lid suture may be due to the poor light transmission of the lids. 15 The lack of response is unlikely to be due to insufficient optical defocus as amblyopia was demonstrated in all of the eight lens-reared animals that were tested. If accommodation is an essential part of the feedback mechanism as previously suggested, 18 it may be that the minimal changes observed here in both the anisometropic and monocularly deprived groups are a function of the feline mechanism of accommodation. However, accommodation is generally considered to be a binocularly linked function, and the axial elongation reported in all other relevant studies on cats and other animal species is confined to the treated eye. It may be that accommodation is not binocularly linked until after the critical period of visual development; but at least in one study, 10 unilateral axial lengthening was reported to have occurred in kittens that were not lid-sutured until up to 80 days of age. The fact that neither the contact lens group (L) nor the ATR group showed a change in axial length sheds no further light on the question of whether accommodation is a mediating, factor in experimental or clinical myopia. The lack of marked and consistent responses to retinal image degradation seen in cats 9101417 as compared with other animal species may be associated with the gross structure of the eyeball. If axial elongation requires some stretching of the sclera, development of myopia may rely on the existence of a relatively weak region of the sclera. Indeed, a thinning of the sclera in the equatorial region of a monkey's eye with induced myopia has been observed. 12 A comparison of the gross structures of the eyes of chicken, cat, monkey, and human is made in Table 2. Note that in the cat's eye, the lens is more posteriorally located than in the other three species, and, in fact, includes the equatorial zone. Smith, Maguire, and Watson 14 have reported the development of a relative myopia averaging 2.2 D (range 0.75-4.5 D) and an increase in axial length averaging 0.76 mm (range 0-1.52 mm) in eight kittens defocussed monocularly with goggles containing negative lenses of magnitude greater than 10 D for 2-3 hr daily from 4 weeks until they were 12 weeks old. Kirby, Sutton, and Weiss 10 reported a consistent increase in axial length but an inconsistent relationship between axial elongation and the degree of relative myopia as measured by retinoscopy in 20 kittens following lid-suture of one eye. In their study, the kittens' eyelids were sutured at various times between 18 and 80 days and maintained for 257-635 days. The average axial length increase recorded was 1.37 mm, and the myopia ranged from 0.5-3.0 D. Ten of the 20 eyes with sutured lids deviated from emmetropia and always in the myopic direction. In Table 2. Proportions of the axial dimensions of the components of the eye Ant. chamber % Lens thickness % Vitreous chamber % Axial length Cat 24.5 3 40 20.5 Chicken 18 25 57 13.5 Cynomolgus monkey 16.5 1 65 1 Human 15 15 70 24.0 The depth of the anterior chamber, the thickness of the lens, and the depth of the vitreous chamber of the species commonly used in myopia development experiments (cat, chicken, monkey, and human) are expressed as percentages of the total axial length. The dimensions used for the cat and chicken are taken from our own studies and that for the cynomolgus monkey from reference 20 and those for the human from reference 21. a study by Smith et al, 14 the ocular dimensions were measured immediately following the final removal of the goggles at 12 weeks, while Kirby et al reported that in their case, ultrasound measurements were made before and during lid closure. It is difficult to reconcile these results with those of the present study and that of Gollender et al. 9 The degree of blur induced in our study was slightly less in most animals than that employed by Smith et al, but the blur was maintained for approximately 8 hr per day and until the cats were at least 16 weeks old. In the study by Smith et al, kittens were maintained in the dark until treatment began, whereas ours and presumably those of Kirby et al were given normal binocular experience. It is possiblebut unlikelythat normalization or emmetropization of the refractive state, which has been found to occur in chickens 13 (also confirmed in our laboratories, unpublished data), may have occurred by the time final measurements were taken in a few cats (such as three of the MD group). Measurements were not recorded until many months after the cessation of lid suture. This explanation, however, is not adequate for MD4 where lid opening coincided with the initiation of ultrasonography and refraction. Also, for most of the animals with optically induced blur, refraction and ultrasonography were performed regularly during the special rearing time and for several months following, and no significant difference was ever seen between the two eyes. In addition, the age at which the atropine and lid-suture treatments were discontinued argues against normalization after that time since by 6 months in normal development, the eye has reached its adult dimension. The literature on the development of myopia in chickens would suggest that accommodative mechanisms may not be the only cause of axial length increase. Indeed, the fact that only peripheral field occlusion is necessary to induce a myopic shift, while

1306 INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / November 1984 Vol. 25 the central retina is receiving sharp images may have some analog in mammalian studies. It is intriguing that the only manifest difference between our studies and those of Smith et al in which a small but consistent myopia was induced, is the means by which the defocus was produced. In our study, contact lenses were employed, with no consequent restriction of visual field. In the other study, goggles were used to hold defocussing lenses. These goggles would cause some degree of restriction of field. It must be concluded that cats are not a useful laboratory animal for the induction of myopia by retinal image degradation. This study has shown that myopia was not induced (within our limits of accuracy) as a result of optical defocus or of lid-suturing, despite the fact that all the behaviorally assessed animals were found to be amblyopic. The unpredictable changes in axial length reported previously 15 are probably not due to relatively poor light transmission through the lids. Key words: retinal image degradation, contact lens, myopia development, cats Acknowledgments The authors would like to thank G. Nissel & Co. (Australia) and the West Australian Contact Lens Manufacturing for supplying contact lenses; N. Noller, C. Tobin, and G. Mason for assistance with ultrasonographic measurements; P. Croucher and P. Brann for supervising the kitten rearing and Vicki Hammond for artwork. References 1. Robb RM: Refractive errors associated with hemangiomas of the eyelids and orbit in infancy. Am J Ophthalmol 83:52, 1977. 2. Rabin J, Van Sluyters RC, and Malach R: Emmetropization: a vision-dependent phenomenon. Invest Ophthalmol Vis Sci 20:561, 1981. 3. Johnson CA, Post RB, Chalupa LM, and Lee TJ: Monocular deprivation in humans: a study of identical twins. Invest Ophthalmol Vis Sci 23:135, 1982. 4. O'Leary DJ and Millodot M: Eyelid closure causes myopia in humans. Experientia 35:1478, 1979 5. Wiesel TN and Raviola E: Myopia and eye enlargement after neonatal lid fusion in monkeys. Nature 266:66, 1977. 6. Sherman SM, Norton TT, and Casagrande VA: Myopia in the lid sutured tree shrew (Tupaia glis). Brain Res 124:154, 1977. 7. von Noorden GK and Crawford MLJ: Lid closure and refractive error in macaque monkeys. Nature 272:53, 1978. 8. Sommers D, Kaiser-Kupfer MI, and Kupfer C: Increased axial length of the eye following neonatal lid suture as measured with A-scan ultrasonography. ARVO Abstracts. Invest Ophthalmol Vis Sci 17(Suppl):295, 1978. 9. Gollender M, Thorn F, and Erickson P: Development of axial ocular dimensions following eyelid suture in the cat. Vision Res 19:221, 1979. 10. Kirby AW, Sutton L, and Weiss H: Elongation of cat eyes following neonatal lid suture. Invest Ophthalmol Vis Sci 22:274, 1982. 11. Yinon V, Rose L, and Shapiro A: Myopia in the eye of developing chicks following monocular and binocular lid closure. Vision Res 20:137, 1980. 12. Wiesel TN and Raviola E: Increase in axial length of the macaque monkey eye after corneal opacification. Invest Ophthalmol Vis Sci 18:1232, 1979. 13. Wallman J, Turkel J, and Trachtman J: Extreme myopia produced by modest change in early visual experience. Science 201:1249, 1978. 14. Smith EL, Maguire GW, and Watson JT: Axial lengths and refractive errors in kittens reared with an optically induced anisometropia. Invest Ophthalmol Vis Sci 19:1250, 1980. 15. Raviola E and Wiesel TN: Effect of dark-rearing on experimental myopia in monkeys. Invest Ophthalmol Vis Sci 17:485, 1978. 16. Young FA: The effect of restricted visual space on the refractive error of the young monkey eye. Invest Ophthalmol Vis Sci 2:571, 1963. 17. Rose L, Yinon V, and Belkin M: Myopia induced in cats deprived of distance vision during development. Vision Res 14:1029, 1974. 18. Young FA: The effect of atropine on the development of myopia in monkeys. Am J Optom Physiol Opt 42:439, 1965. 19. Mitchell DE, Giffen F, and Timney B: A behavioural technique for the rapid assessment of the visual capabilities of kittens. Perception 6:181, 1977. 20. Kaufman PL, Calkins BT, and Erickson KA: Ocular biometry of the cynomolgus monkey. Curr Eye Res 1:307, 1981. 21. Emsley HB: Visual optics. In Optics of Vision, Vol 1, Chap 10, fifth edition. London, Hatton, 1969, pp. 344-346.