Eye Development and Short Sight Reversible If the Visual Restriction Is Removed During the First 6 Weeks of Graham R

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NATURE VOL. 333 23 JUNE 19KH NEWSANDVIEWS,------707 Vision that this is not the case'. Even more intriguing is the finding that this experimentally induced myopia is Eye development and short sight reversible if the visual restriction is removed during the first 6 weeks of Graham R. Martin life'. If pattern vision is restored during this period, then the eye will become CoMPARED with the complex optical own control. The most robust technique emmctropic with the rate of recovery from structures found in invertebrate com has been developed for use with chicks myopia directly related to the degree of pound eyes (see the recent News and and simply involves covering the eye at myopia induced. These results all suggest 1 views article by Mike Land ), the verte birth with a translucent goggle. Pickctt there is a feedback mechanism which brate eye appears to be a simple affair, Seltncr et al. '·' show that a high degree of regulates eye growth and which is in consisting of just two main refractive myopia is induced by this technique in fluenced by the quality of the image components, the lens and the cornea. But chicks as young as 14 days old, indicating present on the retina. the quality of the optical image produced clearly the importance of patterned visual Frank Schaffel and colleagues at Cor in the vertebrate eye is usually far superior input to the eye for its coordinated nell University' have now taken the tech to that of invertebrates. This quality can development during this sensitive period. nique one step further. Instead of simply be achieved only by highly accurate A high degree of myopia can even be inducing myopia by the usc of translucent coordination of the refractive powers of goggles, they raised chicks with lenses in the lens and cornea and of their positions front of their eyes which slightly defocus relative to each other, and to the retina. the image on the retina. The refractive Three groups, who report their results powers of the lenses used are large enough in recent issues of Vision Research'-', are to blur the image in a normal emmctropic beginning to show how this remarkable eye but are within the range such that their coordination of the eye's optical structure optical effects can be compensated by the is achieved, and how it can break down to natural accommodation of the chick eye. produce myopia (short sight). Schaffel et al. show that growing chicks Not all eyes in the same species have the can indeed keep the images on their same optical structure. In their classic retinas in focus when wearing the lenses. comparative study of more than 300 But when the lenses are removed after human eyes, Sorsby et at.' demonstrated between 17 and 26 days, the eye is per that in those eyes judged to be emme manently out of focus- it has developed tropic (normal eyes which when relaxed to take account of the refractive error bring the image of a distant object to a forced upon it by the lens. As in previous focus at the retina), the eye's axial length, studies, Schaffel et al. find that it is eye its corneal and lens refractive powers and length that has changed, just sufficient to its total refractive power, could vary Chick eye with experimentally induced short bring the image and retina into approx between individuals by more than 20 per sight. In the normal eye (a) the image of a imate coincidence. Also, as previously, cent. These authors concluded that "the distant object produced by the lens and cornea they find the refractive components of the emmetropic eye is not the end result of a lies at the surface of the retina. In an experi eye are unchanged and the effect occurs haphazard combination of variable com mental eye (b) the equatorial diameter and the independently in each eye. axial length of the eye are increased, and the ponents, but a co-ordinated organ. The The dependence of the developing vis image of a distant object now lies in front of the individual cmmctropic eye . . . is not retina. Hence the eye is described as 'short ual areas of the brain on normal visual accidental." They also showed that in the sighted' as only the images of objects close to input has been well established. These human eye, refractive errors, such as the eye can be focused at the retina. new results suggest an intriguing devel myopia in which the image is produced in opmental feedback mechanism between the wrong place with respect to the retina, induced by fitting chick eyes with goggles the neural retina, where a patterned are usually the result of anomalous com which restrict the width of the eye's image is detected, the optical system ponents whose values fall outside the visual field'. which produces that image, and growth range of normal variation. But although But what docs the myopia induced by of the eye. These simple techniques the coordinated development of the this technique consist of? What parts of which experimentally induce myopia cmmetropic eye seems to depend on the coordinated development of the eye hold out the promise not only that the normal visual experience, it was not pos arc disrupted? It appears that the whole mechanism underlying the development sible to pin down what exactly underlies eye becomes enlarged so that both its axial of myopia may be understood, but also the apparently uncoordinated develop length and equatorial diameter arc in that the finely coordinated development ment of the myopic eye. or to determine creased but the lens remains unaffected. of the normal eye's optical system might the extent to which the development of an Changes in eye size were evident even be elucidated. 0 cmmetropic eye is controlled by experi within two days of hatching". indicating I. Land. M.F. Narure 332. 15 ( 1988). 2. Pickctt-Seltncr. R.L era/. Vis. Res. 27. 1779-1782 (1987). ential influences as opposed to simple just how sensitive the development of 3. Pickett-Seltncr. R.L era/. Vis. Res. 28.323-328 (1988). growth. The new research is beginning the eye appears to be and showing that 4. Wallman. J. & Adams. J.L Vis. Res. 27. 1139-1163 (1987). 5. Schaffel. F. era!. Vis. Res. 28. 639..(J57 (I 988). to elucidate this problem and shows just this development is dependent on the 6. Sorsby. A. era!. Med. Res. Council (UK) Spec. Rep. Ser. how sensitive the eye's structure is to clarity of vision. Furthermore. the obser 293(1957). anomalous visual input. ved size increase in the eye appears to be 7. Wallman. J. & Turkel. J. Science 201. 1249-1251 (1978). 8. Yinon. U. era!. Vis. Res. 20.557-561 (1980). The new work of Pickett-Scltner eta!. ). .l the result of expansion of the globe result 9. Hodos. W. & Kuenzel. \\l.J. Invest. Opthal. t·is. Sci. 25. and of Wallman and Adams' exploits ing from the accumulation of fluid rather 652-659 (1984). 10. Hayes. B.P. er a!. bn·esr. Opthal. ds. Sci. 27. 981-991 simple techniques'' which can cause a than growth of the eyeball tissue itself. (1986). growing eye to develop a high degree of The observed phenomena could be ------myopia. It is even possible to induce explained in part as the result of an Graham R. Martin is in the departments of Zoology and of Extramural Studies, University myopia in one eye of an animal and not in inflammatory response of the eye to of Birmingham, PO Box 363, Birmingham 10 the other. so an individual can act as its goggle wear , but the new work suggests BJ5 2TT. UK.