Differences in Time Course and Visual Requirements of Ocular Responses to Lenses and Diffusers 575

PURPOSE. Myopia can be induced in chickens by having them wear either negative lenses (lens-compensation myopia [LCM]) or diffusers (form-deprivation myopia [FDM]), whereas positive lenses cause lens-compensation hyperopia (LCH). These three conditions were compared with respect to (i) their early time course and (ii) the effect of two manipulations of the lighting. METHODS. Longitudinal changes in ocular dimensions and refractive error were measured in chicks maintained under three different conditions: (i) wearing either Ϫ15 D lenses or diffusers in a normal light/dark cycle; (ii) wearing either ϩ15 D lenses, Ϫ15 D lenses, or diffusers with brief periods of stroboscopic lights at the beginning and end of the dark period; (iii) wearing either ϩ6 D lenses, Ϫ6 D lenses, or diffusers with the nights interrupted by brief periods of white light. In addition, scleral and choroidal proteoglycan synthesis was measured in eyes that wore positive lenses, negative lenses, or diffusers for 3 hours followed by different periods of darkness. RESULTS. (i) The time course of the changes in axial length over the first 72 hours was significantly faster in LCM than in FDM. Indeed, the diffusers did not begin to significantly affect the total length of the globe for 3 days, although the vitreous chamber had deepened after 9 hours, because the choroid thinned extremely rapidly (within 1 hour) with either diffusers or negative lenses. (ii) Scleral proteoglycan synthesis was higher in eyes with negative lenses than in those with diffusers at 11 hours, but the reverse was true at 27 hours. (iii) Brief periods of stroboscopic light attenuated FDM more than LCM. (iv) In contrast, interruption of the nights by brief periods of light attenuated LCM more than FDM. (v) Neither lighting manipulation affected LCH. (vi) Choroidal proteoglycan synthesis decreased similarly with 3 hours of wearing either diffusers or negative lenses. CONCLUSIONS. Although both negative lenses and diffusers cause similar increases in the rate of ocular elongation, the responses differ in time course and in the effect of manipulations of the daily lighting. The responses to positive lenses differ from both of these. (Invest Ophthalmol Vis Sci. 2001;42:575-583) I n the past three decades, research on experimental myopia in several animal species has increased our understanding of refractive development. All the animal species tested developed myopia under conditions of visual form deprivation (e.g. ). However, whether the eye uses the same mechanism to respond to diffusers and lenses is still open to question and is an issue of considerable importance. One possibility is that the responses to diffusers and to negative and positive lenses are all explicable by a simple rule: obscuring the visual image causes growth toward myopia, whereas sharp vision causes growth toward hyperopia. 10 According to this view, diffusers would obscure vision the most and hence would cause the strongest myopia, negative lenses would obscure vision less, and positive lenses would enhance the sharpness of the images (assuming that the chicks mostly looked at nearby objects and that without positive lenses, vision of near objects would be blurred because of accommodative lag). A test of this hypothesis would be to impose novel visual conditions and see if the responses to negative lenses and diffusers are similarly affected. On the basis of evidence from biochemical studies and from the effects of cutting the ciliary nerve or the optic nerve, Schaeffel et al. There have been three explicit comparisons of FDM and LCM that revealed differences between them. First, at the retinal level, the oscillatory potentials of the electroretinogram are reduced only in the FDM eyes, even though the other components of the electroretinogram are the same in FDM and LCM. 12 Second, it is known that optic nerve section does not prevent the myopia and increased vitreous chamber depth of FDM. 13,14 Wildsoet 15 found that optic nerve section reduces the vitreous chamber elongation of eyes with LCM, but has no effect on eyes with FDM. Third, an abstract from Schmid and Wildsoet 16 asserts that continuous stroboscopic illumination attenuates FDM over a wider range of frequencies than that of LCM. In this article, we first compare LCM and FDM with respect to the early time course in ocular elongation, choroidal thinning, and proteoglycan synthesis. (In the case of proteoglycan synthesis, we examined the response to a brief "pulse" of lens or diffuser wear.) Next we ask whether they are similarly affected by two visual manipulations shown by Nickla 17 to affect FDM: (a) brief periods of stroboscopic light at dawn and dusk and (b) brief periods of light interrupting the night. METHODS Subjects Newly hatched White Leghorn chicks (Gallus gallus domesticus) from Truslow Farms (Chestertown, MD) were raised in temperature-controlled brooders from 1 day of age. The chicks were fed Chick Starter Chow (Purina Mills, St. Louis, MO). White fluorescent lights were on from 8 AM to 10 PM every day. Care and use of the animals conformed to the ARVO Resolution on the Use of Animals in Ophthalmic and Vision Research