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Watching English Change: an Introduction to the Study of Linguistic Change in Standard Englishes in the Twentieth Century (Learning about Language). London and New York: Longman.

Results Only the 0.05), and only small myopic shifts in refraction (. INTRODUCTION Myopia or nearsightedness is a condition in which the eye is too long for its optical power. Severe (high) myopia has been associated with visual impairment and can heavily influence the career and lifestyle choices of affected individuals ().

Interest in understanding how visual guidance of eye growth might lead to myopia is fueled by its increasing prevalence. In the United States, approximately a quarter of the population suffers from myopia, and the prevalence is increasing (, ). Myopia has reached “epidemic” levels in some Asian countries, for example, over 90% in Taiwanese Chinese university students (, ). While the viewpoint that myopia has a genetic origin is apparently supported by the observed higher prevalence of myopia among the children of two myopic parents compared to children with one or no myopic parents () and ethnicity-related differences in myopia prevalence (, ), the case for homeostatic control of refractive error is strongly supported by animal studies (). Of relevance to the current study is the observation that form deprivation induces excessive ocular elongation in young animals, with similar findings in all species studied to-date (tree shrew (); marmoset (); chick (); rhesus macaque (); mice ()).

Likewise, pathologies leading to retinal image degradation in infants and young children, e.g., corneal opacities (, ), cataracts (,,, ), and ptosis (), have been linked to the development of myopia. The consistency of this form deprivation response across different species highlights the importance of a clear (high quality) retinal image for attaining and maintaining emmetropia.

In 2 previous studies of form deprivation myopia, one in chick (), and another in monkey (), diffuser density was manipulated to learn more about the visual requirements of normal refractive development, i.e., emmetropization. The general conclusion from these studies was that form deprivation myopia is a graded phenomenon, that is, the amount of myopia is directly related to the amount of retinal image contrast degradation. In the study involving chicks, increasing retinal image degradation induced increasing myopia, leading to conclude that “emmetropization can take place in the absence of accommodation just by maximizing the average retinal image contrast”. The “graded response” model proposed by rests on the assumption that the underlying retina-derived growth signal encodes changes in image contrast. However, the cross-sectional nature of the data from this study is not an adequate test of the “graded response” model. For example, it is possible that the effect of retinal image degradation is simply to trigger an all-or-none response that is attenuated if and when the limit of the eye’s depth of focus is exceeded as a result of the increased ocular growth, and retinal image contrast is compromised further.

This model also does not take into account the spatial frequency-dependence of emmetropization, as indicated in other studies in chick (,, ). Emmetropization is predicted to fail when the contrast of critical spatial frequencies drops below threshold; here too, an all-or-none response pattern is the predicted outcome of contrast manipulations. The result reported by (i.e., increased myopia with increased retinal image degradation) is predicted for both their graded response and the alternative all-or-none response models, although the temporal patterns of eye growth are expected to be different. The study reported here sought further insight into the inter-relationship between retinal image degradation and eye growth regulation by tracking over time the ocular responses of young chicks exposed to different levels of retinal image contrast degradation. Animals Forty-eight White Leghorn chickens ( Gallus gallus domesticus), obtained as hatchling chicks from a commercial hatchery (Privett Hatchery, New Mexico), were used for this study. They were reared under diurnal lighting conditions (12 hour on/12 hour off light cycle), with access to sifted food and water ad libitum. The food was sifted to remove fine particles that may have interfered with the diffuser treatments (see below).

The cage temperature was kept at approximately 30°C. Experimental procedures were conducted in accordance to the NIH Guidelines of Animal Care and were approved by the Animal Care and Use Committee of the University of California, Berkeley. Form deprivation myopia treatments Transparent plano lenses were combined with each of 4 Bangerter diffusing filters (Fresnel Prism & Lens Co., AZ) spanning the available density range, generating 4 treatment groups, and an additional group wore a plano lens without any filter (no form degradation) as a control.