It is becoming increasingly accepted that myopia is not a disease but a regulatory failure of the guidance of eye-growth. The eyes of children and most animals studied are born myopic or hyperopic (far-sighted) and adjust their growth during the postnatal period to correct their vision (emmetropization). It is clear that in animals this adjustment process is under visual control: Having animals wear positive or negative spectacle lenses causes a rapid compensation for the defocus, returning the eye to its previous refractive state. Understanding emmetropization requires finding out both how the eye can distinguish an image focused in back of the retina (which requires the eye to increase its rate of elongation) from an image focused in front of the retina (which requires the eye to slow its elongation) and how this information in the retina gets transformed into a signal that modulates eye-growth. With respect to the first problem, there is evidence that the eye uses the fact that blue light is focused in front of red light (longitudinal chromatic aberration) to guide its growth. Experiments are proposed to test whether the eye can be shifted toward or away from myopia by manipulations of the chromatic properties of stimuli. With respect to the second problem, two molecular signals change in opposite directions when the eye is exposed to opposite directions of defocus, suggesting that they might be part of the signal cascade that connects the visual signal in the retina to the growth-controlling machinery of the eye. Experiments are proposed in both birds and mammals to discover whether the particular conditions of timing of lens-wear that determine whether or not the eye compensates for lenses also determine whether or not these molecular signals change in the appropriate direction to be controlling the compensatory eye growth. Because the rules controlling how defocus leads to compensatory eye-growth appear to be similar across many different species, it seems likely that the same processes are at work in humans. Because myopia is a widespread condition that seems associated with visual experience, especially reading, it is important to understand how these visually mediated growth control processes work. Such knowledge provides our best opportunity to understand what makes some children develop myopia. ? ? ?
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