Temporal constraints on lens compensation in chicks.

If the effective focal length of a growing eye is modified by spectacle lenses, the eye compensates by altering its growth, thereby keeping images in focus, a process we presume is similar to normal emmetropization. Using chicks, we have investigated how much visual exposure the eye needs to exhibit the two principal components of ocular compensation: altered rate of elongation (a scleral mechanism) and altered choroidal thickness. We have found that surprisingly small amounts of vision through spectacle lenses can elicit robust scleral and choroidal compensation if other visual feedback is limited by keeping the animals in the dark when not wearing lenses. Furthermore, we have found that the amount of vision necessary to induce these responses can be summarized as three rules: First, several brief daily episodes are more effective than a single or a few longer daily episodes, even if the total amount of vision is the same. Second, extremely brief episodes, even if very frequent, are relatively ineffective. Third, when plus and minus lenses are worn successively on the same eye, the plus lens has the dominant effect, even if the minus lens is worn five times longer than the plus lens. In addition, we have shown that the elongation rate and choroidal thickness responses are dissociable, such that brief, infrequent lens-viewing produces only an elongation response in the case of plus lens-wear and only a choroid response in the case of minus lens-wear. We thus show that the emmetropization system does not integrate defocus in a simple, linear fashion. These non-linearities, if present in children, might explain why, although education and reading show an epidemiological correlation with myopia, the total time spent reading and doing other nearwork by individual children generally does not predict the degree of myopia. It may therefore be necessary to quantify more complex temporal patterns of nearwork over the day in order to measure the impact of nearwork on eye growth.