When we found in 1987 that young chickens, treated with spectacle lenses (Figure 1), change their eye axial growth exactly so that they reach again normal refractions - with the lenses still in front of the eye -, we (with my co-authors Adrian Glasser and Howard Howland) believed that we had found "the mechanism" that also drives myopia development in humans (Schaeffel et al, Vision Research 28, 639-657, 1988). We also thought that, after a detailed analysis of the biological mechanisms, myopia in humans could be understood and prevented. While a lot was learned subsequently from such experiments in different laboratories (like that the "visual control of eye growth" is achieved by the retina itself, and that the retina can even determine the sign of dioptric power of the lens) - it remains unclear why children become myopic before they were wearing any lenses. The question remains: what kind of visual experience in daily life is similar to wearing negative lenses? While there is little doubt that "near work" is a factor associated with myopia in most epidemiological studies, the statistical association is not always very high, suggesting that it may not be "the key factor".
It has been measured that most people do not accommodate perfectly when reading a text on the paper, but rather focus a little bit behind - which place the plane of focus behind the retina, just like with a negative lens. However, that this factor relates near work to myopia has not been proven. Other factors, quite unexpected, were recently found that had high associations with myopia in children: in at least four large studies, children had a lower risk of myopia when they were more often outside. Even a dose-response relationship was found. In fact, children with the highest rate of myopia (Singapore) were outside less than 3 hours a week. While one might think that the inhibitory effect of outside activity on myopia is due to the longer viewing distances, and the lack of "near work", our recent experiments in chickens show that the development of deprivation myopia (induced by frosted diffusers in front of the eye) is inhibited by high light levels. If chickens are kept in the lab at different ambient illuminances, they develop the less myopia the higher the illuminance. The lowest amount of myopia developed, however, if they were exposed to day light on the balcony (Ashby and Schaeffel, presented at ARVO 2009). In this case, myopia inhibition has nothing to do with imposed defocus. It could be that temporary exposure to high light may represent a simple way to interfere with myopia also in children - monkey expriments are planned.
There are currently three other major directions in myopia research:
Myopia is not only the worldwide most frequent ocular disorder in adolescence, with still no satisfying rational therapy, but also a very interesting biological problem with many open questions. After we had learned in the past years how "perfectly adapted" the optics of the human eye is to our needs (e.g. Artal & Tabernero, Nature Photonics 2, 586-589, 2008), it is clear that myopia is not a matter of "inaccuracy" in biological design - but somehow intended, in response to an inappropriate input.
Section of Neurobiology of the Eye, Institute of Ophthalmic Research, Centre for Ophthalmology, University Clinics Tuebingen, Germany