In myopia the eye grows too long, generating poorly focused retinal images when people try to look at a distance. Myopia is tightly linked to the educational status and is on the rise worldwide. It is still not clear which kind of visual experience stimulates eye growth in children and students when they study. We propose a new and perhaps unexpected reason. Work in animal models has shown that selective activation of ON or OFF pathways has also selective effects on eye growth. This is likely to be true also in humans. Using custom-developed software to process video frames of the visual environment in realtime we quantified relative ON and OFF stimulus strengths. We found that ON and OFF inputs were largely balanced in natural environments. However, black text on white paper heavily overstimulated retinal OFF pathways. Conversely, white text on black paper overstimulated ON pathways. Using optical coherence tomography (OCT) in young human subjects, we found that the choroid, the heavily perfused layer behind the retina in the eye, becomes about 16 µm thinner in only one hour when subjects read black text on white background but about 10 µm thicker when they read white text from black background. Studies both in animal models and in humans have shown that thinner choroids are associated with myopia development and thicker choroids with myopia inhibition. Therefore, reading white text from a black screen or tablet may be a way to inhibit myopia, while conventional black text on white background may stimulate myopia.

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Emmetropization is controlled by the defocus in the retinal image. It is a classical problem how changes in focus, introduced by accommodation, are taken into account. We have quantified accommodation errors in chickens wearing negative lenses to find out whether they can predict subsequent eye growth. Two groups of chicks, aged 10 to 13 days, wore lenses (-7D) monocularly for 4-7 days. Fellow eyes remained untreated. Vitreous chamber depth (VCD) was measured in alert hand-held chickens with high resolution, using the Lenstar LS 900 (Haag-Streit, Koeniz, Switzerland). Non-cycloplegic refractive state was measured by automated infrared photoretinoscopy with and without the lenses in place. In group 1 (n = 6), measurements were done 5 times a day to obtain detailed VCD growth curves. In group 2 (n = 10), measurements were only taken twice, at 9 am and 4 pm, to reduce the risk of recovery from induced myopia due to the frequent removal of the lenses. As expected from the negative power of the lenses, refractions measured through the lenses were more hyperopic although not as much as predicted by the lens powers, indicating that chickens partially refocused their eyes by accommodation. Among different animals, accommodation errors varied from 1.1 ± 0.9 to 3.6 ± 1.1D (group 1, mean ± 1 standard deviation) and 0.22 ± 1.25 to 1.72 ± 1.23D (group 2). No correlations were found between the magnitude of the accommodation errors in individual animals and subsequent changes in VCD. With negative lenses, VCD grew both during day and night while fellow eyes grew only during the day but shrank during the night. In conclusion, accommodation errors did not predict future eye growth. This raises the question as to why brief periods of clear vision, when lenses are taken off, have a strong inhibitory effect on myopia development while periods of clear vision due to accommodation have apparently no effect. A possible explanation is that, in addition to retina-driven control of eye growth, there is a second neural pathway for the control of eye growth that carries the signal of accommodation - although it is striking that no neuronal and structural correlate has been identified to date.