The following article is presented to you by Andrea Carrillo Alemán
For the general public.
In myopia (short-sightedness), the eye grows too long, and the focus of the image is in front of the retina. The visual scene is defocus when people are looking at a distance. It has been estimated that 2.5 billion people could be affected by myopia by 2020 (1). There is strong association between the level of education and prevalence and development of myopia (2). It is also known that spending time outdoors, where the light is brighter, is associated with less myopia development (3).
What makes children myopic when they read is still not fully understood. One candidate would be the lag of accommodation. Accommodation is the increment of the curvature of the crystalline lens to increase the power and focus objects at different distances. People accommodate less while reading, moving the sharp image behind the retina, which would serve as an error signal (called lag of accommodation) to stimulate eye growth (4). However, the literature is in contradiction. A new unexpected reason why reading may make myopic is the selective stimulation of the ON and OFF pathways of the retina.
Our retinas have cells which compare the brightness in the center and the periphery of their light sensitive area (called receptive fields). These cells send the information to the brain. There are cells that respond mostly to brightness in the center of the receptive field and darkness in the periphery (ON center cells) and cells that respond preferentially to brightness in the periphery (OFF center cells) (review: 5; 6).
Using a software that quantify the relative stimulus strength for ON and OFF cells (figure 1), it has been recently found that dark text on white background (conventional text) stimulates mainly OFF cells and bright text on dark background stimulates mainly ON cells (7).
The layer behind the retina, the choroid, is of particular interest since it has earlier been shown in various animal models (chickens: 8; marmosets: 9; rhesus monkeys: 10) and humans (11)) that changes in choroidal thickness can predict future changes in eye growth. When the choroid thins, the eye typically starts growing; when it thickens, eye growth is inhibited and no myopia will develop. It is possible to measure the thickness of the choroid using optical coherence tomography (OCT) in the living eye with very high resolution. It has been found that after 60 minutes the choroid thickness decreased when reading standard text and increased while reading white text on black background (7). These results suggest that reading text with inverted contrast may be a way to inhibit myopia; however, more studies need to be done in children to confirm the proposed strategy.
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1. Dolgin E. The myopia boom. Nature 2015; 519: 276-278. 2. Jones, L.A., Sinnott, L.T., Mutti, D.O., Mitchell, G.L., Moeschberger, M.L., Zadnik, K. Parental history of myopia, sports and outdoor activities, and future myopia. Invest. Ophthalmol. Vis. Sci. 2007; 48: 3524- 3. Rose, K.A., Morgan I.G., Ip J., Kifley A., Huynh S., Smith W., Mitchell P. Outdoor activity reduces the prevalence of myopia in children. Ophthalmology 2008; 115: 1279-1285. 4. Gwiazda, F. Thorn, J. Bauer, R. Held. Myopic children show insufficient accommodative response to blur. Investigative Ophthalmology & Visual Science 1993; 34 (3): 690-694. 5. Sharpe L.T. and Stockman A. Rod pathways: the importance of seeing nothing. Trends Neurosci 1999; 22 (11):497-504. 6. Schiller PH. The ON and OFF channels of the visual system. Trends Neurosci 1992; 15:86-92. 7. Aleman A.C., Wang M., Schaeffel F. Reading and myopia: Contrast polarity matters. Scientific Reports 2018; 8, 10840. 8. Wallman J. Wildsoet C., Xu A., Gottlieb M.D., Nickla D.L., Marran L., Krebs W., Christensen A.M. Moving the retina: choroidal modulation of refractive state. Vision Res. 1995; 1:37-50. 9. Troilo D., Nickla D.L., Wildsoet C.F. Choroidal thickness changes during altered eye growth and refractive state in a primate. Invest Opthahlmol Vis Sci. 2000; 41 (6):1249-58. 10. Hung L.F., Wallman J., Smith E.L. 3rd. et al. Vision-dependent changes in the choroidal thickness of macaque monkeys. Invest Ophthalmol Vis Sci. 2000; 41(6):1259-69. 11. Chakraborty R. et al. Monocular myopia defocus and daily changes in axial length and choroidal thickness of human eyes. Exp Eye Res 2012; 103:47-54.