Scientists have so far been unable to fully understand myopia on a molecular level, but what they have figured out can provide support for an outdoor light explanation. Studies in the laboratory have shown that bright light can stimulate the secretion of dopamine (dopamine) in the retina, a neurotransmitter that controls the growth of the eye and prevents abnormalities. William Stell, an internist and neurobiologist at the University of Calgary in Canada, said the growth of the eye is the result of a clever balance of dozens of natural chemicals, in which light also plays a role. He likens the process to a car driving up a hill, which can easily go downhill unless the driver applies the brakes. Dopamine and other signals to stop growth are the equivalent of brakes, and exposure to bright light is the equivalent of slamming on the brakes. Steele said the right balance of chemicals secreted in the eye “controls whether the car moves downhill a little bit, or all at once. Compared with natural light, the signal generated by indoor lighting may not be enough to “slam on the brakes” effect to stop the abnormal growth of the eye. On a sunny day, outdoor light intensity ranged from 28,000 to 130,000 lux (international unit of light intensity), while indoor light intensity averaged less than 1,000 lux. Animal experiments have further reinforced the link between light and myopia. Scientists strapped animals with special glasses so that they could only see blurred or distorted views, and this caused the animals to develop myopia. Biophysicist Frank Schaeffel of the University of Tübingen in Germany (Frank Schaeffel) and colleagues reported in 2009 that such myopia does not occur when chicks are exposed to sunlight or bright lights in the laboratory. Scientists at the University of Houston experimented with young macaques with glasses and came up with similar results, published in Ophthalmology Research and Vision in 2012. These two findings provide credible evidence for the outdoor light intensity theory. However, despite the broad scientific consensus that appropriate light intensity and unobstructed visual fields can control normal eye growth, the question remains open. Animal experiments do not accurately reproduce the conditions that occur in children. Steele said that a distorted field of vision can make chicks suffer from myopia, while living only in indoor light does not make them myopic. However, millions of people are still nearsighted without those special glasses. Although the outdoor light theory is a major discovery at the population level, it lacks a complete explanation. “We haven’t determined exactly what’s better about spending time outdoors,” Muti said. In addition to ample sunlight, the outdoors offers a wide field of view, which is quite different from staying indoors. Andy Fischer, a retinal neurobiologist at Ohio State University, points out that the eye is more relaxed outdoors, “it doesn’t have to work hard to bend the light.” In such a relaxed situation, the growth signals that distort the shape of the eye might be called off. The outdoor environment also provides a different set of afterimages (peripheral image, i.e., what is seen in the corner of the eye) than indoors. Although these objects do not appear in the center of the field of view, they also exist in parfocal or scattered focus. Sheffl says, “If you go outside and look into the distance, all the objects in the afterglow will be in the same focal plane.” In other words, the scene reflected in the afterglow is almost the same far and near, easier to focus accurately. The situation indoors is different, Scheufele said, parafocus and bokeh images will be mixed together. Scientists let the animals wear special glasses that can distort or block the afterglow view without affecting the view in the center of the field of vision. The results of the experiment showed that their eyes showed growth stimulation associated with myopia.