Recently I received an email from a man named Allan Wright who sent me a link to an article and video entitled “Busker's eye-opening trick defies science.” It describes a Japanese performer who can move his eyes independently. If he moved his right and left eye as far to the left as possible (that is, the right eye close to and the left eye far from the nose), then he could turn the left eye in, without moving his right eye, so that both eyes were pointing toward his nose. In the video, the eye movements look very unnatural (and uncomfortable!), yet the performer claims they are easy to learn.
To say that the eye trick defies science may be a bit strong, but it does pose a challenge to Ewald Hering’s 19th century “Law of Equal Innervation.” Hering believed that both eyes are controlled by the same neural commands through innate connections from the brain to the eye muscles. His views were challenged by Hermann von Helmholtz who claimed that each eye is controlled separately, and, most importantly, that coordination between the two eyes is not hard-wired but learned. The two men argued bitterly with each other, but they could not resolve their dispute because, at the time, recordings could not be made from the relevant neurons in the brain. But recent research indicates that Hering may have been wrong and Helmholtz right. At least for some eye movements, we control each eye separately.
For more than 100 years, Hering’s views have dominated the way we think about eye movements, visual development, and visual disorders. If, as Hering asserted, our eye movements are controlled by hard-wired connections, then someone with crossed eyes or wall-eyes has a problem that they themselves cannot fix. If, on the other hand, as Helmholtz suggested, we learn how to move our eyes, then a cross-eyed or wall-eyed person may be able to learn how to straighten their eyes with the proper training. This is what happened to me when, after my first 48 years of being cross-eyed, I learned how to coordinate my eyes and see in 3D with optometric vision therapy.
If binocular coordination is learned, then we may need to view some visual behaviors differently. Let’s say that a child is cross-eyed and cannot fuse the images between the two eyes. As result, he or she will see double and will have trouble localizing objects in space. As the child grows up, he or she must find ways to adapt to this confusing situation. In other words, the child has to learn how to see with crossed eyes and may do this by directing the eyes in ways that are not normal.
This may have happened to me. In first grade, I really struggled with reading. A person with normal vision aims both eyes at approximately the same place on the page. But, I was cross-eyed so that my right eye saw letters to the left of where my left eye saw them. Since my two eyes interfered with each other, the letters seemed to move on the page and sometimes change position. Although I am not dyslexic, I distinctly remember wondering if the word I was reading was “saw” or “was.” My mother spent hours reading with me and, by fifth grade, I became a competent but slow reader. It wasn’t until I was an adult and consulted a developmental optometrist, however, that I discovered how I controlled my eyes for reading.
When I looked at the words on the page with my right eye, my left eye turned in, or crossed, by 15º. If I read with my left eye, the reverse happened. The fact that I turned in my eye by 15º reveals how I eliminated the interference between the two eyes, for our “blind spot” is located 15º from the center of the retina. In this region, where the optic nerve leaves the retina for rest of the brain, there are no light-sensing cells. When I read with my right eye, the image of the word fell on the center (the fovea) of the retina of my right eye and on the blind spot of my turned left eye. I could see the letter clearly with my right eye and did not see it at all with my left! The reverse held true when I read with my left eye. Between first and fifth grade, I unconsciously taught myself a way to move my eyes so that they didn’t interfere with each other when I read. All this is possible if we assume that we have independent control of each eye.
At age 48, I underwent optometric vision therapy and learned to aim both eyes simultaneously at the same place in space. My vision became more efficient, and I began to see in 3D. For these changes to happen, I not only had to learn a new way of coordinating my eyes, but I had to unlearn an old way. Helmholtz would have been proud.