Monday, February 2, 2015

Elongating Eyeballs

by Eli Weissman

In A Connecticut Yankee in King Arthur’s Court, Mark Twain wrote, “…...you can’t depend on your eyes when your imagination is out of focus.” (1) Twain was referring to the human tendency to let emotion fog reality. Yet, it is intriguing to consider these words at face value. What if visual acuity depends on the processes that allow us to picture an image in our “mind’s eye”? As an undergraduate, this romantic notion led me on a quest to determine the cause of my own fuzzy vision.


It was the winter of 2004. One of my best friends gave me a book called Relearning to See: Improve Your Eyesight Naturally! by Thomas Quackenbush. My desire to heal my fuzzy vision led me to not only read the book my friend gave me, but to consume as much eyesight improvement literature as I could. I also met with multiple alternative vision “therapists”, bought a strange device with red and blue flashing lights, wore reading glasses (an odd thing for someone with myopia), purchased pinhole glasses (these guys are pretty funny looking, see below), and ceased wearing the negative lenses I needed to see clearly.

In the 1920’s, a ophthalmologist named William Horatio Bates gave birth to much of the anti-glasses culture that exists today. Dr. Bates became displeased with his own practice of prescribing progressively stronger lenses to ametropic (abnormally seeing) patients. His research suggested that visual errors were due to a stressed response to modern industrial societies. Instead of glasses, he prescribed relaxation exercises (e.g. palming, where patients place their hands over their eyes to exclude all light and try to imagine perfect darkness). However, he felt that such exercises were unnecessary if the patient could simply learn to relax, and see without effort.


Despite lengthy attempts to improve my own nearsightedness, I do not have good news to report. Ten years after first learning of Dr. Bates, I am still nearsighted (I have myopia—i.e. my vision is fuzzy when I look in the distance). And while I no longer do any of the relaxation exercises Dr. Bates suggested, there is a part of me that does not believe that glasses are the only solution to visual errors. And, here is why: if I relax my mind, blink lightly, and then look into the distance, I can produce short-term clear vision. While subjective, these moments are enough to grow dendritic trees of doubt in my mind. Why did my eyesight get worse at the end of high school? Was it stress, genetics, excessive near work, or some combination of factors?

Science has no definitive explanation for the cause of errors of refraction (the ability of a lens or eyeball to change the direction light travels). There is, however, research suggesting that eye problems are not a pre-determined genetic destiny:

1.)  Researchers have experimentally produced myopia in a variety of animals (chickens, pig-tailed macaque monkeys, guinea pigs, and others) when subjected to form deprivation (a limiting of the visual field) (2).
2.)  A 2008 study of Australian school children found an association between intense near work and myopia, confirming the findings of other studies that also revealed this correlation. One might conclude that near work is a type of form deprivation (3).
3.)  There is a positive relationship between education level and myopia. A Gutenberg Health Study in Germany found 53 percent of university graduates surveyed to be myopic. This is a dramatically high percentage when compared to the just 23.9 percent of subjects with myopia that had no professional training (4).
4.)  Myopia prevalence has also increased with modernization. Morgan et al. (1975) found this to be the case for an Intuit population in Canada. Forty-five percent of Intuits surveyed between the age of 15 and 29 were myopic while this number falls to just 4.5 percent in individuals aged 30 years or older. The study also suggested that increased school attendance plays a roll in the development of myopia (5).

Ok, so it is apparent that myopia has a strong environmental component, but how does the eye actually change to produce myopia?

First, an understanding of how the eye functions:

The front of the eye (in particular, the cornea and lens) refracts light waves so that they will converge at the retina. Here, waves of light cause changes in the cone and rod cells (associated with day and night vision, respectively). This modifies an electrical signal that the brain interprets. The lens is convex, meaning its center is thicker than its top and bottom. The convex lens causes parallel rays of light to refract and converge at a point behind it. To produce crisp, focused sight the cornea and lens must bend light precisely so that it lands on the retina at the fovea centralis. This is the location of the highest density of cone cells (6).

Unfortunately, this mechanism relies on multiple anatomical features functioning, with little room for error. Thus, the eye is vulnerable to abnormalities. In emmetropic (normally seeing) individuals the eye is the correct length relative to the amount of refraction that it performs, and light converges at the retina. In a myopic individual, light converges in front of the retina. In hyperopia (farsightedness) the opposite is true, and light converges behind the retina. The likely anatomical cause of myopia is a cornea that is too curved, or an axial length (the distance from the front to the back of the eye) that is too long (6).




For an old-fashioned understanding of how the human eye refracts light, please watch this short video (about 30 seconds):


If you have some extra time, here is a slightly more in-depth version (2 minutes):


In Axial Length of Myopia: A Review of Current Research (2010), Meng et al. conclude that as axial length (the distance from the cornea to the retina) increases, the myopic state of the eye increases, as well (7). Light simply refracts too much for it to properly converge on the retina.

Recent research of form deprivation in guinea pigs suggests a mechanism for this increased axial length. The culprit, as it turns out, is a big-name compound in the hormone/neurotransmitter world: dopamine.  Dopamine receptors are common in cells throughout the eye. Guinea pigs treated with translucent eye shields developed myopia and eyes with increased axial lengths. But get this: when guinea pigs with translucent eye shields were injected with L-DOPA, a precursor to dopamine, they did not become as myopic! Not only that, L-DOPA actually reduced eye elongation in these form-deprived rodents (8).  

Alright, we are missing one piece of the puzzle that could help support the theory that limited visual stimuli and less light exposure causes myopia. What if, say, endogenous retinal dopamine levels were affected by form deprivation and light? This would suggest that perhaps this association between near work/form deprivation and myopia could be partially due to decreased levels of dopamine. This is exactly what the L-DOPA researchers found. Form-deprived guinea pigs had less endogenous dopamine in their retinas (8). This evidence was also supported by another group of scientists studying rabbit retinas. They showed that light activates dopaminergic retinal neurons (9). In other words, less light means less dopamine, which increases axial length, which produces myopia.

Before bringing this discussion back to my own nearsightedness, I want to mention that there is a genetic component to eye problems. Identical twins are more likely to both experience errors of refraction than fraternal twin siblings (10).

Hmmmmmm, so does my profile fit the characteristics that have been experimentally and observationally shown to cause myopia?

Excessive near work.....CHECK

Above average time spent in school.....CHECK

Less exposure to sunlight.....HALF-CHECK

Genetic propensity (both my parents and grandparents are myopic).....CHECK

There are likely many mechanisms at play beyond this simple relationship between form-deprivation, dopamine, and myopia. However, the research discussed produces a strong argument for an environmentally-mediated path to the myopic eye.

So, I’m encouraging myself, and anyone reading this blog, to spend more time gazing into the distance, enjoying the outdoors, and appreciating the extraordinary vision evolution has provided us. While I am not certain that having a more focused imagination, as Mark Twain suggests, will lead to clearer vision, I imagine that it couldn’t hurt.
                  

You have reached this blog's conclusion. But, for some more fun, can you identify the hidden images below? Write what you see in the comments section.






Videos and Pictures (in order of appearance):

1. Eye:
2. Pinhole glasses:
3. Palming:
4. Eye refraction:
5. Videos:
6. Guinea pig:
7. Blind spot:
8. Magic eyes:

Papers and Online Resources (in order of appearance):

1. About.com: Classic Literature. “A Connecticut Yankee in King Arthur’s Court” by Mark Twain. Available from:

2. Edwards, M.H. Animal models of myopia. 1996. Acta Opthalmologica Scandinavica 74:213-219.

3. Ip, J.M., S-M. Saw, K.A. Rose, I.G. Morgan, A. Kifley, J.J. Wang, and P. Mitchel. 2008. Role of   Near Work in Myopia: Findings in a Sample of Australian School Children. Investigative Ophthalmology & Visual Science 49(7):2903-2910.

4. Mirashahi, A., K.A. Ponto, R. Hoehn, I. Zwiener, T. Zeller, K. Lackner, M.E. Beutel, and N. Pfeiffer. 2014. Myopia and Level of Education. American Academy of Opthalmology 121(10):2047-2052.

5. Morgan, R.W., J.S. Speakman, and S.E. Grimshaw. 1975. Intuit myopia: an environmentally induced “epidemic”?. Canadian Medical Association Journal 112(5):575-577.

6. Purves D, G.J. Augustine, D. Fitzpatrick, et al., editors. Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates; 2001. Anatomy of the Eye. Available from: http://www.ncbi.nlm.nih.gov/books/NBK11120/

7. Meng, W., J. Butterworth, F. Malecaze, and P. Calvas. 2010. Axial Length of Myopia: A Review of Current Research. Ophthalmologica 225:127-134.

8. Junfeng, M., L. Shuangzhen, Q. Wenjuan, L. Fengyun, W. Xiaoying, and T. Qian. 2010. Levodopa Inhibits the Development of Form-Deprivation Myopia in Guinea Pigs. Optometry and Vision Science 87(1):53-60.

9. Parkinson, D., and R.R. Rondo. 1983. Effect of Light on Dopamine Turnover and Metabolism in Rabbit Retina. Investigative Ophthalmology and Visual Science 24(3):384-388.

10. Tsai M.Y., Lin L.L., Lee V., Chen C.J., and Shih Y.F. Estimation of heritability in myopic twin studies. 2009. Japanese Journal of Ophthalmology 53:615-622.

                                                            


2 comments:

  1. bambi and interlocked donuts! really interesting blog - from someone with perfect vision :)

    ReplyDelete
  2. eToro is the best forex trading platform for newbie and established traders.

    ReplyDelete