As many readers and I myself have noticed there is a discrepancy between my visual acuity without glasses and the correction required as indicated by my focometer. There is a rough correlation that has been found between lines that can be read on a Snellen chart and the required glass lens strength required to improve that person’s vision back to 20/20. Based on this correlation one would expect that if I can read 20/70 on the Snellen chart I would require a lens with a strength of -3D. However, when I measure my refractive state with the focometer it indicates I require a -6D strength lens to read 20/20. I have heard that I should be comparing my prescribed lens strength to my visual acuity in a dim room. I measured my acuity at night in a dimly lit room and I can read 20/200. This is a little bit closer but a disparity still exists.
I believe that this lack of correlation between the two methods is due in part to two factors that are probably related: psychology (or confidence) and muscle memory. I don’t have any scientific evidence for these explanations, only postulations.
The psychological aspect, mental aspect, whatever you want to call it, affects how we view our visual potential. I’ll equate this to running. Let’s say that you haven’t run more than a mile in twenty years and you want to run five miles today. Mentally you will probably start feeling tired after the first mile and think it will be difficult to continue. You may make it to two miles, but most likely you will not run the full five miles. You would have to work up to the five miles to be both physically and mentally able to run the full five miles. Our mental “attitude” has an impact on our abilities. I’m not saying that it’s always the limiting factor; but it could be for some people. With respect to vision I think we experience (to a certain degree) this same effect. I experienced this first hand when I thought I was wearing a reduced pair of glasses and was seeing as if I was wearing the reduced pair. When I realized I was wearing the full strength pair my vision immediately improved to reflect the fact that I was wearing full strength glasses. I have read cases of this happening to other people as well. This goes back to the fact that I think a big impact on our vision is how well we THINK we’re going to see.
The other factor I have considered is muscle memory. Poor vision is in part caused by eyestrain. Eyestrain is caused by tension in the muscles surrounding the eye. Some circles have discussed that the extraocular muscles must maintain a level of strain to keep the eye in an elongated shape in order to see well through a glass lens (i.e. the brain wouldn’t relax the extraocular muscles if the strained muscles were producing clear vision due to the glasses). If you have ever studied control systems this makes a lot of sense; we are getting feedback to our visual cortex from our eyes and vice versa. My theory is that over many many years the muscles in our eyes become accustomed to “performing” this certain task in order for us to see properly with a pair of glasses. When we wear a strong pair of glasses our eyes never get a chance to relax because in order to see well our eyes must maintain a certain degree of strain to keep our vision at an acceptable acuity level; that and most of wear our glasses during all waking hours. When we do not wear our glasses our eyes can relax more because our brains aren’t sending feedback to the muscles to maintain the eyestrain in order to continue seeing well. This is why I think it’s so beneficial and quicker results are obtained if glasses are discarded altogether. Unfortunately I can’t do this. When we don’t wear our glasses our eyes can relax but when we put glasses back on I think our muscles go back to the position that they are accustomed to, which is the level of strain that helps place the eye in the position required to see well with a specific lens strength. It takes time to retrain muscles to perform new tasks (e.g. brush your teeth with your nondominant hand). A good example of this is posture. If we have been sitting in a poor posture for twenty years it will take a while to rehabilitate the muscles to feel correct in the correct posture; the bad pasture will feel correct to us for a while. I don’t believe the physical part of our visual system is much different. Muscle memory is defined as being an unconscious and learned process that manifests from neural pathways that have developed through repetitive practice. The exact mechanisms are unknown, however significant behavioral evidence exists to support its validity. I have also read that attitude affects muscle memory quite significantly. Anyone out there who is a musician is probably familiar with this concept. For example, I can play a piece of music perfectly on the piano until I am in front of an audience of strangers; then I tense up and my abilities are diminished. Stress could be considered an attitude I suppose (albeit sometimes involuntary), so it is reasonable to extrapolate that stress can have a negative impact on vision as I am of the group of people who believes that the extraocular muscles affect visual acuity.
Another related point to muscle memory is visual habits. When wearing glasses we lose the good natural visual habits of shifting, blinking, and saccadic eye movements. It would stand to reason that learning the incorrect muscular habits of staring, not blinking often, and reduction or loss of saccadic eye movements is a result of incorrect muscle memory habits which are unconscious. It is extremely difficult to change unconscious behavior which can be a huge boulder in the road of vision improvement. It takes almost constant attention on a previously unconscious habit which is not easy to maintain over long periods of time. It is equally difficult to then train new muscle memory habits which are correct so that these may become the new unconscious habits.
The very act of putting glasses or contacts over our eyes is a cue to our brains that we are now changing our visual perception. I do not underestimate the power this has over our visual cortex and our attitude and the effect this has on our visual acuity. Putting glasses on my eyes is not something I do unconsciously so I am aware that I should now be seeing better which brings attention to the fact that I expect to see a certain way with a specific pair of glasses on my face. The same is true for using a focometer or other lens over the eye.
In short, I think both our confidence and perhaps more importantly muscle memory play a role in this disparity. As I sit here I can see my vision getting better and worse while wearing my reduced prescription glasses and I think this is a good sign that I am starting to break through the entrenched neural pathways that dictate the muscle tension and poor habits in my extraocular muscles. There is more flexibility in my eyes regardless of the mechanism behind the improvement. If I focus on practicing the correct visual habits with my glasses on I can now see an improvement which I previously never experienced. Being able to see an improvement in my vision while wearing glasses is important to me because it signifies to me that I can improve my vision with or without glasses on my face. Previously I was only seeing changes in my vision without my glasses. Hopefully this means that putting glasses on does not have as strong of a psychological component for me anymore.