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I believe you may be the first to learn of this term, "Troxler Effect," among bullseye shooters. I will present my case and you may form your own conclusions.
My last article related eye dominance and suppression to our red dot focus which led to this topic. I've been curious with the topic of oxygen deprivation and carbon dioxide buildup and its effects to blurred vision, which has been brought up several times, so I proceeded on a little research. My experiment was done in my eye examination room of my optometric practice. I focused on a single 20/40 size letter in a darkened room (similar in size to the bullseye at 50 yards) but with the door opened to allow for some light to enter. This lighting environment was similar to that of my club bullseye league matches held in the evenings when dark.
I first looked at the letter target with only my left dominant eye for 45 seconds while holding my breath. I then looked at the letter with my left dominant eye for 45 seconds while breathing normally. Then I looked at the letter with only my right non-dominant eye, first while holding my breath and then again while breathing normally for 45 second periods. It would be safe to assume that no one would have to hold this long before the shot breaks. The following were my results.
SEEING WITH DOMINANT EYE WHILE HOLDING BREATH:
As I looked at the letter, I was having problems keeping focused after 20 seconds and my first thought was that oxygen deprivation and CO 2 buildup was, in fact, the cause. Then it came back into focus for the duration of the remaining 45 seconds. (How come? Shouldn't the effects worsen?)
SEEING WITH DOMINANT EYE WHILE BREATHING NORMALLY:
The same scenario resulted just as it did when I held my breath. (Why?)
SEEING WITH MY NON-DOMINANT EYE WHILE HOLDING BREATH:
I had problems seeing the letter rather quickly, approximately after only 5 to 8 seconds of holding my breath with the focus fading in and out.
SEEING WITH NON-DOMINANT EYE WHILE BREATHING NORMALLY:
The letter started to fade and became blurred once again after 5 to 8 seconds. (Why again?)
Discussion: My results indicated that whether I held my breath or breathed normally, I had the same results. With my dominant eye, I was able to stay focused for a longer period than with my non-dominant eye, 20 seconds versus 5-8 seconds. Have advocates of the oxygen deprivation theory provided data regarding blood levels of oxygen and carbon dioxide and at what point does vision become affected? I consulted a colleague who graduated from a different university. I also consulted an associate professor from Stanford Medical School who is also a clinical instructor in the Ophthalmology Department at Stanford Hospital. No one was familiar with blurriness due to oxygen deprivation while holding one's breath too long (while sighting). Clearly, we're not referring to extremes when one is at a point of passing out.
Interpretation: What I see happening is a phenomenon known as the "Troxler Effect." This term was named after a Swiss physician who discovered this phenomenon, Ignaz Paul Vital Troxler (1780 - 1866). This is a visual phenomenon in which an object focused onto the retina WITH NO MOVEMENT will fade starting from the periphery. Our visual center in our brain, although very sophisticated, is like a 5 year old child who needs constant stimulation or it becomes bored and shuts down. In my case (yours may be and probably will be different), using my dominant eye, this fading started after 20 seconds of staring but much quicker with the non-dominant eye. As I mentioned in a another article regarding Shooters' Eye Dominance, I'm a believer in using the dominant eye to sight your gun because vision will be more stable for a longer time. You can then work on the other fundamentals when your vision is stable. Shooters are a strong-minded group of individuals and some may have found their ideal method of shooting regardless of which eye is used.
Because I had no binocular cues when using only one eye, there was no depth perception so my eyes started to converge and accommodate somewhat when the Troxler Effect began. As a review, when one converges, one may also accommodate involuntarily and this additional accommodation started to blur the letter target. As the letter was blurring, I started to note that "alternating suppression" was happening as well. I believe that the brain was struggling to see once again and was trying to force me to use whatever means possible. (For those who did not read my previous article, please refer to, "Shooters' Eye Dominance and Suppression, Sighting with Dot Scopes").
Conclusion: We don't experience this Troxler Effect during our day-to-day lives because our eyes are constantly moving from one point to another. Think of it, the only time we tend to stare for any length of time is during target shooting. The reason why we don't usually experience this phenomenon is that our eyes will shift and blink every time a shot breaks. We don't stare long enough in order for the Troxler Effect to begin. Eye movement is needed to keep the brain stimulated so that Troxler Effect does not happen. If we hold our sighting too long, the STATIONARY image of the bull and sight(s) onto the retina will start this Troxler Effect and I don't believe oxygen deprivation and carbon dioxide was ever an influence. Rifle shooters may notice this fading easier because their aim is steadier with their sights while using two hands to hold their rifles. Interesting to note that if your hold is very unsteady, the Troxler Effect will not happen. Don't keep your hold too long has always been good advice for many reasons. I suspect, although I haven't come across any research, that how quickly the Troxler Effect begins may be an indicator of the magnitude of dominance or lack thereof.
I invite you all to try my experiment at home. The best way to see this effect is by looking at a small object against a blank wall. Cut out a letter or small item from a newspaper or a magazine and tape it onto a blank wall. Stand back 2 feet, cover your non-dominant eye and proceed to stare at the letter WITHOUT moving your gaze. When the fading and blurring occur as you stare long enough, you are experiencing this same Troxler Effect while you hold your firearm sight alignment too long and your vision begins to fade. Now cover your dominant eye and stare at the letter once again with the opposite eye and compare your results. Holding your breath and breathing normally will not change results. Now try to hold your breath as long as you can but look around the room and keep your eyes moving. Note that your vision stays in focus. Alternating suppression and convergence/accommodation difficulties may coincide with the Troxler Effect.
Whenever one experiences firearm sight disturbance, look at a different spot, blink, and/or quickly shake your head to regain focus. Continue to use your favorite breathing method as you sight.
Good Vision and Good Shooting To All,
Norman H. Wong, O.D.