Physiological diplopia (and a fun exercise to practice it!)

Physiological diplopia (phys dip) is "a normal phenomenon in which objects not within the area of fixation are seen as double" (source). When someone with normal vision focuses on an object, everything in front of the object appears double, and everything behind the object appears double. This usually goes unnoticed by most people, unless their attention is brought to it.

Interesting side note - if you google "physiological diplopia", you find that many people have accidentally noticed their physiological diplopia, become incredibly concerned, and posted on internet message boards looking for a cure. Here's an example:

I only just found out there is a name for my double-vision, "binocular-diplopia". I am 17 years old and all my life have had a double-vision thing going on with my eyes. I didn't realise there was any problem until I recently looked into it online, discovering the description of this problem fits the description of diplopia. If I cover one eye, it goes away. This goes for both eyes. When I focus on an onject I see it as one, but whatever is behind it it doubled. What I need to know is how serious this can be for me... I have had no previous medical problems with my eyes, or with my head at all, so it has not been caused by any kind of tumors, etc. Basically i'm not sure what to do about this, I haven't told my parents about it. Should I see an eye-doctor of some kind? Please guide me on what I should do...(source)

I've also read that it's not uncommon for ophthalmologists to get a few patients like the one above in their offices from time to time.

Physiological diplopia differs from pathological diplopia. Pathological diplopia is diplopia that results from an abnormality in the visual system, such as strabismus. When someone with strabismus complains of double vision, it's because their eyes are misaligned and pointing in different directions.

If you have strabismus, you don't have physiological diplopia. It takes two eyes pointing at the same thing, and not suppressing, to get physiological diplopia. For a strabismic, developing the ability to achieve and notice physiological diplopia is a great thing. Why? Because physiological diplopia is an excellent way to determine if one is using both eyes, and pointing both eyes at the same place.

This is why we use the Brock string. The Brock string is a physiological diplopia exercise. When you fixate on a bead on the Brock string, the string doubles in front of and behind the bead. It forms an X. This is the normal response that we strabbies are trying to achieve.

I mentioned in a previous post that I had a little trouble with the Brock string. Instead of an X, I would see a Y.

I could see the double image behind the object I was looking at, but I would suppress the double vision in front of the object and only see one string. With practice, I got much better at seeing the X. However, when doing less controlled physiological diplopia activities, I was still terrible at it.

Hold your finger in front of your face. Look at your finger. Your finger should be in focus, and objects in the background should appear double. This has always been pretty easy for me. It's the next trick that is really hard.

Hold your finger in front of your face. Look past your finger, and look at something on the wall. If you do it right, your finger will double. I can't do it, though! It's too hard!

So, I have invented an exercise to help me practice physiological diplopa in free space. My vision therapist really liked it, so I decided to share it. Here it is!

Josh's Special Red/Green Diplopia Exercise of Fun

First, you need targets to fixate on, and the targets must be able to cancel with red/green glasses. Why? Because then you will know for sure if you are actually pointing both eyes at the target. (I often think I'm pointing both eyes at a target, but really I'm only pointing one eye at a target.) I made a hand-held target, and targets for the wall.

 

You also need red/green glasses (of course) and a pen light.

Instructions for diplopia in front of the target
Look at the target (either in your hand or on the wall), and place the pen light between yourself and the target. Focus on the target, and try to make the pen light double around the target. If you do it correctly, there will be two pen lights, one red and one green, around the target.

You can make this activity more "task-y" if you place multiple targets on the wall, and make the pen light double around each of the targets.

Instructions for diplopia behind the target
Hold the pen light in one hand, and hold the hand-held target in the other hand. Place the hand-held target between yourself and the penlight. Focus on the target, and try to make the pen light double behind the target.

End of Instructions

Once you acquire the ability to notice and see physiological diplopia, you can use it to determine if you are pointing both eyes at the same place and not suppressing. For example: you are at school, listening to a lecture by your professor, and you want to practice pointing both eyes at your professor. Hold a pencil close to your eyes, and try to make the pencil appear on both sides of your professors face.

You can also practice this while reading, which I have seen called "bar reading" or "wire reading." Just hold something (a pencil, a wire, etc) between yourself and the book you are reading. Try to make the wire double and straddle the page or the words that you are reading.

My vision therapist told me about an exercise that she likes where the patient must hold a bendy straw in his or her mouth, with the bent part up.

In Guiding Strabismus Therapy, by Lora McGraw (It's a great book! You can find it here), I even saw a physiological diplopia device that could be attached to your glasses. It's called "The Beak", and allows a hands-free (but weird looking) physical diplopia check. I love it!

In conclusion, physiological diplopia is very fun and useful, and there are lots of ways to practice it. Try it out!