Sunday, February 27, 2011

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!


  1. Love this post. Lots of good quote material for my future brock string update. You explain it so well!

    I have been phy-dipping both ways, but it's still easier to get the background to double. I imagine I have less trouble with doubling foreground because of turn-in, not turn-out.

    I also have the reverse of your Y on the Brock string: I can always see the strings running TO the bead, and have more difficulty seeing the far side. Again, this is probably because of turn-in.

    Gotta love that Professor! Where would we be without her? :-)

  2. Ah ... that would be "phys-dipping" (pronounced fizz dipping ;-)

  3. Lynda, I think you're right. I think in theory, and esotrope would have physiological diplopia difficulties that are opposite of mine. Now I need to go draw out your eyeballs and your suppression scotoma, and then I will understand everything... :)

  4. LOL I would love it if you drew out my eyeballs and suppression scotomas ;-) It makes sense that there are supression zones in the retinas.

    I have been mulling your drawings over and have even more questions. . I'm wondering if patching the good right eye for 30 min a day while doing dishes, or near work, would help teach the brain that I want to use my whole left retina. I did this a couple of times figuring it wouldn't hurt, but oh the nausea after awhile ...

  5. Well the suppression scotomas only appear under binocular conditions, so you wouldn't necessarily be addressing that with a patched activity, I think. But who knows? I'm just an English teacher.

    It might be helpful for other reasons, though. You're training the weak eye to be as close to equal as possible to the good eye. Sounds like a good idea to me!

  6. Brilliant exercises Josh. We use the red/green glasses with penlights and distance targets but we don't combine them with red/green targets. Great idea, this must be taxing to work through.

  7. Thanks! Actually I started using the red/green targets because it made it easier for me. I had trouble getting physiological diplopia in front of the object of centration for two reasons - suppression, and not being able to feel if both eyes were pointed at the target. All of the usual advice about "look into the distance" or "look past it" never helped me, because the way I normally look into the distance is to use one eye.

    Using the red/green targets with this exercise solved one of those problems - it revealed if my eyes were pointed at the target. Once I knew that I was pointing them in the right place, I kept pointing them there until the suppression started breaking down.

    Now that some of the suppression is gone, I don't need the red/green glasses and and red/green targets as much. I have enough diplopia now that, with small objects, I can make them double on their own. I then move my eyes until the double images overlap, and I can achieve physiological diplopia in front of the object (like the professor picture).

    (Phys-dip behind an object has always been easier, due to the location of my suppression scotomas, and I have been able to do it since childhood)

    Anyway, the point is - I'm glad you like it! And it may seem like it would be more taxing, but I found the contrary to be true. It took a taxing exercise that I couldn't accomplish and made it doable.

  8. interesting josh!!

  9. Hey, this is #3 on google. Congrats! I just linked to it again in my latest post:

    Hangin' in there!

  10. Dr. Samantha SlotnickAugust 3, 2011 at 7:00 PM

    Thanks for this, Josh.  You are truly an innovator.

    I hope your therapy has been very successful, and it sounds like you don't need the reinforcement of R/G to know when you're suppressing at this point.

    Have you considered working in this field?  You would make an extraordinary therapist (or behavioral optometrist, for that matter)!

  11. Phew. I was like the first person in this article, I was worried about this once I saw an article on double vision online. Lol, thanks!!

  12. I find holding two fingers up on front of my nose and making a 'flying sausage' works as well. I get some strange looks on the bus though... =)

  13. Question: I have been abe to notice this for a very long time and thought it was abnormal but how do people shoot pistols like this! I get very angry when i try to shoot a pitol with both eyes open and I see either two pistols or two targets! How do I knowwhich one is ghost object is real?

  14. This article really helped me! I really actually thought that I had binoculor double vision as I thought the symptoms were exactly as I had

  15. How can we cure physiological diplopia???? Please tell me I'm really worried about this thing since last 2 years.