Fusion

While surfing the ‘net for answers regarding her binocular vision, Mary read my 3D blog and had additional questions.  Since these types of inquiries come up often, I thought it would be best to answer her in another blog!

Mary’s comment is reproduced here:

Mary writes:

I ran into your blog while looking for information about monocular fixation syndrome.  I do not have binocular vision.  From what I understand it is a condition in the brain where the nerves don’t touch.  I have had cataract surgery and was not given the choice of corrected vision since my eyes work independently. Can you explain this better. I have been unsuccessful in locating any information on the web.

Thank you for responding to my 3D blog.  Looking at your question it seems, essentially, that you are asking about the concept of fusion.  I’ll try to break down your issues one by one, but please keep in mind that not having your full eye history at hand and, of course, not having seen you myself, I can merely cover the topic in general and hope that you gain more insight.

Look at the flower above.  If you want to look at the flower, your extraocular muscles (the muscles that move the eyes) move quickly, rotating your eyes so that the image of the flower lands on the fovea.  Positioning that flower onto your fovea is called fixation.  A fixating eye aims straight at the target. You only need one eye to fixate. But we have two eyes, and two foveas.  Here’s an experiment you can try.  Find a red colored piece of plastic, like plastic wrap or a red tinted plastic lid from the kitchen that’s transparent enough to see through, and hold that over your right eye only. Now, cover the right eye and look at the flower.  You are now fixating monocularly with your left eye and the flower probably has a mix of colors.   Now cover your left eye instead and look through the tinted piece at the flower.  You are now monocularly fixating on the flower again, but with your right eye along with the red tint and now the red ring is gone, the blues are darker, and the pinks are faded.  But if you open both eyes, with monocular fixation problems, your brain is confused and only one eye continues to participate for some reason, so you see either the regular mix of colors or the dark blues with no reds or pinks.  You may even rapidly switch back and forth between the eyes, but they won’t work together at the task. Those with binocular vision will see the flower as a strange blended mix of bright colors.   Binocular fixation occurs only when both visual axes, one for each eye, intersect at the flower.

In Figure 1, the image of the flower is landing on the fovea of each eye.  The visual axes of each eye intersect at the flower.  When both eyes see the flower, they fuse that image, as one, and that information goes to your brain as one single binocular image of a flower.  To obtain fusion, you need binocular cooperation with equal vision and straight (aligned) eyes.  If there is a misalignment, your brain “reads” that as either double vision or you learn to suppress that image in one eye.  Here, the visual axis of the right eye is pointing somewhere other than the flower, so fusion cannot occur.

Likewise, if your vision is not equal between the eyes, your brain “reads” this as double or you suppress one eye.  For example, if you have a dense cataract in one eye, or one eye has poor vision for a variety of reasons, your binocular vision can be affected or not even be present.

From your question, it sounds like each of your eyes can see.  They would not have done cataract surgery on a truly non-seeing eye.  But your eyes don’t work together.  Since you are not seeing double, you have learned how to suppress the eye not participating in what you are looking at in that moment.  While you may see this as a problem, this is not unusual at all.  In fact, this is the basic concept of monovision with contact lens wearers.  One eye wears a contact that helps you to see up close.  The other eye wears a lens that sees far away.  Your brain switches back and forth, “listening” to each eye as needed.  As in reading a menu then looking up to see out the window, your brain uses the appropriate eye to do this.  The same concept can be utilized with cataract surgery to minimize the use of reading glasses.  Yes, it can be a bit odd to get used to.  Yes, your depth perception is off.  But it usually works.

It sounds like you have been doing this all your life for reasons unknown, and your cataract surgeon corrected each eye for its best vision.  Then you merely continued to adapt and function like you always have.  If you did not have binocular vision prior to cataract surgery, it’s unlikely that you would obtain it afterwards. You are using both of your eyes.  They just don’t work together.  Depending on the cause of your situation, it may or may not have been correctable if caught at a very young age with various procedures and methods.  But as an adult, you pretty much have to work with what you have.

As to your statement, “…a condition in the brain where the nerves don’t touch.” that’s not true. Your optic nerves, one for each eye, take visual data back to the brain where it is interpreted as one unified message. It’s not a matter of “touching” rather, your brain, for whatever reason, is only “listening” to one channel at a time.  Not unlike our Congress and Senate.  They are all in Washington D.C., even sit together and partake in morning coffee and Danish, but they don’t seem to work together on anything!

I hope this helps!