What is the purpose of prisms in vision therapy?

It's been a week since I'd started using a seven diopter prism, base-in, with the fixation cards exercise, and I've been noticing quite a difference.  In just a week, I'd made quite a lot of progress toward fusion.  Still not yet there, but getting closer and closer.

I am now able to sort of fix with both eyes simultaneously while doing the fixation cards exercise.  It's a little weird because I don't yet have full simultaneous control of both eyes, so there is this sort of confusing feeling of my head existing in two separate realities.  Like my head is in two different rooms, and one of the rooms is drifting around a bit.  But overall it's a nice sensation, because it feels like I am doing the right thing, and this is where I need to be going.  So I'm now fixing, but not quite yet fusing.

So what is the purpose of prisms in vision therapy?  I think a not bad explanation is that they are like a stepping stool.  They make it easier to achieve fusion in the first place, by accommodating for whatever eye turn you may have.  In my own experience, I think an even better explanation for what prisms are in vision therapy is that they are like sticky hands.

Imagine that you are in a big, empty white room.  Except that in the middle of it there is a big white cube, and for some inexplicable reason you feel an incredible desire to get on top of it.  The problem is that the cube is as high as you are.  You cannot see the top face of the cube unless you jump.  In order to get on top of the cube, you're going to have to run toward the cube as fast as possible, jump up as high as possible while slamming your body into the side of the cube, and try to get your hands on top of the cube.  Then you will need to try and grip the top face of the cube with your hands and swing your lower body up to the top of the cube until you can get one leg on top, and then sort of roll the rest of the body over.  Damn.  I'm sweating just thinking about it.

Now imagine that the top of the cube is slippery, and you have no ability to grip the top of the cube.  You can run and jump and slam your body into the cube as much as you want.  Once you get your hands on top of the cube, they're going to slip right off.  If your hands slip off the top of the cube, then nothing else matters.  No amount of planned technique for how you're going to swing your body or what you're going to do with your legs will matter.

In my experience, what prisms have allowed me to do with vision therapy is get a grip.  Prisms have allowed me to grab and hold onto the top of the cube.  And now I can begin to think about what I am going to do next.  The stepping stool analogy isn't bad, but this analogy, although more verbose, is more accurate and precise.

I can speculate why using the prisms work in this way, but I don't frankly know.  I've used them in this way in the past and not with the same results.  But now, for whatever reason, prisms are working for me, and very importantly they make sense to me.  For the longest time, I always had this expectation that stereopsis was suddenly going to pop in for me as Sue describes her steering wheel suddenly popping out at her.  What does that mean?  Does that mean the eyes suddenly go from strabismic to straight?  Part of me, and I'm embarrassed to say it, thought that that's how it worked.  Not really, but kind of.

The prisms seem to free up energy to enable you to build on other skills like fusion and motor ability.  Without the prisms, it's just so much work to get superimposition, nevermind fusion, and the effort takes so much energy, nothing is left for building these important skills.  By freeing up energy, you're enabled to get a grip and build the important skills, and then ramp up a smooth gradient of progress.

Getting closer to fusion with the help of a seven diopter prism

I'm getting quite a bit closer to fusion with the help of my seven diopter prism.

I started using it out of frustration, because I'd been doing the same regimen for about two months, and it seemed like I would be able to, in time, get accommodation synchronized across the eyes with the fixation card exercise.  But I stopped making progress.  Since accommodation and eye angle are linked, I figured I might be able to coax equal eye accommodation via a prism. So that's what I did.  I started using a seven diopter prism base in.  This enabled me to get both double images very close to being in sync--both in terms of fusion and accommodation.

It seemed to free up some energy, and while I don't yet have fusion, it has brought me very close to fusion.  The targets are almost a single fused target at times.  The prism has brought fusion into much closer and more realizable reach.  There are times where I was sure that I had fusion, briefly.

Interestingly, it hasn't hurt my ability to diverge my eyes.  I would have thought it might because my using the prism I'm sort of conceding to my eye's esotropic tendencies, and rendering it even lazier in terms of divergence.  But that doesn't appear to be a problem.  In fact, the Brock String exercise has improved quite a bit since I incorporated the prism in the fixation card exercise.

The rationale is to really practice fusion and teach my brain that this is what it wants.  And then to reduce the prism.  But somehow I doubt that that will be the big part.  I think the big part will be achieving fusion.  In all my years I never had it.  I think my eye's laziness was such an ingrained habit that in all of my years of vision therapy no amount of coaxing would enable me to truly get both eyes working together.  I'd used prisms before.  But now they seem to be working.  Or I had finally started making sense of what I'm doing with prisms.