Suppression scotomas... I've read about them before, but I didn't really find the subject all that interesting. There's a spot on your retina... where you suppress... or something... blah blah blah... who cares. It didn't really interest me. But last night, while doing my vision therapy homework, I suddenly understood suppression scotomas. And I also realized the reason why I often cross my eyes and overconverge when I'm doing vision therapy exercises.
First, what the heck is a suppression scotoma? A scotoma is an area of your retina where images are suppressed. If an image falls on a suppression scotoma, it is suppressed.
This is me, looking at a diamond. I can see the diamond just fine in my right eye. My left eye is being bad, and shooting off into nowhere. The diamond can still be seen peripherally by my left eye, though, and thus I should see two diamonds. But I don't see two diamonds. I only see one diamond. Why? Because the diamond image from my left eye is falling on a suppression scotoma.
The suppression scotoma is that big green spot. Everything that falls on that green spot is suppressed. This suppression scotoma has been custom made by my eyes and brain to prevent double vision. It perfectly fits the way that I use my eyes in every day life. Things in my central field of vision are suppressed, yet things in my left-hand peripheral field are still visible. I do not experience diplopia.
When I do vision therapy exercises, I desperately try to see the input from both eyes, but I have trouble because there's a big scotoma in the way. I need to break down that scotoma and start using that spot on my retina again. But sometimes... I cheat. I peek around the scotoma. How do I do that? Overconvergence!
In this picture, the dotted lines show where my eyes are pointing. They aren't pointing at the diamond, they are pointing in front of it. This causes the image of the diamond to fall outside of my central vision, and I see two diamonds. More importantly, it causes the image of the diamond to fall outside of my suppression scotoma. I receive input from both eyes (hurray!), but it's a diplopic, misaligned image (boo!). I believe this is why, when doing activities with the red/green glasses, I often see a misaligned image like this:
When I realized that I had been overconverging to peek around my suppression scotoma, I was amazed. Brains and eyes are so smart. And tricky. And lazy. And good at finding shortcuts.