I was evaluating a very bright high school student the other day who has been having significant difficulty with standardized tests. She has never had test accommodations in school, but at this point her parents realized that her visual problems are interfering with the sustained demands of the ACT.
“A” (we’ll use only her first initial to preserve confidentiality) exhibited 25^ intermittent alternating esotropia at distance and near, with a preference for left eye fixation. She had a bifocal Rx since early childhood, and her only prior therapy was home-alone HTS. “A” felt like she made gains that quickly dissipated after she stopped therapy.
On the Worth 4 Dot target she reported uncrossed (eso) diplopia on the two circles at the bottom, and as I tried to neutralize her subjective angle with progressively higher amounts of base out prism she made a clever observation:
“You know, every time you move that bar the two circles come closer to each other, and for a moment the one on the right side moved to the left. But then they drift back a little toward the starting position. It makes my eyes feel like two magnets that start to attract each other, but then repel.” Wow! I told her that I was so impressed with her analogy that I was going to quote her. So think about that for a moment.
I’ve never quite thought about efforts at binocular vision in the way that “A” described it, and it’s quite elegant. For those of us with normal binocular vision, the impulse to fuse is so strong that when we’re dissociated briefly our fusion reflex acts like two magnets that re-couple as soon as the images are brought back within Panum’s Fusional Area. But for someone who is struggling to maintain fusion, there are competing forces between coupling and de-coupling the eyes. The prism bar is manipulating the two images like an electro-optical magnet, setting up conditions that either attract or repel fusion.