We get this question a lot, so we figured it was worth explaining properly. If someone has a homonymous hemianopia, meaning they've lost the same side of vision in both eyes, why would we only put the Peli prism lens in one eye? The vision loss affects both eyes. Shouldn't the help go in both eyes too?
It's a fair question. Here's what's actually going on.
Two Eyes, One Shared Blind Spot
With a homonymous hemianopia, a person loses the same side of vision in each eye. So someone with a left hemianopia is missing the left side of the picture, in both eyes at once. What they're left with is half a visual field on each side, and their brain combines those two half-fields into one overall picture of the world.
The goal of the Peli lens is to expand that visual field out toward the blind side, giving the person more warning of what's approaching from the side they can't naturally see.
Why One Eye Sees More Clearly Than Two Eyes Together
Here's where it gets a little counterintuitive. When we test vision, we test each eye on its own. A right eye test and a left eye test each give us a clear picture of what that single eye sees. But that's not quite the same as what the person sees with both eyes open and working together.
When we fit a Peli lens on one eye, say the left eye, for a person with a left hemianopia, the prism takes information from the blind side and shifts it into view on the seeing side of that same eye. That information has to come from somewhere, so the area it used to occupy in that eye's field is now empty. If we tested that eye alone afterward, we would see a gap where that information used to be, along with the new expanded field out to the side.
The other eye, left untouched, still has its normal field.
What Happens When You Fit Both Eyes
Now think about fitting the Peli lens on both eyes instead of just one. Doing that expands the field toward the blind side in both eyes, which sounds like it should be better. But it also creates that same gap, that scotoma, in the same spot in both eyes at once.
Fit one eye, and one eye keeps its normal straight-ahead field while the other eye covers the side. Fit both eyes, and you gain field toward the blind side, but you lose field on the seeing side in both eyes together. Because that gap shows up in the same location in both eyes when you fit two, it can't be filled in by the other eye the way it would be if only one eye had the prism.
The end result is straightforward, even if the reasoning takes a minute to follow: fitting the Peli lens on just one eye gives most patients a larger overall field of view than fitting both.
