P3D Re: Exotropia and stereo

[Linda Nygren <lnygren@xxxxxxx>]

Mark Langenfeld wrote:

> I suffer from a chronic visual condition called exotropia (more commonly
> known as "lazy eye"). Exotropia is marked by the lazy eye deviating
> outward (mine also deviates slightly upward), as opposed to inward
> (diplopia) in the cross-eyed individual. I have been pretty thoroughly
> examined in connection with obtaining a waiver for an aviation medical
> certificate, and the findings were essentially: (a) I look out of one eye
> at a time ("alternating") to avoid double vision; and (b) I should
> not be able to achieve image fusion.
> 
> But I have just recently discovered that everything from View-Masters to
> antique parlor stereoscopes give me splendid 3D results. (Interestingly, a
> co-worker also afflicted with "lazy eye" also tells me he gets good
> results with a V-M). This was a startling discovery for me, bordering on a
> visual epiphany. Needless to say, I am smitten.
> 
> Does anyone have a good eplanation for how and why someone supposedly
> without binocular vision can find himself so thoroughly enjoying the 3D
> world?
> 
> Regards,
> 
> Mark
Hi, Mark. You may have seen a recent thread about *exophoria*, which is
a milder tendency for the eyes to deviate outward when one is blocked
and fusion is broken. As you know, *exotropia* is when the eye deviates
out persistently and image fusion cannot be achieved. Comparable terms
for eyes that turn in are *esophoria* and *esotropia*. *Stabismus* is a
term including both exotropia and esotropia, and is sometimes referred
to as *lazy eye* since one eye doesn't cooperate and fix on the same
object as the other eye. More properly, *lazy eye* refers to actual loss
of visual acuity (also called *amblyopia*) that occurs in an unused eye,
such as if a person with strabismus only uses one eye and ignores the
other image to avoid double vision (the technical term for double vision
is *diplopia*).
In Mark's case, since he has alternating fixation, he apparently
maintained visual acuity in both eyes. The reason kids with strabismus
are treated with eye patches, as early as possible, is to prevent this
visual loss in the less dominant/weaker eye (actually, it is the brain
that loses the ability to process the visual input if it is deprived of
this input during brain growth and development.)

Anyway, the neat thing about what Mark is saying is that the prisms in
the viewers apparently direct his line of sight inward enough that he
can fuse the images and see stereo (presumably he does the vertical
correction himself).  Sometimes people have some prismatic correction
put into their eyeglass prescription and this would potentially allow
binocular vision in everyday life as well if you are actually able to
fuse the images.(It would probably take some getting used to.) Has your
eye doctor ever had you try a prismatic correction during an eye exam,
Mark? Also, I wonder whether you have tried parallel freeviewing of old
stereo cards. It might work well for you. -Linda


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