I haven’t yet asked Strabby, the vision therapy blogging patient par excellence, which practice she goes to for her vision therapy sessions. Frankly I’m enjoying being kept in some suspense, as Strabby masterfully relates her progress through her skilled vision therapist, Jeri, functioning as a combination visual tour guide and personal trainer. In her latest installment Strabby describes the use of an office-based computerized VT program pioneered by optometrist Dr. Jeffrey Cooper. She writes that instead of seeing a proper 3D square, she saw 3D-ish areas. What an elegant description! To understand this a bit more, we’ll have to go back to the basics of the vision therapy target that is used for both the in-office version that Strabby is using, Computer Orthoptics, and its companion version for home, HTS.
The in-office version uses liquid crystal glasses that controls which part of the computer screen is seen by the right eye and which part is seen by the left eye. When both eyes are working together effectively, a virtual image in the form of a square is seen on the screen, a unique product of the summation between right and left eyes. Because the target is virtual, someone not wearing the liquid crystal glasses but watching the screen will simply see the blue section and the red section of the screen either sliding toward each other and overlapping or sliding away from each other. They won’t see the 3D square in the center. Strabby’s job when she sees the square is to decide in which direction it has shifted – up, down, left or right, and to make her response with a joystick in that direction.
How is it then that Strabby doesn’t really see a square, but a sense of something that seems 3D-ish? Let’s go back to the basic chairside test of Random Dot Stereopsis, the Random Dot “E”. A patient with well adapted constant strabismus has no business seeing this target. Why not? Because it requires precise alignment of the “foveas” or centers of the two eyes simultaneously which, by definition, a strabismus is lacking. As Strabby notes, she was able to see other kinds of stereo targets reasonably well in 3D because these targets provide a little wiggle room allowing for their perception even in the face of slight misalignment between the corresponding centers of the two eyes. Not so the Random Dot Stereogram. Or perhaps we should learn to qualify that statement better.
Let’s take a page again out of Strabby’s notebook. In the booklet pictured here she was able to see which circle and animals were popping out quite well on each row. However, she added parenthetically, I could barely perceive any 3D-ness with the Random-Dots on the right-hand side of the test.
Yet take the Random Dot “E” Test plates above, show them to Strabby, and ask her to pick which of the two plates looks totally blank, and which one looks “different”, even if it doesn’t really look like an “E”. I’ll bet you dollars to doughnuts that Strabby gets it right every time. How is that possible? Is she somehow picking up one eyed clues to depth? Nope. Because if you ask Strabby to cover one eye while she’s looking through the polaroid filters, the two plates will now appear identical and blank. Uncover one eye and the something in the “E” plate will re-emerge. Ultimately that something 3D-ish will coalesce into form.
This is the exciting transition that patients like Strabby embark on when they arrive at the junction she’s crossing in her VT program. The binocular centers in her brain are beginning to coincide well enough that she doesn’t have to settle for stereovision that is an anomaly. As the great vision scientist David Marr might have said, she is working on an upgrade to her “2.5 D” vision sketch, or what Strabby describes as an area that is 3D-ish. While perceiving Randot forms sharply may be considered by some to be the epitome of 3D-ness or bifoveality, the view from binocular space is so much richer than that. Continue reading Strabby’s description of her yoked prism experiences, and you’ll find out just how much richer it can be.
– Leonard J. Press, O.D., FCOVD, FAAO
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