The Dual Nature of Stereopsis – Part 4

Let’s put together some of these concepts into therapy principles.  In part 3  we noted Professor Super’s observations that the flexiblity of registering divergent as well as convergent disparity, as well as the speed of accurate stereoscopic projection holds significance.  One common tool in vision therapy is vecctograms, and the spiraling stereoscopic effect of the Springle affords the same level of phased stereoscopic appreciation that I reviewed in part 3.

The first level of appreciation might be that the letters aren’t all the same plane.  Once this registers, the sprial effect begins to emerge, as well as the localization of each letter as being on plane, receded, or popped out relative to its own box.  When set at ortho on the scale, the letter “E” is on plane and the letter “I” has the most convergence disparity, measuring apx. 7^ BO at a 40 cm viewing distance.  If you separate the vectograms to “C” on the scale at the bottom, the letter “E” will have 3^ of divergent (BI) disparity and the letter “I” will have 4^ of convergent disaprity (BO).  Once the patient is able to appreciate that she is diverging to “E” and converging to “I”, qualititive aspsect of stereopsis should be reinforced.  The letter “E” should appear larger than letter “I” due to SILO.  When noting motion parallax, the letters “E” and “I” will move in opposite directions.

Next, have the patient condition the speed of stereoscopic response by either looking away and then looking back at the target, or doing a cover/uncover procedure.  When you break fusion by covering one eye, note the speed with which accurate stereoscopic perception is recovered.  Is it equal when uncovering the right eye as it is when uncovering the left eye?  When you count to ten before uncovering one eye, is the delay much longer than when counting to three?  As with many aspects of vision therapy, though we may not attain perfect symmetry right and left eyes, we want to approach symmetry of reflex fusion, thereby sharpening stereoscopic perception.

An alternative to the Spirangle vectogram, also affording simultaneous convergence and divergent disparity is the Vortex Polarized Variable Vectogram.



There are other vectograms that provide simultaneous convergent and divergent disparity.  One is the Chicago Skyline vectogram.  Another way to do develop flexibility between convergent and divergent stereoscopic projection is to use the two-tiered vectogram with the disparity in opposite directions, one Base In and the other Base Out.  Another way to extend flexibility is to have the patient switch between a near vectogram with convergent stereoscopic disparity and a projected overhead vectogram with divergence stereoscopic disparity.

There are other ways to achieve stereoscopic speed and flexibility.  The Life Saver Card has built in simultaneous base in and base out disparity.  On the second row, for example, in the word “CLEAR”, when coverging the letter “E” is on plane, the letter “L” is forward and the letter “A” is behind the plane.  The VTS-3 can also be used to attain projected stereoscopic flexibility.   Lastly, what happens when the patient is doing these procedures on a balance board or walking rail?  Does activating the movement and vestibular systems help or hinder the patient’s stereoscopic perception?

– Leonard J. Press, O.D., FCOVD, FAAO

2 thoughts on “The Dual Nature of Stereopsis – Part 4

  1. I’ve enjoyed reading this thread. Seems we have two major groups- form and motion stereopsis. With the work in infantile monocular motion processing asymmetries, it seems that motion is a critical component. I had the opportunity to see David Cook’s seminar, wonderful RCS on Stereopsis…..emphasis was alot in the area of target motion, especially in the periphery. Thanks again!

    • You’re welcome,. Curt. Sorry I didn’t get to experience Dave’s seminar info on target motion in the periphery. I will say however that I’ve always emphasized patients dealing with stereo cues in motion. In fact, I’ll throw that into a part 5.

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