Dr. Curt Baxtrom related a nice insight after Part 4 that prompts me to do a Part 5. He noted the duality of stereopsis in terms of form vs. motion processing, and related that Dave Cook gave an RCS on developing stereopsis in strabismus and amblyopia, with emphasis on peripheral stereopsis. This is another feature of the dual nature of stereopsis, from the standpoint of form vs. motion and center vs. periphery that likely has neuroanatomic correlates in different “V” centers of the cortex.
There are many useful procedures that Dave as well as Bob Sanet cover in their seminars that use stereopsis to help an individual localize in space. They also make the point that that our clinical tests of stereopsis based on booklets such as the RDS and Wirt Circle Tests really aren’t a good index of how a person utilies stereopsis for day to day function “in the real world”.
I want to therefore highlight something I mention in Part 4, which is the use of something like Eccentric Circles. When projecting the acetate circles into space using divergence, it is one thing to be able to perceive form stereopsis. It is quite another to achieve motion stereopsis, particualrly in the periphery. So we always emphasize a phases process like this:
1) attain fusion of the center set of circles with use of a pointer
2) attain fusion of the center set of circles and remove the pointer
3) trombone the circles inward and outward while maintaining stereoscopic localization of the inner and outer circles.
4) rotate with the circles clockwise, then counterclockwise, while maintaining stereoscopic localization. Begin with small arcs of the circles and them progressively widen the arcs into the periphery.
Here’s a crucial point. We always begin ECs with as formless a background as possible so that the field is uncluttered. A light, blank wall or blue sky is preferable. This minimizes the effect of “optic flow”. Once the patient is able to do this, they can seek to have objects in the background, which increases optic flow of the surround relative to the target. It functions as a cognitive load of sorts, as the patient in many instances struggles to maintain fusion if not stereoscopic localization. The patient does this sitting or standing initially, but then proceeds to walk while doing the circles.
Another variation on the procedure is for the patient to maintain stereoscopic projection while slowly rotating their head about the three axes in space:
a) chin up vs.chin down
b) head tilted L vs. head tilted R
c) head rotated L vs. head rotated R
That integrates the visual-vestibular interactions (semi-circular canal changes) with stereoscopic perception. In addition to patients transitioning from strabismus to binocular perception, this is an important phase for patients recovering from acquired brain injury.
I wrote about this extensively in the chapter that Bob Sanet and I co-authored in the textbook co-edited by Penelope Suter and Lisa Harvey.
Lastly I’ll return to a point I made in Part 2, which is the wealth of information on stereopsis in Chapter 21 of Borish’s Clinical Refraction (2006) by Dr. J.J. Saladin. His summary of controller system theory relates directly to the principles I discussed about therapy in Part 4, in terms of building reflex fusion incorporating stereopsis. To paraphrase (p, 957):
Vision therapy decreases and stabilizes fixation disparity and therefore aids stereopsis. Vision therapy increases the sensitivity of the disparity detectors, decreases suppression, and therefore increases disparity detector gain. Note that the same disparity detectors that serve disparity vergence also serve stereopsis. Vision therapy also increases the gain and quickens the SVA (slow vergence adaptation) mechanism. Both of these events take a load off of the disparity detectors, thereby decreasing fixation disparity and improving stereopsis (especially stereoscopic threshold). This also permits the reflex, disparity driven fusional vergence mechanism to precisely control disparity through the feedback loop. This precise control in turn stabilizes fixation disparity and improves the reliability (quality) of stereopsis. One can now appreciate that the quantity and quality of stereoscopic ability are intertwined with the binocularity of the visual system, and that stereopsis is indeed the barometer of binocularity.