The August cover photo of the journal of Proceedings of the National Academy of Sciences features the genetics of lager beer and is pretty, but isn’t as interesting as the article in the current issue on perceptual learning of stereopsis in adult patients with strabismus who were formerly stereoblind. The conclusion of the article, that the stereopsis developed by these subjects has reduced resolution and precision based on clinical random dot stereogram (RDS) targets, but has substantial benefit in everyday life, is crucial. Let’s take a closer look at the training paradigm.
Each training trial began with a binocular-fusion assisting frame show in “A”. Interestingly, even though three of the four patients with strabismus were alternators on cover testing, they were all left eye dominant on sensory testing. As you can see in frame “A”, the contrast is reduced to the dominant left eye or DE (LE), and maintained at its original contrast level of the non-dominant right eye or NDE (RE). The reduced contrast shows as the faded outer frame or border of the target, as well as the faded inner portion which has a central suppression cue. The four inner squares are common to both eyes, but the lines within the square are not. The top/right lines are only seen by the RE and the bottom/left lines only by the LE.
By adjusting the vertical and horizontal position of the image to each eye, and lowering the contrast to the DE (LE) sufficiently, fusion was attained. Once fusion was achieved, the target in “B” was presented stereoscopically with the patient focused on identifying the float in the square wave grating pattern.
These principles of fusion are embodied in several chapters of the textbook, Applied Concepts in Vision Therapy. There are numerous ways to bias the brain into favoring the NDE, most of which involve some sort of penalization or degrading of the image of the NDE as compared to the DE. An instrument that’s been around for ages is the Synoptophore, in which the image to the DE can be de-tuned by decreasing brightness. It has the added advantage of breaking suppression by enabling photic stimulation to flicker the image. It also makes it easy to align the images not only taking into account vertical and horizontal alignment, but to rotate the target allowing for cyclotorsion.
Now here’s a low tech way to reduce the contrast of the DE: Bangerter Foils. You can take any set of vectograms, have the patient view with polaroid filters, and use a Bangerter occlusion foil to reduce contrast to the DE until the patient is able to appreciate stereofusion. The easy way to do this diagnostically is to use a Bangerter foil bar over the DE that allows you to titrate contrast up and down.
One can use a plus lens over the DE to adjust the level of contrast reduction as well. Here’s one of the interesting and surprising results of the study. The authors note that to attain a fusion lock, contrast reduction to the DE was necessary. However, to attain the stereo effect, equality of the offset pair of images (as you can see in “B” above) yielded the best result. While they speculate about the reason why this might be so, you can try it clinically with your patients who find it difficult to appreciate stereopsis centrally.
I’m not clever enough to do this stuff on the computer, but I’ll borrow an image from Squnity Josh and you’ll have to visualize a bit. Ding and Levi’s research would predict that if you washed out the ring of a Quoit vectogram corresponding go the DE (and Lord knows our vectos seem to fade all on their own!), but insert a central target that has sterefusion offset (represented here by the red square), the patient who normally can’t appreciate central stereopsis might be able to do so. Perhaps a simple way to do it is place the Bangerter Foil on the outer or peripheral portion of the vectogram corresponding to the DE, and leave the inner stereo stimuli equal to both eyes. I’d have to play with it, but I’m thinking Dr. Lederer’s vectogram target might lend itself to this if the central portion of the target has stereo.
The beauty of this article is that the individuals with strabismus reported nice “pop out” quality of life changes involving things like 3-D movies, judging stairs, and sports. And for that, doing well on central stereopsis and booklet tests such as RDS probably aren’t as relevant, whereas for demanding central judgments like banging in a nail or threading a needle, developing central stereo might be very useful.
– Leonard J. Press, O.D., FCOVD, FAAO