At the end of Part 1 I suggested an experiment in which you would tilt your head toward one shoulder and see how much offset you could tolerate before losing the stereo view in the picture below. Then tilt your head in the opposite direction and see if the amount of tilt before you lose it just about the same.
The amount that you should be able to tilt your head to the right or left before losing fusion should be roughly equal — about 12 degrees before you experience double vision or suppression, and about 8 degrees before you recover fusion. As Dr. Margareten pointed out in his comments following Part 1, that doesn’t leave much wiggle room. Even if you’re not an optometrist, you can probably visualize how small an angle of torsion or tilt 12 degrees is from your school days protractor (I know, I’m dating myself).
I like a term that David Guyton used regarding the ability to tolerate fusional imbalances of this is nature, which is perturbation. (You can read Guyton’s article here.) I’m convinced after many years of practice, after having done routine cheiroscopic tracings and red Maddox rod testing in free space, that the prevalence of patients with binocular instability the includes cyclovertical imbalance is under-recognized.
It is my contention that sustained S3D viewing will unmask these patients because their ability to compensate either while wearing 3D polarized glasses or even in auto-stereoscopic viewing such as you can do with the photo above, will be compromised. These types of binocular imbalances may be primary or secondary effects of postural skews. And normally it is a head-to-toe problem that includes every pivot point in the body such as head, neck, shoulders, pelvis, feet — and don’t forget to count the eyes as ball-in-socket pivot points. As pointed out by Guadalupe Mejia in the Journal of Behavioral Optometry, this ultimately translates into problems with visual-vestibular integration, and induces feelings of dizziness or discomfort.
So what can we do about this?