Stereopsis enhances our visual world by literally and figuratively adding another dimension to our view of nature, and no one addresses these qualitative features better than Sue Barry in Fixing My Gaze. As noted in Part 6 stereopsis is also a quale that serves as a barometer of binocular efficiency, and a compass of sorts when it comes to spatial judgment. In today’s terms we might say that it’s an important feature of the visual system’s GPS (Global Positioning System).
Here’s one practical example. I was walking down the block this morning and heard the sound of jackhammering in the distance. My attention was drawn to a work area in the region of the red awning, which is the building that I wanted to enter. Is the work being done at the entrance, so that I would be better served by entering the rear of the building?
If you have excellent stereopsis you can make that judgment from a considerably greater distance than if your stereoscopic localization is poor. Coarse/qualitative/magno pathway or second order stereoscopic judgment is what we’re talking about here. Booklet tests of stereopsis like RDS figures or Wirt Circles yielding fine/parvo pathway/first order stereoacuity won’t necessarily be a good index of your judgment here.
As you draw nearer to the building, particularly if your stereopsis failed you at a distance, the fact that the work area is beyond the red awning becomes much more obvious. If the work area was indeed within the red awning, it may have been only a nuisance that you walked an extra block to find this out, and would have to backtrack and re-route to the rear entrance of the building. If you’re driving, and you have to make an analogous judgment, getting it wrong may cause you to stop short or swerve into the next lane to avoid encroaching on the work space. Now your compromised stereoscopic perception can be a potential road hazard to other drivers instead of a mere nuisance.
Let’s finish with a flourish by introducing a few more key articles and concepts. Acknowledgment is due to Dr. Samantha Slotnick, who pointed out the great review article on stereopsis performance by J.J. Saladin, in which he gives extensive treatment to qualitative aspects that we’ve highlighted such as speed of response, reliability-robustness, and strength of percept. We tend to approach vision therapy dealing with stereropsis in three primary vectors of space. However in a dynamic world, where qualitative stereopsis predominates, there are many sub-vectors of space that are operative as we look at different angles. That’s why we emphasize in therapy doing binocular procedures at different viewing angles. This can be done initially with Brock String in different positions of gaze, as well as keeping the string angle constant, but having the patient change the viewing angle by purposely tilting her head to the left or right. We then ultimately extend this procedure to free space fusion targets with stereo cues.
Schreiber and Schor do a great job of representing the kinematic complexity of these movements. They use a virtual ophthalmotrope designed to illustrate Helmholtz, Fick, and rotation vector coordinates, as well as Listing’s extended law. Another marvelous reference for understanding the geometry of stereoscopic space volume, paticularly at different angles and in different planes of gaze, is Howard and Rogers brilliant Volume 2 on Depth Perception in Seeing in Depth.
One final thought. We have a favorite expression in Optometry that “A pair of eyeballs never walks into your office”. However, there is no escaping the fact that we are talking about eyeballs here, and they are not incidental to the discussion. The references cited on kinematics reminds us that the motor command signals from the visual centers in the brain telling the eyes what to look for, and the feedback loops involving the integrated pulley systems of the extraocular muscles, pivot about a center of rotation with multiple vectors.
These brief clips of Poincare/Bloch spheres call to mind the dual nature of stereopsis that toggles between states: the fine and the coarse, the quantitative and the qualitative, the central and the peripheral, the static and the dynamic, the magno and the parvo pathways. This represents a quantum aspect of directed eyeballs searching as coordinated spheres within and without their orbits, with stereopsis as a key element of this global positioning system.
– Leonard J. Press, O.D., FCOVD, FAAO